Vitamins, Cofactors, Minerals and Trace Elements Quiz

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Which description best defines a cofactor in enzyme chemistry?

A A molecule that reduces enzyme activity by binding to it
B A non-protein helper required for a particular enzyme reaction ✓
C The protein component of an enzyme once its cofactor has been removed
D A reactant that is converted into the reaction product
E A protein segment that forms the enzyme active site without outside help

The answer is B: A non-protein helper required for a particular enzyme reaction

Explanation

Why B is correct

See Cofactors Make Enzymes Work. Option B is correct because a cofactor is a non-protein helper that an enzyme needs for a particular reaction. An enzyme's amino-acid side chains (the chemical groups sticking out from the protein) cannot perform every chemical reaction alone, so many enzymes borrow extra chemistry from a cofactor. A cofactor can help by carrying electrons or chemical groups, by stabilising charge, by holding the substrate in the right position, or by taking part directly in the catalysis. Some cofactors are organic molecules called coenzymes, and many of these are made from vitamins, for example riboflavin gives FMN and FAD and niacin gives NAD and NADP. Others are inorganic ions, such as magnesium or zinc. The enzyme protein without its required cofactor is the apoenzyme, and the protein with its cofactor attached is the active holoenzyme. The usual trap is to confuse the cofactor with the substrate or with an inhibitor, so remember the roles: the enzyme is the worker, the substrate is the material, and the cofactor is the helper.

Cofactor: a non-protein helper that an enzyme needs for a particular reaction.
Coenzymes: organic cofactors, many made from vitamins, such as FMN, FAD, NAD and NADP.
Inorganic ions: such as magnesium or zinc, also act as cofactors.
Roles: the enzyme is the worker, the substrate is the material and the cofactor is the helper.

Why the other options are incorrect

A. A molecule that reduces enzyme activity by binding to it: A molecule that slows or blocks an enzyme is an inhibitor. A cofactor has the opposite effect, because the enzyme needs it in order to work.
C. The protein component of an enzyme once its cofactor has been removed: This describes the apoenzyme, the protein part left behind when the cofactor is missing, which is inactive. The cofactor is the removed part and not the protein that remains.
D. A reactant that is converted into the reaction product: A reactant that the enzyme converts into product is the substrate. The substrate is used up by the reaction, whereas a cofactor helps the reaction and is reused.
E. A protein segment that forms the enzyme active site without outside help: The active site is the pocket of the enzyme protein where the reaction takes place. A protein segment that works alone is part of the enzyme itself, whereas a cofactor is a separate non-protein helper that the enzyme requires.

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1
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2
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