Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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inactivity. (See Dietary Fiber: Physiological Effects;

Effects of Fiber on Absorption.)

0021 -Predigested, chemically defined formulas Pre-

digested, chemically defined formulas are also called

elemental formulas or diets. These preparations are

used in the nutritional management of patients with

impaired gastrointestinal function, such as severe im-

pairment of luminal nutrient hydrolysis or substan-

tially reduced functional absorptive capacity (e.g.,

short-bowel syndrome, exocrine pancreatic insuffi-

ciency). Predigested diets contain carbohydrate in the

form of glucose polymers, peptides and amino acids as

the nitrogen source, fat, usually as a mixture of

medium-chain and long-chain triglycerides, and the

full complement of vitamins and minerals.

0022 -Disease-specific formulas Specific dietary formula-

tions have been developed for patients with condi-

tions such as liver disease, renal disease, or

respiratory disease.

0023 Liver disease Patients with encephalopathy and cir-

rhosis have abnormal plasma amino acid profiles

characterized by raised levels of aromatic amino

acids (i.e., tyrosine and phenylalanine) and depressed

levels of branched-chain amino acids (i.e., leucine,

valine, and isoleucine). These changes in the pattern

of amino acid levels have been implicated in the

pathogenesis of hepatic encephalopathy. Enteral

feeding preparations containing low levels of aro-

matic amino acids and higher levels of branched-

chain amino acids are available for oral or tube

feeding: the administration of these diets may be of

benefit to patients with encephalopathy associated

with acute or chronic liver disease. (See Liver: Nutri-

tional Management of Liver and Biliary Disorders.)

0024 Renal disease Formulas containing all of the essen-

tial amino acids plus histidine, without the non-

essential amino acids, have been developed for

patients with impaired renal function. It is suggested

that these diets reduce urea production and promote

the reutilization of nonessential amino acids in the

liver by transamination. At present, there is little

evidence to support their chronic use to delay pro-

gression of chronic renal disease.

0025 Respiratory disease The level of energy from carbo-

hydrate in enteral formulas can result in further in-

creased production of carbon dioxide and respiratory

distress in some patients. In order to reduce the pro-

duction of carbon dioxide, the use of enteral formulas

which have a higher fat:carbohydrate ratio has been

suggested for patients with respiratory disease. These

formulas may facilitate more rapid weaning from

ventilatory support.

0026-Modular diets Modular diets allow specific com-

ponents of a diet to be altered according to the indi-

vidual patient requirement. These include products

that provide various types of isolated carbohydrate,

protein, and fat compounds.

Initiation and Contraindications of Enteral Feeding

0027Some care teams administer small volumes of diluted

enteral feeds during the initial stages of nutrition

support in order to minimize any gastrointestinal

side-effects. Some studies report that the use of dilute

‘starter regimens’ does not decrease the frequency of

gastrointestinal side-effects but may result in reduced

energy and nutrient intake and thus may adversely

affect the nitrogen balance. Enteral nutrition is

contraindicated in patients with an inaccessible or

nonfunctioning gastrointestinal tract (e.g., paralytic

ileus, intestinal obstruction).

Complications of Enteral Nutrition

0028Malposition of enteral feeding tubes may occur, but

this can usually be avoided by strict adherence to the

protocol for the insertion of feeding tubes, and con-

firmation of the position of the tube. Unintentional

removal of feeding tubes is commonly noted in

agitated, disoriented patients. Blockage of tubes can

occur unless they are flushed regularly.

0029Diarrhea is the most commonly reported side-

effect, and the factors implicated are high osmotic

loads, concomitant antibiotic therapy, and exces-

sively rapid infusion. Problems of lactose intolerance

are limited, since the majority of commercial enteral

formulas are low in lactose or lactose-free. Tube-

feeding-related diarrhea occurs in up to 30% of

patient, but its etiology is unclear. Nausea, vomiting,

bloating, cramps, and abdominal distension can

occur in some patients, especially following bolus

feeding or when the feed is infused at a high flow

rate. Pulmonary aspiration and regurgitation may be

reduced by elevating the head of the patient’s bed.

