Chapter 20:Enzymes

Spencer L. SeagerMichael R. Slabaugh

www.cengage.com/chemistry/seager

Jennifer P. Harris

ENZYME CHARACTERISTICS• Enzymes are proteins that catalyze chemical reactions. They

speed up chemical reactions by lowering activation energies.

ENZYME CHARACTERISTICS (continued)

• Enzymes are specific in the type of reactions they catalyze.• Absolute specificity acts only on one substance.• Relative specificity acts on structurally related

substances.• Stereochemical specificity distinguishes between

stereoisomers.

ENZYME CHARACTERISTICS (continued)

• Enzyme activity can be regulated.• The cell controls rates of reactions.• The cell controls amount of any product formed by

regulating the action of enzymes.

CLASSIFYING AND NAMING ENZYMES

• The earliest enzymes have names with –in to indicate their protein composition.

• Examples:• pepsin• trypsin• chymotrypsin

CLASSIFYING AND NAMING ENZYMES (continued)• Many known enzymes created the need of a systematic

nomenclature system (Enzyme Commission (EC) system), which:• has six major classes based on type of reaction catalyzed.• names the specific substrate and functional group acted

upon as well as the type of reaction catalyzed.• ends the name in –ase.

• A substrate is the substance that undergoes a chemical change catalyzed by an enzyme.

CLASSIFYING AND NAMING ENZYMES (continued)

CLASSIFYING AND NAMING ENZYMES (continued)• Enzymes also have common names, which:• are shorter than EC name.• can be formed by one of the following methods:• adding –ase to the name of the substrate.• adding –ase to a combination of the substrate name

and type of reaction.• include examples, such as the enzyme for:• the substrate urea, which has a common name of

urease.• the substrate alcohol and a dehydrogenation reaction

type, which has a common name of alcohol dehydrogenase.

CLASSIFYING AND NAMING ENZYMES (continued)

ENZYME COFACTORS• Some enzymes require a second substance present

(cofactor) in order to be active, not a true prosthetic group (only weakly bound to the enzyme).

• Cofactors can be a nonprotein molecule or ion.• If the cofactor is an organic molecule, it is called a

coenzyme. • An apoenzyme is the catalytically inactive protein formed by

the removal of the cofactor.

• Coenzymes are often derived from vitamins.

Apoenzyme + cofactor (coenzyme or inorganic ion) → active enzyme

ENZYME COFACTORS (continued)

ENZYME MECHANISM• All enzymes have an active site – the location on the enzyme

where a substrate binds and catalysis occurs.• Enzymes complex with the substrate and the chemical

reaction proceeds.

E + S ⇆ ES → E + Penzyme substrate enzyme- enzyme product

substrate complex

ENZYME MECHANISM (continued)• Specific example:

ENZYME MECHANISM (continued)• There are two main theories on active sites:• Lock-and-key theory states that the substrate has a

shape that exactly fits the active site. This explains enzyme specificity.

ENZYME MECHANISM (continued)• Induced-fit theory states that the conformation of the active

site changes to accommodate an incoming substrate.

ENZYME ACTIVITY• Enzyme activity is the rate at which an enzyme catalyzes a

reaction.• Turnover number is the number of substrate molecules

acted on by one enzyme molecule per minute.• Enzyme international unit is the quantity of enzyme that

catalyzes the conversion of 1 µmol of substrate per minute.• An enzyme assay is an experiment that measures enzyme

activity.

ENZYME ACTIVITY (continued)

FACTORS AFFECTING ACTIVITY• The more enzyme present, the higher the enzyme

concentration and the faster substrate reacts.

FACTORS AFFECTING ACTIVITY (continued)

• Increasing substrate concentration increases the reaction rate until enzymes become saturated (Vmax).

FACTORS AFFECTING ACTIVITY (continued)

• Enzymes have an optimum temperature range (usually 25-40°C), above or below which they begin to denature.

FACTORS AFFECTING ACTIVITY (continued)

• Enzymes have optimum pH values (usually around 7), above and below which the rate decreases.

FACTORS AFFECTING ACTIVITY (continued)

ENZYME INHIBITION• Inhibitors decrease enzyme activity.• Irreversible inhibitors covalently bond with the enzyme and

render it inactive.• Many poisons are irreversible inhibitors.

