Enzymes continued

• Some enzymes work together to complete a series of reactions to produce a given product:

• A + B ------- C ------- D ------------ E

• Collectively, these steps are known as a metabolic pathway

Enzyme 1 Enzyme 2 Enzyme 3

A metabolic pathway

The entire set of all chemical reactions being performed by an organism is

called its metabolism

• Anabolism = chemical reactions that expend energy to make or transform chemical bonds

• Catabolism = chemical reactions that harvest energy by breaking chemical bonds

Anabolism involves endergonic reactions

Catabolism involves exergonic reactions

Control of Metabolism

• Coenzymes

• Inhibitors

• Allosteric regulation

• Feedback Inhibition

• Cooperativity

Coenzymes

• Coenzymes are bound at the active site in order to interact with the substrate and play an essential role in the catalysed reaction.

• They act as carriers of a variety of chemical groups. .

Most water-soluble vitamins are components of coenzymes

Vitamin Coenzyme Deficiency

Thiamine (B1)Thiamine

pyrophosphateBeriberi (weight

loss,other problems

Riboflavin (B2) FAD+ Mouth lesions, dermatitis

Nicotinic acid (niacine)

NAD+ Pellagra (dermatitis, depression)

Pantohtinic acid Coenzyme A Hypertension

Biotin Biotin Rash, muscle pain

Inhibitors

• Specific for an enzyme

• Can be reversible or non-reversible

• Competitive inhibitors

• Non-competitive inhibitors

Competitive vs. Non-competitive inhibitors

Many inhibitors are used as pesticides or drugs

Regulating the rates of enzyme-driven reactions

• Cells use inhibitors and activators to turn off and on enzymes

• Many enzymes are controlled by an allosteric site remote from the active site

Allosteric regulation of enzyme activity

Many allosterically controlled enzymse show quaternary structure

Enzyme 1 Enzyme 2 Enzyme 3Inter-mediate

Inter-mediate

X ProductStart ofpathway

Presence of product inhibits enzyme 1

Feedback inhibition

Many enzymes are actually regulated by the end products of the reaction they catalyze

This prevents too much product from being made

An example of Feedback inhibition