Duodenal (transpyloric) feeding may be beneficial in

patients at risk for aspiration.

0030Abnormalities of liver function tests are generally

minor and reversible with an enteral feeding regime.

Regular monitoring of patients enables early detec-

tion of biochemical imbalances, including electro-

lyte and fluid balance abnormalities. Drug–nutrient

interactions can occur with many medications, in-

cluding theophylline, warfarin, methyldopa, and

digoxin.

ENTERAL NUTRITION 2117

Monitoring Patients Receiving Enteral

Nutrition

0031 Monitoring enterally fed patients is an essential as-

pect of providing safe and effective nutrition support.

Fluid Balance Charts and Weight

0032 Continuous fluid-balance records enable an accurate

review of the patient’s actual intake of enteral for-

mula compared to the prescribed regimen, as well as

other fluid intake. The prescribed regimen and the

actual intake may not be the same, and without

records, patients may receive suboptimal (inadequate

or excessive) levels of nutrition support. All patients

receiving enteral nutrition support should be weighed

regularly. The frequency of the weight assessment

depends upon the patient’s age, nutritional status,

and medical condition.

Biochemical and Hematological

0033 Plasma glucose, electrolytes, calcium, phosphate, and

magnesium should be monitored regularly, and more

frequently in severely malnourished patients. Plasma

proteins are useful markers of nutritional status, trans-

ferrin and prealbumin in the short-term, and albumin

in the long-term. Hemoglobin and the white blood cell

and lymphocyte count should be monitored regularly

and often. (See Nutritional Assessment: Biochemical

Tests for Vitamins and Minerals.)

Anthropometric and Dynamometric

0034 Measurements of mid-arm circumference and triceps

and subscapular skinfolds should be taken regularly

and are commonly used for research purposes. (See

Nutritional Assessment: Anthropometry and Clinical

Examination.)

Ending Enteral Tube Feeding

0035 As the patient is able to increase oral intake of regular

food, the administration of the enteral tube feed is

decreased. Oral and enteral tube intake should be

monitored using food record charts to document the

nutritional quality of the total intake. Enteral feeding

should not be discontinued until adequate nutrition

can be taken from food and/or oral dietary supple-

ments, and normal hydration and weight maintained.

Nutrition Team

0036 Team management of enterally fed patients reduces

associated morbidity and optimizes nutrition

support. This interdisciplinary team usually includes

a dietitian/nutritionist, nurse, and physician.

See also: Carbohydrates: Requirements and Dietary

Importance; Dietary Fiber: Physiological Effects; Effects

of Fiber on Absorption; Energy: Measurement of Food

Energy; Fats: Requirements; Liver: Nutritional

Management of Liver and Biliary Disorders; Malnutrition:

The Problem of Malnutrition; Minerals – Dietary

Importance; Nutritional Assessment: Anthropometry

and Clinical Examination; Biochemical Tests for Vitamins

and Minerals; Protein: Requirements; Deficiency;

Vitamins: Overview

Further Reading

Alpers DH and Klein S (1999) Approach to the patient

requiring nutritional supplementation. In: Yamada T,

Alpers DH, Laine L, Owyang C and Powell DW (eds)

Textbook of Gastroenterology 3rd edn, pp. 1081–1107.

Philadelphia, PA: Lippincott Williams & Wilkins.

DeLegge MH and Copenhaver (1999) Enteral and paren-

teral nutrition. In: Brandt LJ and Daum F (eds) Clinical

Practice of Gastroenterology, 1st edn, vol. 2, pp. 1567–

1575. Philadelphia, PA: Churchill Livingstone.

Forchielli ML, Hendricks KM and Lo CW (1999) Home

enteral parenteral nutrition. In: Walker WA and Watkins

JB (eds) Nutrition in Pediatrics: Basic Science and

Clinical Applications, 2nd edn, pp. 727–733. Hamilton,

Ontario: B.C. Decker.

Mascarenhas MR, Tershakovec AM and Stallings VA

(1999) Parenteral and enteral nutrition. In: Wyllie R

and Hyams JS (eds) Pediatric Gastrointestinal

Disease, 2nd edn, pp. 741–757. Philadelphia, PA: W.B.

