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Enzymes have protein nature and molecular weights ranging from about 12 000 to over 1 million. The large molecule of enzymes is flexible for binding natural and unnatural substrates at their active site. The active site contains moieties consisted with amino acid residues. Although the activity of some enzymes requires no chemical groups other than their amino acid residues, others require an additional chemical component called cofactor. A cofactor, also called a coenzyme, is either one or more inorganic ions, such as Fe²⁺, Mg²⁺, Mn²⁺, or Zn²⁺ (Table 1.2) [9], or an organic or metallo‐organic molecule. Coenzyme are often derived from vitamins and organic nutrients required in small amounts in the diet (Table 1.3) [9]. The cofactor binds to the active site, in some cases covalently and in others noncovalently, which serves as transient carriers of redox equivalents, such as NAD(P)H or chemical energy (ATP) and is essential for the catalytic action of those enzymes that require cofactors.

Table 1.2 Some inorganic metal ions as cofactor of enzymes.

Source: Based on Nelson and Cox [9].

Fe2+ or Fe3+	Cytochrome oxidase, catalase, peroxidase
K+	Pyruvate kinase
Mg2+	Hexokinase, pyruvate kinase, enolase
Mn2+	Arginase, ribonucleotide reductase
Ni2+	Urease
Zn2+	Carbonic anhydrase, alcohol dehydrogenase, carboxypeptidases A and B

Table 1.3 Some coenzymes as transient carriers of specific atoms or functional groups.

Source: Based on Nelson and Cox [9].

Coenzyme	Chemical groups transferred	Dietary precursor in mammals
Biocytin	CO2	Biotin
Coenzyme A	Acyl group	Pantothenic acid and other compounds
5’‐Deoxyadenosylcobalamin (coenzyme B12)	H atoms and alkyl groups	Vitamin B12
Flavin adenine dinucleotide	Electrons	Riboflavin (vitamin B2)
Lipoate	Electrons and Acyl groups	Not required in diet
Nicotinamide adenine Dinucleotide	Hydride ion (:H−)	Nicotinic acid (niacin)
Pyridoxal phosphate	Amino groups	Pyridoxine (vitamin B6)
Tetrahydrofolate	One‐carbon groups	Folate
Thiamine pyrophosphate	Aldehydes	Thiamine (vitamin B1)

For some enzymes, a coenzyme is required for their activity. A coenzyme or metal ion that is bound to the enzyme protein at the active site is called a prosthetic group. The protein part of such an enzyme is called the apoenzyme or apoprotein, and the entire enzyme is called a holoenzyme. Most of these cofactors are relatively unstable molecules. We will consider various coenzymes throughout the text in more detail for those related enzyme‐catalyzed reactions.