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Enzymes are proteins, composed of polypeptide chains and non-protein
groups. Their function is to help with the reactions of many cells and
molecules by serving as catalysts. A catalyst is a substance that allows the
activation energy required for a reaction by forming a temporary association
with the molecules that are reacting. During this process, the catalyst itself is
not permanently altered in the process, and so it can be used over and over
again. Because of catalysts, cells are able to carry out chemical reactions at a
great speed and at comparative low temperatures.
Almost 2,000 different enzymes are now known, each of them capable
of catalyzing a specific chemical reaction. The molecule (s) on which an
enzyme acts is known as its substrate. For example, sucrose is the substrate
for the enzyme sucrase. Enzymes have specific structures that only its
specific substrate will fit into. The polypeptide chains of an enzyme are
folded in such way that they form a grove or pocket on the surface. The
substrate fits in to this grove, which is the site of reactions catalyzed by the
enzyme, or active site. Recent studies of enzyme structure have suggested
that the active site is flexible. The binding between enzymes and substrate
appears to alter the shape of the enzyme. This induces a close fit between the
active site and the substrate. It is also believed that this may put some strain
on the substrate molecule facilitating the reaction.
Another characteristics of enzymes are competitive inhibition and
non-competitive inhibition. Competitive inhibition is the binding of a
competitive molecule to the active site of the enzyme. This prevents the
proper substrate from reacting with the enzyme. In non-competitive
inhibition the binding of a non-competitive inhibitor to another site on the
enzyme induces an allosteric change, or conformational change, that prevents
the active site from binding to the proper substrate. An allosteric change
could be either a negative modulator or a positive effector. There’s also
cooperativity. This is the binding at an active site that makes the others more
reactive. An example to this is hemoglobin. Co-enzymes are also factors of
enzymes. These are heme-groups or prosthetic groups that bind to the
enzymes loosely as part of the reactions, such as vitamins.
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