Cofactors

Many enzymes cannot function without nonprotein partners called cofactors—for example, iron, copper, zinc, magnesium, or calcium ions. By binding to an enzyme, a cofactor may stimulate it to fold into a shape that activates its active site. Coenzymes are organic cofactors usually

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

84 Part One Organization of the Body

A + B

  • substrates)
  • C + D

Enzyme (products)

Substrate A

Substrate A

Substrate B

Enzyme

(a) Enzyme and substrates

Substrate B

Product C Ü

Product D

Enzyme

Product D

  • a) Enzyme and substrates
  • b) Enzyme-substrate complex
  • c) Enzyme (unchanged) and reaction products

Figure 2.27 The Three Steps of an Enzymatic Reaction. (a) One or more substrate molecules bind to the enzyme's active sites. (b) The substrates and enzyme form a temporary enzyme-substrate complex and the substrates react chemically with each other. (c) The enzyme releases the reaction products and is available to catalyze the same reaction again.

derived from niacin, riboflavin, and other water-soluble vitamins. They accept electrons from an enzyme in one metabolic pathway and transfer them to an enzyme in another pathway. For example, cells partially oxidize glucose through a pathway called glycolysis. A coenzyme called NAD+,28 derived from niacin, shuttles electrons from this pathway to another one called aerobic respiration, which uses energy from the electrons to make ATP (fig. 2.28). If NAD+ is unavailable, the glycolysis pathway shuts down.

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