Enzymes are organic molecules that speed up chemical reactions
without being used up or altered.
Almost all enzymes are proteins,
but some have nonprotein components as well
(and a rare few are made of RNA).
Enzymes can be used to regulate biochemical pathways.
Enzyme molecules are recycled, so they are needed in very small amounts only.
- Catalyst: Something that speeds up a chemical reaction (by lowering
the energy of activation), but is not used up or altered in the reaction.
- Enzyme: An organic catalyst, made largely or entirely of protein.
- Substrate: A chemical substance on which an enzyme acts.
Many enzymes are named after their substrate plus -ase.
Example: the
enzyme lactase digests the sugar lactose.
- Structure: Every enzyme has an active site that must bind to the substrate;
small structural changes here have large effects and may inactivate the enzyme.
The rest of the enzyme holds the active site in the proper position;
small changes here are usually less critical.
- Isoenzymes: Most enzymes exist in several varieties, called isoenzymes or isozymes.
These isoenzymes function alike, but they can be distinguished using electrophoresis,
a technique which separates proteins based on their speed of migration
through an electric field.
- Function: Enzymes speed up the rate of the reaction that they catalyze.
The enzyme binds to the substrate, forming an enzyme-substrate complex.
Reaction products are then released, and the enzyme molecule is recycled to react with
another substrate molecule. Most enzymes function best at a particular
temperature optimum or pH optimum.
- Specificity: Most enzymes match their substrates, and are thus highly specific
to a given substrate.
The lock-and-key hypothesis describes this type of enzyme specificity.
Some substrates may change the shape of their enzyme to make them fit
(the induced fit hypothesis).
- Conformation: the exact shape of an enzyme molecule.
- Allostery: Enzyme activity often depends on a slight degree of controlled movement (flexibility)
in the enzyme molecule. This slight change in conformation is called allosteric control or allostery.
- Inhibitors: Inhibitors are molecules that can inactivate enzymes by attaching to the active site,
by altering the shape of the enzyme molecule, or by interfering with allostery.
All enzymes can be destroyed by denaturing their proteins (as by boiling).
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