coenzymes and cofactors

coenzymes

• coenzymes are organic molecules that are required by certain enzymes to carry out catalysis.

• They bind to the active site of the enzyme and participate in catalysis but are not considered substrates of the reaction.

• coenzymes often function as intermediate carriers of electrons, specific atoms or functional groups that are transfered in the overall reaction. An example of this would be the role of NAD in the transfer of electrons in certain coupled oxidation reduction reactions.

some coenzymes and the reactions they are involved in are shown in the following table.

coenzymes in group transfer reactions

coenzyme	abbreviation	entity transfered
nicotine adenine dinucelotide	NAD - partly composed of niacin	electron (hydrogen atom)
nicotine adenine dinucelotide phosphate	NADP -Partly composed of niacin	electron (hydrogen atom)
flavine adenine dinucelotide	FAD - Partly composed of riboflavin (vit. B2)	electron (hydrogen atom)
coenzyme A	CoA	Acyl groups
coenzymeQ	CoQ	electrons (hydrogen atom)
thiamine pyrophosphate	thiamine (vit. B1)	aldehydes
pyridoxal phosphate	pyridoxine (vit B6)	amino groups
biotin	biotin	carbon dioxide
carbamide coenzymes	vit. B12	alkyl groups

cofactors

Cofactors are often classified as inorganic substances that are required for, or increase the rate of, catalysis.

Examples of some enzymes that require metal ions as cofactors is shown in the table below

cofactor	enzyme or protein
Zn++	carbonic anhydrase
Zn++	alcohol dehydrogenase
Fe+++ or Fe++	cytochromes, hemoglobin
Fe+++ or Fe++	ferredoxin
Cu++ or Cu+	cytochrome oxidase
K+ and Mg++	pyruvate phosphokinase

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