Regulation of Enzyme Activity

Ronnie Z. Valenciano Jr.

BSE 3B

Group 4

January 10, 2012

Mechanisms for Regulating Enzyme Activity

1. Allosteric Enzymes• Enzymes whose activity can be changed

by molecules (effector molecules) other than substrate.

• Allosteric means "other site" or "other structure".

• The interaction of an inhibitor at an allosteric site changes the structure of the enzyme so that the active site is also changed.

2 Processes Involving the Allosteric Enzyme

1. Negative Allosterism

- effector binding sites alters the shape of the active site of the enzyme making it to an inactive configuration.

2. Positive Allosterism

- effector binding sites that alters the shape of inactive site of enzyme to an active configuration.

Therefore, binding of the effector molecule regulates enzyme activity by determining whether it will be active or not.

2. Feedback Inhibition• An enzyme regulation process in

which formation of a product inhibits an earlier reaction in the sequence.

It controls the allosteric enzymes.

• This occurs when an end-product of a pathway accumulates as the metabolic demand for it declines.

• This end-product in turn binds to the regulatory enzyme at the start of the pathway and decreases its activity - the greater the end-product levels the greater the inhibition of enzyme activity.

Feedback Inhibition

3. Proenzymes (Zymogen)

• The inactive form of enzyme which can be activated by removing a small part on their polypeptide chain.

• Mostly are the digestive enzymes and blood clotting enzymes.

• Why is it that digestive enzymes are in inactive state before it becomes active?

• This is necessary to prevent digestion of pancreatic and gastric tissues.

Zymogen

• Pepsinogen• Trypsinogen • Prothrombin

Active Form of Enzyme

• Pepsin• Trypsin• Thrombin

4. Protein Modification

• Another mechanism that can on and off the enzyme.

• This a process in which a chemical group is covalently added to removed from the protein.

• Phosphorylation, whereby a phosphate is transferred from an activated donor (usually ATP) to an amino acid on the regulatory enzyme, is the most common example of this type of regulation.

Inhibition of Enzyme Activity

• Enzyme inhibitors are chemicals that can bind to enzyme and either eliminate or drastically reduce their catalytic ability.

Classification

1. Reversibility

- Deals whether the inhibition will eventually dissociate from the enzyme releasing it in the active form.

2. Competition

- Refers whether the inhibitor is a structural analog or look – alike of the natural substrate.

- If so, the inhibitor and substrate will compete for the enzyme’s active site.

Enzyme Inhibitors

1. Irreversible Inhibitors

2. Reversible Inhibitors

a. Competitive Inhibitors

b. Uncompetitive Inhibitors

c. Mixed Inhibitors

d. Non – competitive Inhibitors

1. Irreversible Inhibitors

• usually covalently modify an enzyme, and inhibition can therefore not be reversed.

• Often contains highly reactive functional group such as aldehydes, alkenes, haloalkenes, etc.,

Examples:

snake’s venom

poison

Nerve gases (DPF)

Reaction of the irreversible inhibitor diisopropylfluorophosphate (DFP) with a serine

protease

2. Reversible Inhibitors

• bind to enzymes with non-covalent interactions such as hydrogen bonds, hydrophobic interactions and ionic bonds.

• generally do not undergo chemical reactions when bound to the enzyme and can be easily removed.

a. Competitive Inhibitor• They are molecules that resemble the

structure and charge distribution of the natural substrate for a particular enzyme.

• The inhibition is competitive because the inhibitor and substrate compete for the binding to the enzyme’s active site.

b. Uncompetitive Inhibition

• the inhibitor binds only to the substrate-enzyme complex, it should not be confused with non-competitive inhibitors.

c. Mixed Inhibition• The inhibitor can bind to the enzyme at the

same time as the enzyme's substrate. However, the binding of the inhibitor affects the binding of the substrate, and vice versa.

• Although it is possible for mixed-type inhibitors to bind in the active site, this type of inhibition generally results from an allosteric effect where the inhibitor binds to a different site on an enzyme.

d. Non – competitive Inhibitor

• is a form of mixed inhibition where the binding of the inhibitor to the enzyme reduces its activity but does not affect the binding of substrate.

• Binds to R groups of amino acids or perhaps to the metal ion cofactor.

• Modifies the shape of the active site.

Specificity of the Enzyme – Substrate

Complex

4 Classes of Enzyme Specificity1. Absolute Specificity

- An enzyme that catalyzes the reaction of only one substrate.

2. Group Specificity

- an enzyme that catalyzes processes involving similar molecule containing the same functional group.

e.g., hexokinase

3. Linkage Specificity

- an enzyme that catalyze the formation of breakage of only certain binds in molecule.

e.g., proteases – hydrolyzes peptide bonds

5. Stereochemical Specificity

- an enzyme that can distinguished an enantiomer from the other.