factors affecting enzymatic activity enzyme inhibition regulation of enzyme activity clinical...

Factors Affecting Enzymatic Activity Enzyme Inhibition Regulation of Enzyme Activity Clinical Applications of Enzymes

Enzymes

Factors Affecting Enzymatic Activity

Enzymes are most active at an optimum temperature (usually 37°C in humans).

Enzymes show little activity at low temperatures. Enzymes lose activity at high temperatures as

denaturation occurs.

Temperature and Enzyme Action

pH and Enzyme Action

Each enzyme exhibits peak activity at narrow pH range (optimum pH).

Enzymes contain R groups of amino acids with proper charges at optimum pH.

Enzymes lose activity in low or high pH as tertiary structure is disrupted.

Optimum pH reflects

the pH of the body

fluid in which the

enzyme is found.

pH and Enzyme Action

Optimum pH Values

Most enzymes have an optimum pH of about 7.4 In certain organs, enzymes operate at different

pH values.

Enzyme Concentration The rate of reaction increases as enzyme

concentration increases (at constant substrate concentration).

At higher enzyme concentrations, more substrate binds with enzyme.

Substrate Concentration The rate of reaction increases as substrate

concentration increases (at constant enzyme concentration).

Maximum activity occurs when the enzyme is saturated.

Enzyme Inhibition

Inhibitors: Are molecules that cause a loss of catalytic

activity. Prevent substrates from fitting into the active

sites.

E + S ES E + P

E + I EI no P

Enzyme Inhibition

Reversible Competitive Inhibition

A competitive inhibitor: Has a structure like the

substrate. Competes with the

substrate for the active site.

Has its effect reversed by increasing substrate concentration.

A noncompetitive inhibitor: Has a structure different than

the substrate. Distorts the shape of the

enzyme, which alters the shape of the active site.

Prevents the binding of the substrate.

Cannot have its effect reversed by adding more substrate.

Noncompetitive irreversible Inhibition

Malonate and Succinate Dehydrogenase

Malonate: Is a competitive inhibitor of succinate

dehydrogenase. Has a structure that is similar to succinate. Inhibition is reversed by adding succinate.

Loss of all enzymatic activity Toxic substance (irreversible inhibitor) forms a

covalent bond with an amino acid in the active center. Prevents the substrate from entering the active

site. Prevents the catalytic activity.

Examples: insecticides and nerve gases inhibit the enzyme acetylcholinesterase (needed for nerve conduction).

Irreversible Inhibition In irreversible inhibition, a substance destroys

enzyme activity by bonding with R groups at the active site.

Zymogens (proenzymes)

Are inactive forms of enzymes. Are activated when one or more peptides are

removed. Such as proinsulin is converted to insulin by

removing a small peptide chain.

Digestive Enzymes

Produced as zymogens in one organ and transported to another such as the pancreas when needed.

Activated by removing small peptide sections.

Allosteric Enzymes

An allosteric enzyme is an enzyme in a reaction sequence that binds a regulator substance.

A positive regulator enhances the binding of substrate and accelerates the rate of reaction.

A negative regulator prevents the binding of the substrate to the active site and slows down the rate of reaction.

Feedback Control

A product acts as a negative regulator. An end product binds with the first enzyme (E1) in a

sequence, when sufficient product is present.

Clinical Applications of Enzymes

Diagnostic Enzymes

The levels of diagnostic enzymes determine the amount of damage in tissues.

Serum Enzymes used in diagnosis of tissue damage

Hepatitis

A 36-year old man was admitted to a hospital following episodes of nausea, vomiting, and general malaise.

His urine was darker than usual. Upon examination it was discovered that his liver

was enlarged and tender to palpation.

Clinical examples and case studies.

Hepatitis Liver function tests were abnormal:

Plasma ALT (Alanine aminotransferase) was 1500 U/L (6.0 – 21 U/L).

Plasma AST (Aspartate aminotransferase) was 400 U/L (7.0 – 20 U/L).

During the next 24 hours the man developed jaundice, and his plasma total bilirubin was 9.0 mg/dL (0.2 – 1 mg/dL).

A diagnosis of hepatitis was made.

Clinical examples and case studies.

Hepatitis

Clinical examples and case studies.

What reactions are catalyzed by AST and ALT? What is the coenzyme?

Case discussion:Hepatitis is an inflammation of the liver

Transaminases (amino acids metabolism)– Catalyze the transfer of α-amino groups from

amino acid to a α-keto acid through the intermediary coenzyme pyridoxal phosphate (derived from the B6 vitamin, pyridoxine)

– Amino acids enter into the Krebs cycle for oxidation to CO2 and H2O

– Amino acid X + keto acid Y ↔ amino acid Y + keto acid X

– ALT and AST