What are 'Enzymes'?

One molecule of carbonic anhydrase can process one million molecules of CO2 each second.

* Acetylcholinesterase. It catalysis the breakdown of the neurotransmitter acetylcholine at several types of synapses as well as at the neuromuscular junction - the specialized synapse that triggers the contraction of skeletal muscle.

One molecule of acetyl cholinesterase breaks down 25,000 molecules of acetylcholine each second. This speed makes possible the rapid "resetting" of the synapse for transmission of another nerve impulse.

Most of these interactions are weak and especially so if the atoms involved are farther than about one angstrom from each other. So successful binding of enzyme and substrate requires that the two molecules be able to approach each other closely over a fairly broad surface. Thus the analogy that a substrate molecule binds its enzyme like a key in a lock.

This requirement for complementarity in the configuration of substrate and enzyme explains the remarkable specificity of most enzymes. Generally, a given enzyme is able to catalyse only a single chemical reaction or, at most, a few reactions involving substrates sharing the same general structure.

Competitive inhibition

The necessity for a close, if brief, fit between enzyme and substrate explains the phenomenon of competitive inhibition.

One of the enzymes needed for the release of energy within the cell is succinic dehydrogenase.

It catalysis the oxidation (by the removal of two hydrogen atoms) of succinic acid (a). If one adds malonic acid to cells, or to a test tube mixture of succinic acid and the enzyme, the action of the enzyme is strongly inhibited. This is because the structure of malonic acid allows it to bind to the same site on the enzyme (b). But there is no oxidation so no speedy release of products. The inhibition is called competitive because if you increase the ratio of succinic to malonic acid in the mixture, you will gradually restore the rate of catalysis. At a 50:1 ratio, the two molecules compete on roughly equal terms for the binding (=catalytic) site on the enzyme.

Enzyme cofactors

Many enzymes require the presence of an additional, nonprotein, cofactor.

* Some of these are metal ions such as Zn2+ (the cofactor for carbonic anhydrase), Cu2+, Mn2+, K+, and Na+.

* Some cofactors are small organic molecules called coenzymes. The B vitamins

o thiamine (B1)

o Riboflavin (B2) and

o Nicotinamide

are precursors of coenzymes.

Coenzymes may be covalently bound to the protein part (called the apoenzyme) of enzymes as a prosthetic group. Others bind more loosely and, in fact, may bind only transiently to the enzyme as it performs its catalytic act.

Lysozyme: a model of enzyme action

A number of lysozymes are found in nature; in human tears and egg white, for examples. The enzyme is antibacterial because it degrades the polysaccharide that is found in the cell walls of many bacteria. It does this by catalyzing the insertion of a water molecule at the position indicated by the red arrow. This hydrolysis breaks the chain at that point.