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Chemical Thermodynamics and the Equilibrium State

Chemical Kinetics


The Michaelis-Menten equation expresses the relationship between the concentration of substrate and reaction rate. When the concentration is small, the rate is proportional to substrate concentration. When the concentration of substrate so large that it saturates the enzyme, the rate is maximal and no longer increases with greater substrate concentration. The equation rests on a model of enzyme activity where the rate is conceived as depending on the concentration of enzyme substrate complex, a concentration which is one side of the thermodynamic equilibrium between substrate and enzyme. The maximal rate (Vmax) occurs when the enzyme sites are saturated with substrate. Allosteric enzymes do not conform to the Micheaelis-Menten model because the binding of substrate to one active site can affect the properties of other active sites on the same enzyme molecule. Additionally, competitive inhibition does not change Vmax (the competitive inhibitor can be overcome by high substrate concentration), although with noncompetitive inhibition, Vmax does change.

While you definitely need to have a basic familiarity with Michaelis-Menten kinetics, very difficult enzyme kinetics questions would have a hard time finding their way onto the MCAT because it would produce too great an advantage for students who have taken biochemistry.

Heat and Temperature

Chemical Kinetics


In model physical science systems, the rate of a chemical reaction typically doubles every 10oC increase in temperature. In biological systems, however, where biochemical reactions tend to be enzyme catalyzed, the dependence of rate on temperature is more complex. A particular enzyme will possess an optimal temperature. For example, ectotherms (i.e. 'cold blooded') can either occupy a habitat of constant temperature for optimal enzyme activity, such as the deep ocean, or their physiological processes may have adapted for a range of temperature. Temperature regulation in endothermic animals depends on the internal generation of heat to maintain a chemical environment for optimal enzyme function.

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