8885dc06_190-2371/27/047:13 AM Page19 6mac76:385 Chapter 6 Enzymes TABLE 6-2 Some Coenzymes That Serve as Transient Carriers of Specific Atoms or Functional Grou zyme Examples of chemical groups transferred Dietary precursor in mammals Biocytin Biotin Coenzyme A Acyl groups Pantothenic acid and other compounds 5-Deoxyadenosylcobalamin H atoms and alkyl groups Vitamin B12 penzyme B, Flavin adenine dinucleotide Electrons Riboflavin(vitamin B2) Electrons and acyl groups Not required in diet Nicotinamide adenine dinucleotide Hydride ion(:H) Nicotinic acid(niacin Pyridoxal phosphate Amino groups ridoxine(vitamin B6) Tetrahydrofolate One-carbon groups Folate Thiamine pyrophosphate Aldehydes Thiamine(vitamin B1) Note The structures and modes of action of these coenzymes are described in Part IL and one or more metal ions for activity. A coenzyme or tion, before the specific reaction catalyzed was known metal ion that is very tightly or even covalently bound For example, an enzyme known to act in the digestion to the enzyme protein is called a prosthetic group. a of foods was named pepsin, from the greek pepsis, " di- complete, catalytically active enzyme together with its gestion, and lysozyme was named for its ability to lyse bound coenzyme and/or metal ions is called a holon- bacterial cell walls. Still others were named for their zyme. The protein part of such an enzyme is called the source: trypsin, named in part from the greek tryein, apoenzyme or apoprotein Coenzymes act as tran-"to wear down, was obtained by rubbing pancreatic sient carriers of specific functional groups. Most are de- tissue with glycerin. Sometimes the same enzyme has rived from vitamins, organic nutrients required in small two or more names, or two different enzymes have the amounts in the diet. We consider coenzymes in more same name. Because of such ambiguities, and the ever- detail as we encounter them in the metabolic pathways increasing number of newly discovered enzymes discussed in Part Il. Finally, some enzyme proteins are biochemists, by international agreement, have adopted modified covalently by phosphorylation, glycosylation, a system for naming and classifying enzymes. This sys and other processes. Many of these alterations are in- tem divides enzymes into six classes, each with sub volved in the regulation of enzyme activity. classes, based on the type of reaction catalyzed ( Table 6-3). Each enzyme is assigned a four-part classification Enzymes Are Classified by the Reactions number and a systematic name, which identifies the re- They Catalyze action it catalyzes. As an example, the formal system- atic name of the enzyme catalyzing the reaction Many enzymes have been named by adding the suffix ase"to the name of their substrate or to a word or ATP+ D-glucose→→ADP+ D-glucose6 phosphate phrase describing their activity. Thus urease catalyzes is ATP: glucose phosphotransferase, which indicates that hydrolysis of urea, and dna polymerase catalyzes the it catalyzes the transfer of a phosphoryl group from ATP polymerization of nucleotides to form DNA. Other en- to glucose. Its Enzyme Commission number (E.C. zymes were named by their discovers for a broad func- number) is 2.7.1.1. The first number (2) denotes the TABLE 6-3 International Classification of Enzymes Class type of reaction catalyzed Oxidoreductases Transfer of electrons(hydride ions or H atoms) 123456 Group transfer reactions Hydrolases Hydrolysis reactions(transfer of functional groups to wate Lyases Addition of groups to double bonds, or formation of double bonds by removal of groups Transfer of groups within molecules to yield isomeric forms Formation of C--C,C-S,C-0, and C-N bonds by condensation reactions coupled to ATP cleavag Note: Most enzymes catalyze the transfer of electrons, atoms, or functional groups. They are therefore classified, given code numbers, and assigned names according to the type of transfer reaction, the group dono, and the group acceptor.and one or more metal ions for activity. A coenzyme or metal ion that is very tightly or even covalently bound to the enzyme protein is called a prosthetic group. A complete, catalytically active enzyme together with its bound coenzyme and/or metal ions is called a holoen￾zyme. The protein part of such an enzyme is called the apoenzyme or apoprotein. Coenzymes act as tran￾sient carriers of specific functional groups. Most are de￾rived from vitamins, organic nutrients required in small amounts in the diet. We consider coenzymes in more detail as we encounter them in the metabolic pathways discussed in Part II. Finally, some enzyme proteins are modified covalently by phosphorylation, glycosylation, and other processes. Many of these alterations are in￾volved in the regulation of enzyme activity. Enzymes Are Classified by the Reactions They Catalyze Many enzymes have been named by adding the suffix “-ase” to the name of their substrate or to a word or phrase describing their activity. Thus urease catalyzes hydrolysis of urea, and DNA polymerase catalyzes the polymerization of nucleotides to form DNA. Other en￾zymes were named by their discovers for a broad func￾tion, before the specific reaction catalyzed was known. For example, an enzyme known to act in the digestion of foods was named pepsin, from the Greek pepsis, “di￾gestion,” and lysozyme was named for its ability to lyse bacterial cell walls. Still others were named for their source: trypsin, named in part from the Greek tryein, “to wear down,” was obtained by rubbing pancreatic tissue with glycerin. Sometimes the same enzyme has two or more names, or two different enzymes have the same name. Because of such ambiguities, and the ever￾increasing number of newly discovered enzymes, biochemists, by international agreement, have adopted a system for naming and classifying enzymes. This sys￾tem divides enzymes into six classes, each with sub￾classes, based on the type of reaction catalyzed (Table 6–3). Each enzyme is assigned a four-part classification number and a systematic name, which identifies the re￾action it catalyzes. As an example, the formal system￾atic name of the enzyme catalyzing the reaction ATP
D-glucose 88n ADP
D-glucose 6-phosphate is ATP:glucose phosphotransferase, which indicates that it catalyzes the transfer of a phosphoryl group from ATP to glucose. Its Enzyme Commission number (E.C. number) is 2.7.1.1. The first number (2) denotes the 192 Chapter 6 Enzymes Coenzyme Examples of chemical groups transferred Dietary precursor in mammals Biocytin CO2 Biotin Coenzyme A Acyl groups Pantothenic acid and other compounds 5-Deoxyadenosylcobalamin H atoms and alkyl groups Vitamin B12 (coenzyme 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) Note: The structures and modes of action of these coenzymes are described in Part II. TABLE 6–2 Some Coenzymes That Serve as Transient Carriers of Specific Atoms or Functional Groups No. Class Type of reaction catalyzed 1 Oxidoreductases Transfer of electrons (hydride ions or H atoms) 2 Transferases Group transfer reactions 3 Hydrolases Hydrolysis reactions (transfer of functional groups to water) 4 Lyases Addition of groups to double bonds, or formation of double bonds by removal of groups 5 Isomerases Transfer of groups within molecules to yield isomeric forms 6 Ligases Formation of COC, COS, COO, and CON bonds by condensation reactions coupled to ATP cleavage Note: Most enzymes catalyze the transfer of electrons, atoms, or functional groups. They are therefore classified, given code numbers, and assigned names according to the type of transfer reaction, the group donor, and the group acceptor. TABLE 6–3 International Classification of Enzymes 8885d_c06_190-237 1/27/04 7:13 AM Page 192 mac76 mac76:385_reb: