Factors Effecting Enzymes

Factors Affecting Enzyme Action

The activity of enzymes is strongly affected by changes in pH and temperature. Each enzyme works best at a certain pH left graph and temperature right graph, its activity decreasing at values above and below that point. This is not surprising considering the importance of

* tertiary structure i.e. shape in enzyme function and

* non covalent forces, e.g., ionic interactions and hydrogen bonds, in determining that shape.

Examples:

* the protease pepsin works best as a pH of 1-2 (found in the stomach) while

* the protease trypsin is inactive at such a low pH but very active at a pH of 8 (found in the small intestine as the bicarbonate of the pancreatic fluid neutralizes the arriving stomach contents .Changes in pH alter the state of ionization of charged amino acids e.g., Asp, Lys that may play a crucial role in substrate binding and/or the catalytic action itself. Without the unionized -COOH group of Glu-35 and the ionized -COO- of Asp-52, the catalytic action of lysozyme would cease.

Hydrogen bonds are easily disrupted by increasing temperature. This, in turn, may disrupt the shape of the enzyme so that its affinity for its substrate diminishes. The ascending portion of the temperature curve red arrow in right-hand graph above reflects the general effect of increasing temperature on the rate of chemical reactions graph at left. The descending portion of the curve above blue arrow reflects the loss of catalytic activity as the enzyme molecules become denatured at high temperatures.

Enzymes

Enzymes are catalysts. Most are proteins. (A few ribonucleoprotein enzymes have been discovered and, for some of these, the catalytic activity is in the RNA part rather than the protein part. Link to discussion of these ribozymes.)

Enzymes bind temporarily to one or more of the reactants of the reaction they catalyze. In doing so, they lower the amount of activation energy needed and thus speed up the reaction.

Link to a discussion of free energy (G) and "Ð"G".

Examples:

* Catalyse. It catalyzes the decomposition of hydrogen peroxide into water and oxygen.

2H2O2 -> 2H2O + O2

One molecule of catalyse can break 40 million molecules of hydrogen peroxide each second.

* Carbonic anhydrase. It is found in red blood cells where it catalyzes the reaction

CO2 + H2O H2CO3

It enables red blood cells to transport carbon dioxide from the