Enzymes as Biological Catalysts Nomenclature and Names of Enzymes Nature of Enzymes Enzyme Cofactors Classification of Enzymes Mechanism of Action Enzyme Specificity Enzyme Catalyzed Reaction

Enzymes

Large molecules which most are proteins Activity lost if denatured Catalysts for biological reactions Have unique three-dimensional shapes

that fit the shapes of reactants. They remain unchanged at the end of the

reaction

Enzymes

Enzymes are Biological Catalysts

Enzymes are proteins that: Increase the rate of reaction by lowering the energy

of activation. Catalyze nearly all the chemical reactions taking

place in the cells of the body.

The name of an enzyme: Usually ends in –ase. Identifies the reacting substance.

Sucrase catalyzes the hydrolysis of sucrose. Lipase catalyzes the hydrolysis of lipids.

Describes the function of the enzyme. Oxidases catalyze oxidation. Hydrolases catalyze hydrolysis.

Could be a common name, particularly for the digestion enzymes such as pepsin and trypsin.

Nomenclature and Names of Enzymes

Simple enzymes:composed of whole protein e. g. Ribonuclease

Complex enzymes:composed of protein PLUS a relatively small organic / inorganic compound

Nature of Enzymes

Holoenzyme:(Apoenzymes + Prosthetic group / Coenzyme /

Cofactor) A prosthetic group describes a small organic

molecule bound to apoenzyme by covalent bond Coenzyme: When the binding between the

apoenzyme and non-protein components is non-covalent bond, the small organic molecule is called a coenzyme, e. g FAD [Flavine Adenine Dinucleotide] and vitamin B-complex .

Cofactor: Inorganic compounds (e.g. metal ions such as Fe++, Zn++ or Cu++) can be bound to the enzyme and are called cofactors

Nature of Enzymes

Enzyme Cofactors• A simple enzyme is an active enzyme that consists

only of protein.• Many enzymes are active only when they combine

with cofactors such as metal ions or small molecules.

• A coenzyme is a cofactor that is a small organic molecule such as a vitamin.

Metal Ions as Cofactors

• Many active enzymes require a metal ion.• Zn2+, a cofactor for carboxypeptidase, stabilizes the

carbonyl oxygen during the hydrolysis of a peptide bond.

Some Enzymes and Their Cofactors

Enzymes are classified according to the reaction they catalyze.

Classification of Enzymes

Reactions catalyzed Class

Oxidation-reduction. Add or remove hydrogen atoms. Oxidoreductases

Transfer groups or atoms between donor and acceptor molecules. Transferases

Hydrolysis by adding water across a bond. Hydrolases

Add/remove atoms to/from a double bond.

(Adding water, ammonia, or carbon dioxide across double bonds or remove them forming double bonds).

Lyases

Rearrange atoms Isomerases

Two chemical groups are joined (or ligated) with the use of energy from ATP Ligases

Oxidoreductases and Transferases

Oxidoreductases

Include all enzymes that catalyze oxidation reduction reaction between two substrates Dehydrogenases Oxidases Reductases

Transferases

Catalyze the transfer of a functional group from one molecule (donor) to another (acceptor) Transaminases catalyze transfer of an amino

group

Kinases (phosphotransferases) catalyze transfer a phosphate group

Hydrolases and Lyases

Isomerases and Ligases

Active Site Is a region within an enzyme that fits the shape of

molecules called substrates.

Contains amino acid R groups that align and bind the substrate.

Releases products when the reaction is complete.

Mechanism of Action

Lock & Key Model

In the lock-and-key model of enzyme action: The active site has a rigid shape. Only substrates with the matching shape can fit. The substrate is a key that fits the lock of the active

site.

Induced-fit Model

In the induced-fit model of enzyme action: The active site is flexible, not rigid. The shapes of the enzyme, active site, and

substrate adjust to maximum the fit, which improves catalysis.

There is a greater range of substrate specificity.

Enzymes may recognize and catalyze: A single substrate. A group of similar substrates. A particular type of bond.

Enzyme Specificity

Isoenzymes Isoenzymes catalyze the same reaction in different

tissues in the body. Lactate dehydrogenase, which converts lactate to

pyruvate, (LDH) consists of five isoenzymes.

Enzyme Catalyzed Reaction

Enzyme Catalyzed Reaction

The proper fit of a substrate (S) in an active site forms an enzyme-substrate (ES) complex.

E + S ES Within the ES complex, the reaction occurs to

convert substrate to product (P).ES E + P

The products, which are no longer attracted to the active site, are released.

Overall, substrate is convert to product. E + S ES E + P

Enzyme Catalyzed Reaction

K1 [E][S]=K-1[ES] +K2 [ES]

Keq = K1 [E] [S]-K-1[ES] -K2[ES]= 0

Michaelis-Menten equationV° = Vmax [S]/[S]+Km

Michaelis-Menten ConstantKm = K2 + K-1 / K1

PEESSE 2

1

1k

k

k

Michaelis-Menten Curve

Substrate Saturation of an Enzyme

A. Low [S] B. 50% [S] or Km C. High, saturating [S]