Biochemistry: The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Enzymes: Protein Catalysts of the Cell
Mechanisms of Enzymatic Catalysis
Proximity and Orientation Effects

Despite the remarkable achievements of enzymology and our detailed knowledge of the Spatial Structure of certain Enzymes from X-ray crystallography, the precise mechanism by which these extraordinary catalysts operate remains largely enigmatic. Neither Acid-Base Catalysis nor Covalent Catalysis alone appears sufficient to account for the immense rate enhancements observed in enzyme-catalyzed reactions. What other factors, then, are responsible for the remarkable catalytic efficiency of enzymes?

One of the earliest hypotheses suggested that enzymes simply bring reactants together and hold them in close proximity long enough for reactive groups to collide and ultimately react. While intuition suggests that this proximity effect should be of paramount importance, early quantitative assessments concluded that its contribution was relatively minor. Later, Page and Jencks demonstrated that these initial evaluations were flawed; they showed that a rate acceleration of 10³-fold or greater can be expected solely from the reduction in Entropy of two reactants when they are brought together and immobilized On the surface of an enzyme molecule [60, 61]. Because binding is accompanied by a decrease in entropy, the enthalpy of this process must necessarily be high, implying that substrate binding itself largely constitutes the driving force for catalysis. This concept was first proposed by Westheimer [62], who suggested that enzymes act as 'entropy traps' through their substrate-binding capacity.

The loss of translational and rotational entropy—estimated by Page and Jencks to range from —160 to —210 kJ∙mol^-1∙K^-1—fully compensates for the unfavorable activation entropy typically characteristic of bimolecular reactions.

In recent years, greater attention has been directed toward understanding how precise the orientation of substrates must be for rapid reactions to occur [63, 63a]. To this end, A number of model reactions have been investigated, such as the spontaneous formation of an internal ester (lactone) accompanied by the elimination of Water:

Class="center">

reacts significantly faster due to substantial conformational constraints [64, 65]: the reaction rate is 10" times higher than that of reaction (6-90), likely because these constraints lead to a dramatic increase in the collision frequency between the —COO^- and —OH groups. Three methyl groups are brought into close proximity to form a trialkyl 'lock.' These findings strongly suggest that orientational effects can play a major role in Enzymatic Catalysis.

According to another theory explaining the high rates of enzyme-catalyzed reactions, enzymes are capable of inducing strain or distortion within the substrate molecule, leading to the weakening of specific bonds (see Chapter 7, Section B, 4, a on Lysozyme). This strain may either be accompanied by a conformational change in the protein molecule itself or arise as a direct consequence of such a change. Another important factor to consider is that certain reactions proceed much faster in media of low dielectric constant than in water. It is possible that the polar groups of the substrate undergo partial dehydration upon binding to the Active Site, thereby enhancing their reactivity.

In many enzymatic reactions, The formation of the Transition State is accompanied by a significant volume change (∆V^≠). It is likely that these volume changes stem primarily from alterations in the degree of Hydration of groups situated on the enzyme surface and play a crucial role in enzymatic catalysis [65a].