Saunders.

Mascarenhas MR, Kerner JA and Stallings VA (2000) Par-

enteral and enteral nutrition. In: Walker WA, Durie PR,

Hamilton JR, Walker-Smith JA and Watkins JB (eds)

Pediatric Gastrointestinal Disease, 3rd edn, pp.

1705–1752. Hamilton, Ontario: B.C. Decker.

Shike M (1999) Enteral feeding. In: Shils ME, Olson JA,

Shike M and Ross AC (eds) Modern Nutrition in Health

and Disease, 9th edn, pp. 1643–1656. Philadelphia, PA:

Williams & Wilkins.

Smith P, Zeitzer J and Leibowitz AB (1995) Enteral nutri-

tion. In: Haubrich WS, Schaffner F and Berk JE (eds)

Gastroenterology, 5th edn, pp. 3209–3220. Phila-

delphia, PA: W.B. Saunders.

Stoker TW and Castle JL (1999) Special diets. In: Walker

WA and Watkins JB (eds) Nutrition in Pediatrics: Basic

Science and Clinical Applications, 2nd edn, pp.

727–733. Hamilton, Ontario: B.C. Decker.

Torosian MH (ed.) (1995) Nutrition for the Hospitalized

Patient: Basic Science and Principles of Practice. New

York: Marcel Dekker.

2118 ENTERAL NUTRITION

Enterobacteriaceae See Escherichia coli: Occurrence; Detection; Food Poisoning; Occurrence and

Epidemiology of Species other than Escherichia coli; Food Poisoning by Species other than Escherichia coli

Enzymatic Browning See Browning: Nonenzymatic; Toxicology of Nonenzymatic Browning; Enzymatic

– Biochemical Aspects; Enzymatic – Technical Aspects and Assays

Enzyme Immunoassay See Immunoassays: Principles; Radioimmunoassay and Enzyme

Immunoassay

ENZYMES

Contents

Functions and Characteristics

Uses in Food Processing

Uses in Analysis

Functions and Characteristics

J R Whitaker, University of California, Davis, CA, USA

Introduction

0001 Enzymes are absolutely essential for life. They occur

in all animals, plants, and microorganisms. Nearly all

chemical reactions in living organisms are a result of

enzyme activities. Enzymes cause reactions to occur

rapidly at ambient temperatures with a high degree

of specificity. As an example, it takes 24–72 h at

100–110



C and 6 N hydrochloric acid to hydrolyze

a protein. Gastrointestinal-tract enzymes accomplish

this in 2–4 h at 37



C. There are thousands of

different enzymes in living organisms.

Definition of an Enzyme

0002 An enzyme is a protein with catalytic properties due

to its power of specific activation. Some enzymes also

require cofactors for activity. The properties and

characteristics of enzymes are described below. A

few ribonucleic acids, called ribozymes, also act as

catalysts for a limited number of hydrolytic reactions,

but they do not fit the definition of an enzyme as

defined above.

History

0003Although the effects of enzymes, such as fermenta-

tion, digestion, milk clotting and meat tenderization,

were known for centuries, the first clear recognition

of an enzyme as a unique compound was in 1833,

when Payen and Persoz reported that an alcohol

precipitate of malt extract contained a thermolabile

substance ‘diastase’ (now known as amylase) because

of its ability to separate starch from insoluble envel-

opes of starch grains. They identified some character-

istics of enzymes – precipitation with alcohol,

thermolability, and specificity – and designed a

naming system (stem of substrate name plus the suffix

‘-ase’) that is used universally today. Schoenbein, in

1855, discovered peroxidase in plants that, in the

presence of hydrogen peroxide, causes a brown

solution of gum guaiac to turn blue. Later (1856),

Schoenbein also discovered polyphenol oxidase in

mushrooms, which causes them (and many other

plant tissues) to turn brown. Berthelot in 1860

ENZYMES/Functions and Characteristics 2119

discovered invertase in yeast, which causes an inver-

sion in the optical rotation of a sucrose solution.

Several thousand enzymes are now well known.

0004 The relationship between enzymes and living cells

was a controversial topic between 1875 and 1900.

