(二)按酶的化学组成分类简单蛋白酶：指酶的活性仅仅决定于它的蛋白质结构。如脲酶、至蛋白酶、淀粉酶、脂肪酶等。结合蛋白酶：这些酶只有在结合了非蛋白组分（辅助因子）后，才表现出酶的活性。几个概念：酶蛋白-apoenzyme（脱辅酶-apoprotein）、辅助因子和全酶(holoenzyme）、辅基(prosthetic group）、(cofactor)辅酶(coenzyme)
（二）按酶的化学组成分类 n 简单蛋白酶：指酶的活性仅仅决定于它的蛋白质结 构。如脲酶、蛋白酶、淀粉酶、脂肪酶等。 n 结合蛋白酶：这些酶只有在结合了非蛋白组分（辅 助因子）后，才表现出酶的活性。几个概念：酶蛋 白-apoenzyme（脱辅酶-apoprotein）、辅助因子 （cofactor） 和全酶(holoenzyme)、辅基(prosthetic group)、 辅酶(coenzyme)
Many enzymes need non-proteincofactors to help in catalysisThe cofactors can be inorganic ions or coenzymes(complex organic or metal-organic molecules).n Some cofactors bind to the enzyme protein verytightly (non-covalently or covalently), they are thuscalled prosthetic groups.Only the combination of an apoenzyme with itsncofactor (i.e., a holoenzyme) is operative (aholoenzyme also refers to the assembled form of amultiple subunit protein).Coenzymes usually function as transient carriers ofnspecific function groups.Vitamins (organic nutrients required in small amountin the diet) have been found to often act as precursorsof coenzymes
Many enzymes need non-protein cofactors to help in catalysis n The cofactors can be inorganic ions or coenzymes (complex organic or metal-organic molecules). n Some cofactors bind to the enzyme protein very tightly (non-covalently or covalently), they are thus called prosthetic groups. n Only the combination of an apoenzyme with its cofactor (i.e., a holoenzyme) is operative (a holoenzyme also refers to the assembled form of a multiple subunit protein). n Coenzymes usually function as transient carriers of specific function groups. n Vitamins (organic nutrients required in small amount in the diet) have been found to often act as precursors of coenzymes
Some Inorganic Elements That Serve as Cofactorsfor EnzymesCu2+Cytochromeoxidase（细胞色素氧化酶）Fe2+ or Fe3+Cytochromeoxidase，catalase，peroxidase（过氧化氢酶）K+Pyruvatekinase（丙酮酸激酶）Mg2+(已糖激酶）Hexokinase，glucose6-phosphatase,pyruvatekinaseMn2+（核苷酸还原酶）（精氨酸酶Arginase，ribonucleotidereductaseMoDinitrogenase（四氧化二氮酶）Ni2+Urease(尿素酶)SeGlutathioneperoxidase（谷胱甘肽酶）Zn2+（碳脱水酶）Carbonicanhydrase,alcoholdehydrogenas（乙醇脱氢酶）carboxypeptidasesAand B（羧肽酶）
（细胞色素氧化酶） （过氧化氢酶） （丙酮酸激酶） （己糖激酶） （精氨酸酶） （核苷酸还原酶） （尿素酶） （四氧化二氮酶） （谷胱甘肽酶） （碳脱水酶） （乙醇脱氢酶） （羧肽酶）
SomeCoenzymesThatServeasTransientCarriersofSpecificAtomsorFunctionalGroups*ExamplesofchemicalDietaryprecursor inCoenzymegroupstransferredmammals(VitaminsCO2BiocytinBiotin维生素H生物胞素Coenzyme AAcylgroupsPantothenicacidand酰基othercompoundsH atoms andVitamin B125'-Deoxyadenosylcobalamin(coenzymeB12)alkyl groupsElectronsFlavin adenine dinucleotideRiboflavin(vitaminB2)LipoateElectronsandNot required in dietacylgroupsNicotinamide adenineHydride ion (:H-)Nicotinic acid (niacin)dinucleotideAminogroupsPyridoxalphosphatePyridoxine(vitaminBe)FolateTetrahydrofolateOne-carbongroupsThiaminepyrophosphateAldehydesThiamine(vitamin B,)
(Vitamins) 生物胞素 维生素H 酰基
2 H202 →2 H20 + O2Beef Liver CatalaseNADPHProtoporphyrin IX(a prosthetic group)Catalase
(a prosthetic group) 2 H2O2 → 2 H20 + O2 200,000 catalytic events/second/subunit (near the diffusion-controlled limit). The reaction is sped up by a billion fold! (tetramers)