Enzymes Are the Agents of Metabolic Function

Chapter 14

Enzyme Kinetics

Living organisms seethe with metabolic activity. Thousands of chemical reactions are proceeding very rapidly at any given instant within all living cells. Virtually all of these transformations are mediated by enzymes, proteins (and occasionally RNA) specialized to catalyze metabolic reactions. The substances transformed in these reactions are often organic compounds that show little tendency for reaction outside the cell. An excellent example is glucose, a sugar that can be stored indefinitely on the shelf with no deterioration. Most cells quickly oxidize glucose, producing carbon dioxide and water and releasing lots of energy:

C₆H₁₂O₆ + 6 O₂ ® 6 CO₂ = 6 H₂O + 2870 kJ of energy

(-2870 kJ/mol is the standard free energy change [DG°'] for the oxidation of glucose; see Chapter 3). In chemical terms, 2870 kJ is a large amount of energy, and glucose can be viewed as an energy-rich compound even though at ambient temperature it is not readily reactive with oxygen outside of cells. Stated another way, glucose represents thermodynamic potentiality: its reaction with oxygen is strongly exergonic, but it just doesn’t occur under normal conditions. On the other hand, enzymes can catalyze such thermodynamically favorable reactions so that they proceed at extraordinarily rapid rates (Figure 14.1). In glucose oxidation and countless other instances, enzymes provide cells with the ability to exert kinetic control over thermodynamic potentiality. That is, living systems use enzymes to accelerate and control the rates of vitally important biochemical reactions.

Figure 14.1 •Reaction profile showing large D G^‡ for glucose oxidation, free energy change of -2,870 kJ/mol; catalysts lower D G^‡, thereby accelerating rate.

Enzymes Are the Agents of Metabolic Function

Acting in sequence, enzymes form metabolic pathways by which nutrient molecules are degraded, energy is released and converted into metabolically useful forms, and precursors are generated and transformed to create the literally thousands of distinctive biomolecules found in any living cell (Figure 14.2). Situated at key junctions of metabolic pathways are specialized regulatory enzymes capable of sensing the momentary metabolic needs of the cell and adjusting their catalytic rates accordingly. The responses of these enzymes ensure the harmonious integration of the diverse and often divergent metabolic activities of cells so that the living state is promoted and preserved.

Figure 14.2 •The breakdown of glucose by glycolysis provides a prime example of a metabolic pathway. Ten enzymes mediate the reactions of glycolysis. Enzyme 4, fructose 1,6, biphosphate aldolase, catalyzes the C-C bond- breaking reaction in this pathway.

Date: 2016-01-03; view: 1198