Pasteur, a microbiologist and enologist, argued that

fermentation was inseparable from living cells. Lie-

big, a chemist who demonstrated the action of pepsin

on proteins, held that enzymes are chemical sub-

stances, active in the absence of cells. Bu

chner, in

1897, separated broken yeast cells from liquid, show-

ing that the cell-free extract carried out fermentation,

thereby ending the controversy. In 1878, Ku

hne pro-

posed the name ‘enzyme’ (Greek for ‘in yeast’) for the

substance transforming one compound to another.

(See Fermented Foods: Origins and Applications.)

Protein Nature of Enzymes

0005 Enzymes are proteins. This was generally accepted in

the late 19th and early 20th centuries. However, this

idea was challenged in the late 1920s by Willsta

tter. He

purified peroxidase until no protein was detectable,

but appreciable activity remained. Therefore, he con-

cluded that enzymes could not be proteins. Sumner, in

1926, crystallized urease from Jack Bean meal and

showed it to be a protein. This set off a polemic debate

between Willsta

tter and Sumner that involved many

scientists. In 1930, the famous biochemist Haldane

indicated that, with the single exception of urease,

almost nothing was known about the chemical nature

of enzymes. However, soon thereafter, scientists at the

Rockefeller Institute in New York, led by the distin-

guished biochemist, John Northrop, crystallized

pepsin, trypsin, chymotrypsin, and carboxypeptidase

A, among others. All were proteins. More than 600

enzymes, all proteins, have now been crystallized.

0006 As proteins, enzymes have specific structures. Each

has a fixed molecular size and a specific amino acid

sequence (primary sequence), as determined by the

gene for that enzyme. The primary structure is folded,

via a-helices, b-pleated sheets, b and g bends and

random-coil segments, to give secondary structure.

Further folding, giving a tertiary structure, results

from thermodynamic and kinetic requirements that

most of the hydrophobic amino acid residues must be

inside the protein, away from water, and most of

the hydrophilic amino acid residues must be on the

surface. (See Protein: Chemistry.)

0007 Many enzymes consist of single polypeptide mol-

ecules. Others have two or more subunits (identical or

different) per molecule, giving rise to a quaternary

structure. Some enzymes associate to form macro-

molecular structure systems to convert substrate to

product(s) more efficiently. Escherichia coli pyruvate

dehydrogenase is a complex of three different

enzymes and five cofactors (M

of 4.44 10

Da).

0008Early on, researchers investigated water-soluble

enzymes. Now, much emphasis is on structurally

bound enzymes.

Solvation of Enzymes

0009Water is an important factor in determining native

enzyme structure, as well as activity. Hydrophobic

amino acid residues are largely inside the enzyme

molecule, away from water, and most of the hydro-

philic amino acid residues are on the outside. Enzymes

contain 30–40% (w/v) of bound surface water

(170–220 mol of water per 10 000 gram of protein).

The relatively high surface hydrophobicity of a folded

polypeptide chain, and its lack of hydration by water

molecules, determines whether an enzyme has a qua-

ternary structure or not. In less polar solvents than

water, but miscible with water, an enzyme probably

would not fold in the same way, and is most likely

inactive. Enzymes can be more stable suspended in

immiscible organic solvents than in water. (See Water

Activity: Effect on Food Stability.)

Size

0010Enzymes are large molecules. Ribonuclease (M

13 683) and lysozyme (14 100) are small enzymes,

alkaline phosphatase (80 000) and mushroom poly-

phenol oxidase (128 000) are of intermediate size,

and b-galactosidase (520 000) and glutamate dehy-

drogenase (2 000 000) are large enzymes.

Catalytic Nature of Enzymes

0011The most unique feature of enzymes is their ability to

bind compounds (called substrates) stereospecifically

and to convert them to other compounds (called prod-

ucts) very efficiently. Two measures of enzyme effi-

ciency are turnover number and rate enhancement.

