10 - Coenzymes - Coenzymes and Vitamins Learning Objectives...

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Coenzymes and Vitamins • Coenzymes • Metal ions in enzymes • NAD + , FAD, CoA • Certain vitamins are precursors of coenzymes • Examples of vitamins Learning Objectives
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Cofactors: Metal Ions and Coenzymes • Certain chemical reactions can be catalyzed by enzymes only in association with cofactors that act as “chemical teeth” of the enzymes. • There are two types of cofactors: metal ions and coenzymes. • Most enzymes are proteins. An enzyme is geometrically complementary to the corresponding substrate . • Standard amino acids from which proteins are composed have only a few types of functional groups. These groups and their combinations can catalyze many but not all chemical reactions that occur in living organisms.
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Metal Ions as Cofactors • Certain trace elements required by organisms are metal-ion cofactors for enzymes. • Some heavy metals are toxic because they replace metal-ion cofactors. For example, Zn 2+ , Cd 2+ and Hg 2+ are in the same group of the periodic table. •Zn 2+ is a cofactor in certain enzymes, e.g., RNA polymerase. Replacement of Zn 2+ by toxic Cd 2+ and Hg 2+ makes the enzyme inactive. Fig. 11-13
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Coenzymes • Coenzymes are relatively small organic molecules that bind to enzymes either permanently or transiently . • Coenzymes that are permanently bound to the enzymes (by covalent bonds or strong nonbonded interactions) are called prosthetic groups (e.g., the heme prosthetic group of cytochrome C, the iron-containing protein involved in cell respiration). • A catalytically active enzyme-cofactor complex is called a
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10 - Coenzymes - Coenzymes and Vitamins Learning Objectives...

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