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How are holoenzymes and apoenzymes different?
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Enzyme
In the induced-fit theory of enzyme-substrate binding, a substrate approaches the surface of an enzyme (step 1 in box A, B, C) and causes a change in the enzyme shape that results in the correct alignment of the catalytic groups (triangles A and B; circles C and D represent substrate-binding groups on the enzyme that are essential for catalytic activity). The catalytic groups react with the substrate to form products (step 2). The products then separate from the enzyme, freeing it to repeat the sequence (step 3). Boxes D and E represent examples of molecules that are too large or too small for proper catalytic alignment. Boxes F and G demonstrate binding of an inhibitor molecule (I and I′) to an allosteric site, thereby preventing interaction of the enzyme with the substrate. Box H illustrates binding of an allosteric activator (X), a nonsubstrate molecule capable of reacting with the enzyme.
In biochemistry, the primary difference between a holoenzyme and an apoenzyme lies in their catalytic activity and composition.
- Apoenzyme: This is the protein component of an enzyme. By itself, an apoenzyme is catalytically inactive and cannot perform biochemical reactions. It determines the specificity of the enzyme system for a particular substrate.12
- Holoenzyme: This is the complete, catalytically active form of the enzyme. It is formed when an apoenzyme combines with its necessary cofactor.31
The cofactor acts as a nonprotein component essential for biological activity. When this cofactor is removed from a holoenzyme, the remaining apoenzyme loses its ability to function as a catalyst. Depending on the enzyme, these cofactors may be bound firmly (prosthetic groups) or loosely (coenzymes) to the apoenzyme.1