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Enzymes and cofactors
Nearly all enzymes are made of protein. Some enzymes are functional protein-only molecules, but many require additional non-protein components, called cofactors, to function. Cofactors can be subclassified as either inorganic ions (e.g. Zn2+) or complex organic molecules called coenzymes (many of which are vitamins). Many enzymes need several cofactors in order to function. Where a cofactor is needed for enzyme function, the enzyme (protein) component is called the apoenzyme. The cofactor often completes the active site or makes the active site more reactive by assisting enzyme-substrate interactions. Substrate 1
Substrate 2
One substrate combines with the coenzyme to form a coenzyme/ substrate complex
The coenzyme/substrate complex combines with the second substrate to form the product (P).
PR E O V N IE LY W
Lactate dehydrengase with bound NAD+ shown in red.
P
Coenzyme
Apoenzyme
If cofactors are not permanently bound to the enzyme, they can detach after the reaction to participate in other reactions. Neither the apoenzyme nor the cofactor has catalytic activity on its own. Example: dehydrogenases + NAD. NAD is the coenzyme form of the vitamin niacin (B3). Many coenzymes are vitamin derivatives.
2. How do enzymes lower the activation energy for a reaction?
3. Why are enzymes referred to as "biological catalysts"?
4. Describe the difference between digestion and protein synthesis in terms of the energy released or required :
5. What is a cofactor?
CL
7. How do cofactors enable an enzyme's catalytic activity?
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6. Describe the difference between the two different broad categories of cofactors:
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