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
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84 Part One Organization of the Body
(a) Enzyme and substrates
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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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.