enzyme

MIT Biology Hypertextbook: Enzyme Mechanisms: "Not all proteins are enzymes, but most enzymes are proteins (the exception is catalytic RNA). A catalyst is a molecule which increases the rate of a reaction but is not the substrate or product of that reaction. A substrate (A) is a molecule upon which an enzyme acts to yield a product (B).

A ------> B
Enz

The free energy of this reaction is not changed by the presence of the enzyme, but, for a favored reaction (where delta G is negative), the enzyme can speed it up."

Enzymes couple with substrates in transitional states, effecting conformational changes (3D structure) that facilitates transition to products.

MIT Biology Hypertextbook on Enzyme Biochemistry : Chemical Energetics : Enzyme Mechanisms : Enzyme Kinetics : Feedback Inhibition