Page 151-159.docx

1/27/10 10:01 PM Page 151-159 Enzymes speed up metabolic reactions by lowering energy barriers A spontaneous chemical reaction occurs without any requirement for outside energy, but it may occur so slowly that it is imperceptible Enzyme Macromolecule that acts as a catalyst In the absence of regulation by enzymes, chemical traffic through the pathways of metabolism would become terribly congested because many chemical reactions would take such a long time Activation energy The amount of energy needed to push the reactants over an energy barrier so the reaction can begin Enzymes can only speed up reactions that would occur anyway Effects of Temperature and pH on Enzyme activity All enzymes have an optimal temperature and an optimal pH that they are most active Temperature: The rate of enzymatic reaction increases with increasing temperature Above the ideal temp, the speed of the reaction slows dramatically Most human enzymes optimal temp.s between 35-40 degrees C. pH most between pH6-8 some exceptions pepsin: pH 2 trypsin: pH 8 cofactors nonprotein helpers for catalytic activity coenzyme if the cofactor is also organic competitive inhibitors: reduce the productivity of enzymes by blocking substrates from enzyme active sites noncompetitive inhibitors do not directly compete with the substrate impede enzymatic reactions by binding to another part of the enzyme Allosteric Regulation Any case in which a protein?s function at one site is affected by the binding of a regulatory molecule to a separate site May result in either inhibition or stimulation of an enzyme?s activity Cooperativity Amplifies the response of enzymes to substrates: one substrate molecule primes an enzyme to accept additional substrate molecules more readily Feedback inhibition A metabolic pathway is switched off by the inhibitory binding of its end product to an enzyme that acts early in the pathway Some enzymes and enzyme complexes have fixed locations within the cell and act as structural components of particular membranes