Example turnover numbers are 10

–10

mol of sub-

strate converted to product per second per mole of

enzyme active site for chymotrypsin and catalase,

respectively. Some examples of rate enhancement

(based on the same temperature and concentration)

are: hydrogen peroxide is converted to water and

oxygen by catalase 3.5 10

times faster than non-

catalytically; invertase hydrolyzes sucrose to glucose

and fructose 5.6 10

times faster than does 1 M

hydrochloric acid; urease hydrolyzes urea to carbon

dioxide and ammonia 4.2 10

times faster than

does 1 M hydrochloric acid. This remarkable rate

enhancement makes life possible at ambient tem-

peratures.

2120 ENZYMES/Functions and Characteristics

001 2 Enzymes increase rates of conversion of substrate

to product by lowering the activation energy, E

. E

the minimum energy that a substrate must acquire

above the ground-state energy to go to product. In

two of the examples above, catalase lowers E

from

18.0 to 6.4 kcal mol

1

(3.5  10

rate enhancement)

for the oxidoreduction of hydrogen peroxide to prod-

ucts and urease lowers E

from 24.2 to 8.7kcalmol

1

(4.2  10

rate enhancement) in the hydrolysis of

urea.

001 3 Enzymes convert substrates to products by well-

known chemical mechanisms. Because of their effi-

ciency, it was once considered that they must have

magical qualities. We now know that several well-

known factors can account for this efficiency. These

are: (1) conversion from inter- to intramolecular

reaction; (2) proximity and specific orientation of

substrate in the active site of the enzyme; (3) catalysis

by distortion of bonds; (4) general acid–general base

catalysis; and (5) nucleophilic–electrophilic catalysis.

These will be explained below.

Active Sites of Enzymes

0014 Reactions occur at active sites of enzymes. The

active site is a small area, a cavity or hole on the

surface of the enzyme. The active site consists of 10–

15 amino acid residues brought together by folding

from different parts of the primary structure of the

protein. One part of the active site is responsible for

stereospecific binding of the substrate in proximity

to the (second) transforming part of the active site. In

a-chymotrypsin, residues Ser189, Gly216, and

Gly226 are responsible for binding the peptide sub-

strate, and residues His57, Ser195, and Asp102 are

responsible for catalyzing hydrolysis of the scissile

peptide bonds. The amino group of Ile16 and the

carboxyl group of Asp194 assist in maintaining

the shape of the active site. The binding site recog-

nizes l-Tyr, l-Phe or l-Trp residues of the substrate.

Peptide bonds involving other l-amino acids or d-

amino acids are not hydrolyzed. There must be very

close complementarity, following any induced fit, be-

tween the shape of the substrate and the active site for

binding to occur. (See Amino Acids: Properties and

Occurrence.)

Proximity and Orientation Effects

0015 Proximity and orientation effects include (1) intra- vs

intermolecular catalysis, (2) reaction entropy, and (3)

effective concentration of reactive groups. An absolute

requirement for catalysis is that the substrate must

bind stereospecifically in a correct orientation at the

active site to form the enzyme–substrate complex, as

shown in eqn (1), where E is the enzyme, S is the

substrate, ES is the enzyme–substrate complex, EP

is the enzyme–product complex, and P is the product.

The obligatory ES complex converts an intermolecu-

lar reaction to an intramolecular reaction. This in-

creases the rate of an enzyme-catalyzed reaction

compared with a noncatalyzed reaction by 10

,and10

times for bimolecular, trimolecular,

and termolecular reactions, respectively.

E + S E

S E

P E + P.

−1

−2

−3

(1)

0016Binding of the substrate at the active site involves

multiple (a minimum of three and up to 12) contacts

between the substrate and the enzyme (Figure 1).

0017As a result of ES, catalytic groups of the enzyme

are 0.1–0.2 nm from the substrate bond to be

transformed, usually by general acid–general base or

nucleophilic–electrophilic catalysis (Figure 2). This

proximity effect is equivalent to about a 10 M

concentration of the corresponding intermolecular

catalyst (His57 in Figure 2).

Enzyme

(a) (b)

fig0001Figure 1 Schematic representation of several possible modes

of orientation of ethanol on the surface of alcohol dehydrogen-

ase. A, B, and C are binding points for two Hs, OH, and CH

ethanol on the surface of the enzyme. Only (a) shows the ethanol

bound properly with the enzyme surface.

