Chapter 3rd Enzymes 1

Enzymes TOPICS: Introduction and history Structure of enzymes Mechanism of Enzyme Catalyzed Reactions Factors affecting enzyme action Enzymes naming & classification 2

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2 Structure of enzymes Enzymes Complex or holoenzymes (protein part and nonprotein part – cofactor) Simple (only protein) Apoenzyme (protein part) Cofactor Prosthetic groups usually small inorganic molecule or atom; usually tightly bound to apoenzyme Coenzyme -large organic molecule -loosely bound to apoenzyme 6

APOENZYME and HOLOENZYME 2 APOENZYME and HOLOENZYME The enzyme without its non protein moiety is termed as apoenzyme and it is inactive. Holoenzyme is an active enzyme with its non protein component.

2 Types of Cofactors Coenzyme: The non-protein component, loosely bound to apoenzyme by non-covalent bond. Examples : vitamins or compound derived from vitamins. Prosthetic group The non-protein component, tightly bound to the apoenzyme by covalent bonds is called a Prosthetic group.

Example of prosthetic group 2 Example of prosthetic group Example of metalloenzyme: carbonic anhydrase contains zinc Metalloenzymes contain firmly bound metal ions at the enzyme active sites (examples: iron, zinc, copper, cobalt).

Coenzyme classification 2 Coenzymes Coenzymes act as group-transfer reagents Hydrogen, electrons, or groups of atoms can be transferred Coenzyme classification (1) Metabolite coenzymes - synthesized from common metabolites Vitamin-derived coenzymes - derivatives of vitamins Vitamins cannot be synthesized by mammals, but must be obtained as nutrients 10

2 Examples of metabolite coenzymes ATP can donate phosphoryl group S-adenosylmethionine donates methyl groups in many biosynthesis reactions S-adenosylmethionine 11

2 Vitamin-Derived Coenzymes Vitamins are required for coenzyme synthesis and must be obtained from nutrients Most vitamins must be enzymatically transformed to the coenzyme Deficit of vitamin and as result correspondent coenzyme results in the disease 12

2 NAD+ and NADP+ Nicotinic acid (niacin) an nicotinamide are precursor of NAD and NADP Lack of niacin causes the disease pellagra NAD and NADP are coenzymes for dehydro-genases 13

Enzyme Specificity 2 Enzymes may recognize and catalyze: A single substrate. A group of similar substrates. A particular type of bond.

2 Active Site The 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 Enzyme Catalyzed Reactions 3 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

Example of An Enzyme Catalyzed Reaction 3 Example of An Enzyme Catalyzed Reaction

Mechanism of Enzyme Action: 3 Mechanism of Enzyme Action: 1.Lock-and-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.

2. Induced-fit Model 3 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.

Factors affecting enzyme action: 1- Temperature and Enzyme Action 4 Enzymes: Are most active at an optimum temperature (usually 37°C in humans). Show little activity at low temperatures. Lose activity at high temperatures as denaturation occurs.

Factors affecting enzyme action: 2- pH and Enzyme Action 4 Enzymes: Are most active at optimum pH. Contain R groups of amino acids with proper charges at optimum pH. Lose activity in low o high pH as tertiary structure is disrupted. r

Factors affecting enzyme action: 4- Enzyme Concentration The rate of reaction increases as enzyme concentration increases (at constant substrate concentration). At higher enzyme concentrations, more substrate binds with enzyme.

Factors affecting enzyme action: 5- Substrate Concentration 4 The rate of reaction increases as substrate concentration increases (at constant enzyme concentration). Maximum activity occurs when the enzyme is saturated.

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