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Some enzymes require other molecules in order to function

Many enzymes require the presence of nonprotein chemical “partners” in order to function (Table 8.2):

table 8.2 Some Examples of Nonprotein “Partners” of Enzymes

Type of molecule	Role in catalyzed reactions
Prosthetic groups
Heme	Binds ions, O₂, and electrons
FAD	Carries electrons/protons
Retinal	Converts light energy
Inorganic cofactors
Iron (Fe²⁺ or Fe³⁺)	Oxidation/reduction
Copper (Cu⁺ or Cu²⁺)	Oxidation/reduction
Zinc (Zn²⁺)	Stabilizes DNA binding structure
Coenzymes
Biotin	Carries —COO^–
Coenzyme A	Carries —CO—CH₃
NAD	Carries electrons/protons
ATP	Provides/extracts energy

Prosthetic groups are distinct, non–amino acid atoms or molecular groupings that are permanently bound to their enzymes. An example is flavin adenine dinucleotide (FAD), which is bound to succinate dehydrogenase, an important enzyme in cellular respiration (see Key Concept 9.3).
Inorganic cofactors include ions such as copper, zinc, and iron that are permanently bound to certain enzymes. For example, the enzyme alcohol dehydrogenase contains the cofactor zinc.
A coenzyme is a nonprotein carbon-containing molecule that is required for the action of one or more enzymes. It is usually relatively small compared with the enzyme to which it binds.

A coenzyme moves from enzyme to enzyme, adding or removing chemical groups from the substrate. A coenzyme is like a substrate in that it does not permanently bind to the enzyme: it binds to the active site, changes chemically during the reaction, and then separates from the enzyme to participate in other reactions. There is actually no clear distinction between the functions of coenzymes and some substrates. For example, ATP and ADP have been described as coenzymes, even though they are really substrates that gain or lose phosphate groups during chemical reactions. The term “coenzyme” was coined before the functions of these molecules were fully understood. Biochemists continue to use the term, and for consistency with the field, we use it in this book.