ENZYMES/Functions and Characteristics 2121

Catalysis by Distortion

0018 There is a close complementarity between the struc-

ture and shape of a substrate and its binding position

in the active site. This led Emil Fischer to the ‘lock

and key’ analogy for the fit between a substrate and

an enzyme. Later data established that enzymes

permit some (small) latitude in the structure and

shape of a substrate, leading to the ‘rack mechanism’

(Lumry) or the ‘induced-fit’ concept (Koshland). This

results in a decrease in the activation energy needed,

with a rate enhancement of 10

–10

0019Based on the factors above, an enzyme increases

rates of reactions by 10

–10

, depending on how

many substrates are involved in the reaction. Bender

et al. in 1964 experimentally and theoretically

determined rates of hydrolysis of N-acetyl-l-trypto-

phanamide by the three catalysts hydroxide ion, imi-

dazole, and a-chymotrypsin (Table 1). There is

remarkable agreement between the rate calculated

by theory (Table 1, 4d) and determined experimen-

tally (Table 1, 3).

Effect of Environmental Factors on Rates

of Enzyme-catalyzed Reactions

0020Time, enzyme concentration, substrate concentra-

tion, nature of the substrate (including physical

state), pH, temperature, solvent, activators, and in-

hibitors all affect rates of enzyme-catalyzed reactions.

Asp-102

Ser-195

His-57



Asp-102

Ser-195

His-57



Asp-102

Ser-195

His-57



(A)

Asp-102

Ser-195

His-57

(D)

(B) (C)

Asp-102

Ser-195

His-57

(F)

Asp-102

Ser-195

His-57

(E)

(a) Acylation

(b) Deacylation

fig0002 Figure 2 Proposed mechanism for a-chymotrypsin-catalyzed reactions. (a) Acylation. (b) Deacylation.

tbl0001 Table 1 Factors responsible for the rate enhancement of

chymotrypsin-catalyzed hydrolysis of N-acetyl-

L-tryptophan-

amide relative to the hydroxide ion and imidazole

(1) Rate constant for hydroxide ion

catalysis

3  10

4

1

(2) Rate constant for imidazole catalysis 4.8  10

10

1

(3) Experimental rate constant for

a-chymotrypsin catalysis

4.4  10

2

1

(4) Calculated rate constant for

a-chymotrypsin catalysis

(cumulative):

(a) Conversion of imidazole catalysis

to intramolecular reaction (10

)

4.8  10

9

1

(b) Nucleophilic catalysis by

serine-OH(10

)(see Figure 2)

0.8  10

7

1

(10

)

4.8  10

4

1

(d) General acid catalysis by

imidazole(10

)(see Figure 2)

4.8  10

2

1

At 25



C; data from Bender ML, Kezdy FJ and Gunter CR (1964) The

mechanism of action of proteolytic enzymes. XXXIII. The anatomy of an

enzymatic catalysis. a-Chymotrypsin. Journal of the American Chemical

Society 86: 3714–3721.

2122 ENZYMES/Functions and Characteristics

Time

0021 Enzyme-catalyzed reactions are time-dependent. Ini-

tially, the concentration of product, [P], formed is

linearly related to reaction time. How long the linear

relation holds depends on the initial substrate concen-

tration, [S]

, in relation to the Michaelis constant

([S]

at which v

¼ 0.5V

max

; see eqn (2)), enzyme

stability, pH, temperature and activators and revers-

ibility of reaction. A tangent drawn to the initial part

of the curve gives the initial velocity, v

( ¼ dP/dt), an

invaluable experimental parameter in enzymology.

Enzyme Concentration

0022 For most enzyme-catalyzed reactions, v

is directly

proportional to the enzyme concentration, [E]

Doubling [E]

doubles v

. This is analytically very

convenient since enzyme concentrations in biological

systems are based on determination of v

under

standard conditions. However, there are several

cases where direct relations do not hold, as described

by Whitaker in 1994.