Examples: CN-, Hg2+, and Pb2+

• Some antibiotics are irreversible inhibitors.• Examples: Sulfa drugs and penicillins inhibit specific

enzymes essential to the life processes of bacteria.• Penicillins interfere with transpeptidase, an enzyme

that is important in bacterial cell wall construction.• Inability to form strong cell walls prevents the bacteria

from surviving.

ENZYME INHIBITION (continued)

ENZYME INHIBITION (continued)• The cyanide ion:• is an irreversible enzyme inhibitor.• is extremely toxic.• acts very rapidly.• interferes with the operation of an iron-containing enzyme

(cytochrome oxidase) by forming a very stable complex.• does not allow the enzyme to function properly.• stops cellular respiration.• causes death in minutes.

ENZYME INHIBITION (continued)• The cyanide poisoning antidote:• must be administered quickly.• can be sodium thiosulfate (same substance known as

“hypo” in developing photographic film), which:• converts the cyanide ion to a thiocyanate ion, which:• does not bind to the iron of cytochrome oxidase.

ENZYME INHIBITION (continued)• Heavy metal toxicity:• is due to ability to render the protein part of enzymes

ineffective.• occurs when metals combine with the –SH groups found

on many enzymes.• causes nonspecific protein denaturation.

• Mercury and lead poisoning can cause permanent neurological damage.

ENZYME INHIBITION (continued)• Heavy-metal poisoning treated by administering chelating

agents (substances that combine with the metal ions and hold them very tightly).

• An example of a chelating agent is ethylenediaminetetraacetic acid, EDTA , which:• chelates all heavy metals except mercury.

• The calcium salt of EDTA administered intravenously.• Calcium ions are displaced by heavy-metal ions that bind to

the chelate more tightly.• The heavy metal-EDTA complex is soluble in body fluids and

is excreted in the urine.

ENZYME INHIBITION (continued)• Reversible

inhibitors reversibly bind with enzymes.

• Competitive reversible inhibitors compete with substrate for binding at the active site.

• Action can be reversed by increasing substrate

concentration (LeChâtelier’s principle).

ENZYME INHIBITION (continued)• Sulfa drug on bacteria is an example of competitive enzyme

inhibition.• Folic acid normally synthesized within the bacteria by

process that requires p-aminobenzoic acid.• Sulfanilamide resembles p-aminobenzoic acid and competes

with it for the active site of the bacterial enzyme.• Sulfanilamide can prevent bacterial growth.

ENZYME INHIBITION (continued)• Noncompetitive reversible inhibitors bind to the enzyme at

a location other than the active site.• Substrate concentration doesn’t affect inhibitor action.

ENZYME REGULATION (continued)

ENZYME REGULATION• Zymogens or proenzymes are an inactive precursor of an

enzyme.• Some enzymes are stored as inactive zymogens.• They are released when needed and activated at the location

where the reaction occurs.

ENZYME REGULATION (continued)

ENZYME REGULATION (continued)• Allosteric regulation of allosteric enzyme activity is altered

by the binding of a modulator.• Modulators can increase allosteric enzyme activity

(activator) or decrease it (inhibitor).• Feedback inhibition is an example of a modulator

decreasing the activity of an allosteric enzyme.

ENZYME REGULATION (continued)• The synthesis of isoleucine is a five-step process.• Threonine deaminase (enzyme for first step) is subject to

inhibition from isoleucine (final product).• Isoleucine and threonine have very different structures;

therefore, this is an example of a noncompetitive inhibitor.• Isoleucine binds to an allosteric site, not an active

site.

ENZYME REGULATION (continued)• Enzyme induction is the synthesis of an enzyme in

response to a cellular need.• This is an example of genetic control.• The synthesis of -galactosidase is an example of

enzyme induction.

MEDICAL APPLICATIONS• Changes in blood serum concentrations of specific enzymes

can be used to detect cell damage or uncontrolled growth (cancer).

• The measurement of enzyme concentrations in blood serum has become a major diagnostic tool, particularly in diagnosing diseases of the heart, liver, pancreas, prostate, and bones.

MEDICAL APPLICATIONS (continued)