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Enzymes Enzymes as Biological Catalysts

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Presentation on theme: "Enzymes Enzymes as Biological Catalysts"— Presentation transcript:

1 Enzymes 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

2 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

3 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.

4 Nomenclature and Names of Enzymes
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.

5 Nature of Enzymes Simple enzymes: Complex enzymes:
composed of whole protein e. g. Ribonuclease Complex enzymes: composed of protein PLUS a relatively small organic / inorganic compound

6 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

7 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.

8 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.

9 Some Enzymes and Their Cofactors

10 Classification of Enzymes
Enzymes are classified according to the reaction they catalyze. 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

11 Oxidoreductases and Transferases

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

13 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

14 Hydrolases and Lyases

15 Isomerases and Ligases

16 Mechanism of Action 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.

17 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.

18 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.

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

20 Isoenzymes Isoenzymes catalyze the same reaction in different tissues in the body. Lactate dehydrogenase, which converts lactate to pyruvate, (LDH) consists of five isoenzymes.


22 Enzyme Catalyzed Reaction

23 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

24 Enzyme Catalyzed Reaction
K1 [E][S]=K-1[ES] +K2 [ES] Keq = K1 [E] [S]-K-1[ES] -K2[ES]= 0 Michaelis-Menten equation V° = Vmax [S]/[S]+Km Michaelis-Menten Constant Km = K2 + K-1 / K1

25 Michaelis-Menten Curve

26 Substrate Saturation of an Enzyme
A. Low [S] B. 50% [S] or Km C. High, saturating [S]

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