Substrate Concentration

0023Because of the obligatory E S (eqn (1)), the relation

between [S]

and v

gives a right-hyperbolic plot

(Figure 3). The extent of saturation of an enzyme

with a substrate, to form ES, is dependent on the

relation of [S]

to K

.At[S]

<< K

, the reaction

rate is first order with respect to [S]

(dP/dt ¼k[E]

[S]

). At [S]

>> K

, the reaction rate is zero order

with respect to [S]

(dp/dt ¼k[E]

), thereby giving the

maximum velocity, V

max

, under the conditions used.

At [S]

¼K

, v

¼0.5V

max

.When[S]

0.01K

100K

, the reaction rate is a mixture of first and

zero order.

0024Enzyme-catalyzed systems that follow Figure 3

obey Michaelis–Menten kinetics

¼ k½E

½S

=ðK

Þþ½S

Þ: ð2Þ

0025Substrate activation and inhibition and allosteric

behavior cause deviations from this relation.

0026A plot of 1/v

vs 1/[S]

gives a linear relationship

(Lineweaver–Burk equation), permitting V

max

and

to be determined readily (Figure 3 insert).

4 8 12 16 20 24

Initial velocity

0.5 V

max

Substrate concentration

0.5

1.0

max

)/K

1/v

1/(A

)

max

fig0003 Figure 3 Relation between initial velocity, v

, and initial substrate concentration, [S]

, for an enzyme-catalyzed reaction. Insert:

effect of three different concentrations of a competitive inhibitor on 1/v

versus 1/[S]

plot. [I

]

,[I

]

,[I

]

, and [I

]

are 0, 0.5, 1.5, and 3.0

times K

, respectively.

ENZYMES/Functions and Characteristics 2123

0027 Enzyme-catalyzed reactions are sensitive to the pH of

the reaction, resulting in a pH optimum (maximum

). Examples of pH optima are: porcine pepsin, pH 2;

horse-radish peroxidase, pH 6; bovine trypsin, pH 8;

bovine milk alkaline phosphatase, pH 10; catalase,

pH 4–9. The relation between pH and v

, bell-shaped

or sigmoidal, results from the effect of pH on the

ionization of essential catalytic or binding groups in

the active site of the enzyme and the effect of pH on

stability of the enzyme as a protein. (See pH – Prin-

ciples and Measurement.)

Temperature

0028 Temperature affects enzyme-catalyzed reactions by

influencing v

, the stability of the enzyme as a protein

and various equilibria, including K

and pK

of cata-

lytic groups. There is a logarithmic effect of tempera-

ture on the rate of reaction (Figure 4, curve A), as

predicted by the Arrhenius relation:

k ¼ A

E

=RT

, ð3Þ

where k is a rate constant, A is a proportionality

constant, T is temperature (degrees Kelvin), R is the

universal gas constant, and E

is the Arrhenius acti-

vation energy. At higher temperatures, depending on

the specific enzyme, enzymes lose activity because of

instability (see curve B, Figure 4), resulting in a

decrease in activity. The temperature at which

the maximum v

is obtained is the temperature

optimum (apex of curve C, Figure 4) under the con-

ditions used.

Enzyme Activation

0029Any compound that increases v

when added to

an enzyme–substrate system is an activator. These

compounds can be cofactors (coenzymes or pros-

thetic groups), allosteric effectors, conformational

effectors, or converters of proenzymes to enzymes

(by proteolysis, usually).

0030Cofactors are small organic or inorganic molecules

essential for enzyme activity. Organic cofactors

are often built around the B vitamins (pyridoxine,

thiamin, riboflavin, niacin, and pantothenic acid),

whereas inorganic cofactors include Zn

2þ

,Mn

2þ

5þ

,Mo

6þ

,Cu

2þ

,Mg

2þ

,Co

2þ

,Ca

2þ

,Fe

2þ

,Fe

3þ

and Cl



. For example, more than 150 enzymes

require Zn

2þ

as a cofactor. Coenzymes are bound

via noncovalent bonds, whereas prosthetic groups

generally are covalently bound to the protein. (See

Coenzymes.)

Enzyme Inhibition

0031Any compound that decreases v

when added to

an enzyme–substrate system is an inhibitor. Inacti-

vation caused by adverse pH, temperature or solv-

ents is not included. Inhibitors occur naturally in

many biological materials, may be produced syn-

thetically, or may be metal ions (particularly Hg

2þ

,Ag

,Cu

2þ

). Enzyme inhibitors are important

in pathogenic microorganism control, food storage,

and food-quality preservation. Many pharmaceut-

icals are designed to inhibit specific microbial

enzyme systems. Fungicides, herbicides, and insecti-

cides are targeted to specific enzymes. Products of

enzyme reactions are often inhibitors, since they

resemble substrates. (See Fungicides; Pesticides and

Herbicides: Types, Uses, and Determination of

Herbicides; Storage Stability: Mechanisms of Deg-

radation.)

Naming of Enzymes

0032Enzymes are named by adding the suffix ‘-ase’ to the

stem of the substrate name, e.g. amylum (Latin for

‘starch’)/amylase, maltose/maltase, carbohydrate/car-

bohydrase, lipid/lipase. However, there are several

enzymes that act on a carbohydrate, for example, or

a single enzyme may act on several substrates. Other

trivial names are derived from the source of the

Temperature

Velocity (or k) for lines A and C

1/velocity (for line B)

fig0004 Figure 4 Effect of temperature on v

of an enzyme-catalyzed

reaction. Curve A shows the effect of temperature on the velocity

of substrate conversion to product. Curve B shows the effect of

temperature on the stability of the enzyme. Curve C shows the

actual effect of temperature on the velocity of substrate convers-

tion to product.

2124 ENZYMES/Functions and Characteristics

enzyme or the color of the purified enzyme. In 1955,

the International Union of Biochemistry appointed

members from many countries to a commission on

enzymes, with the charge to develop a systematic

method of naming enzymes. In 1961, a classification

was adopted, based on dividing ‘the enzymes into

groups on the basis of the type of reaction catalysed

and this, together with the name(s) of the substrate(s),

provided a basis of naming individual enzymes’.

Retention of the trivial name (already in general use)

and addition of a four-digit numbering system were

also adopted.

003 3 All enzymes fit one of six types of chemical reac-

tions. These are (with the first digit of the number and

a generalized reaction):

EC1. Oxidoreductases

AX þ B Ð A þ BX, where X is H or e



;Bis

oxygen etc.

EC2. Transferases

AY þ B Ð A þ BY, where Y is methyl, acyl,

glycosyl, etc.

EC3. Hydrolases

AB þ H

O Ð AH þ BOH, where water is a

substrate.

EC4. Lyases

EC5. Isomerases

CR′R

CR′

R, i.e

D− L−, cis

trans, keto enol, etc.

EC6. Ligases (synthetases)

A þ B þ AJP Ð AB þ ADP þ P

(or AMP þ PP),

where P

is inorganic phosphate.

0040 The first digit of the EC number indicates the class to

which the enzyme belongs, the second digit the sub-

class, the third digit the subsubclass, and the fourth

digit serial number of the enzyme in its subsubclass.

For example, the trivial name of the enzyme catalyz-

ing the reaction shown in eqn (4) is alcohol dehydro-

genase, the systematic name is alcohol:NAD

oxidoreductase, and the number is EC 1.1.1.1. The

first digit indicates that it is an oxidoreductase, the

second that the donor is a primary alcohol, the third

that the acceptor is NAD

or NADP

, and the fourth

that the enzyme is alcohol dehydrogenase.

OH þ NAD

ÐCH

CHO þ NADH þ H

ð4Þ

0041The naming of an enzyme is complete when the

organism, organ, and isozyme (if present) are indi-

cated. For example, one of the two pig pancreas

a-amylases is porcine pancreatic a-amylase-1, and

the other is porcine pancreatic a-amylase-2. The iso-

zyme number is assigned consecutively, on the basis

of the fastest moving isozyme towards the anode by

electrophoresis, as ‘1’, and so on.

See also: Amino Acids: Properties and Occurrence;

Coenzymes; Fermented Foods: Origins and

Applications; Fungicides; Pesticides and Herbicides:

Types, Uses, and Determination of Herbicides; pH –

Principles and Measurement; Protein: Chemistry;

Storage Stability: Mechanisms of Degradation; Water

Activity: Effect on Food Stability