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In chemistry, the term "turnover number" has two distinct meanings.. In enzymology, the turnover number (k cat) is defined as the limiting number of chemical conversions of substrate molecules per second that a single active site will execute for a given enzyme concentration [E T] for enzymes with two or more active sites. [1]
This constant is a measure of catalytic efficiency. The most efficient enzymes reach a k 2 / K M {\displaystyle k_{2}/K_{M}} in the range of 10 8 – 10 10 M −1 s −1 .
In the field of biochemistry, the specificity constant (also called kinetic efficiency or /), is a measure of how efficiently an enzyme converts substrates into products.A comparison of specificity constants can also be used as a measure of the preference of an enzyme for different substrates (i.e., substrate specificity).
The distribution of known enzyme catalytic rates (k cat /K M). Most enzymes have a rate around 10 5 s −1 M −1. The fastest enzymes in the dark box on the right (>10 8 s −1 M −1) are constrained by the diffusion limit. (Data adapted from reference [1])
The specificity constant / (also known as the catalytic efficiency) is a measure of how efficiently an enzyme converts a substrate into product. Although it is the ratio of k cat {\displaystyle k_{\text{cat}}} and K m {\displaystyle K_{\mathrm {m} }} it is a parameter in its own right, more fundamental than K m {\displaystyle K_{\mathrm {m} }} .
Moreover, superoxide dismutase has the largest k cat /K M (an approximation of catalytic efficiency) of any known enzyme (~7 x 10 9 M −1 s −1), [24] this reaction being limited only by the frequency of collision between itself and superoxide. That is, the reaction rate is "diffusion-limited".
The basis for a graphical rate law rests on the rate (v) vs. substrate concentration ([S]) plots discussed above. For example, in the simple cycle discussed with regard to different-excess experiments a plot of v / [A] vs. [B] and its twin v / [B] vs. [A] can provide intuitive insight about the order of each of the reagents.
In organic chemistry, kinetic resolution is a means of differentiating two enantiomers in a racemic mixture.In kinetic resolution, two enantiomers react with different reaction rates in a chemical reaction with a chiral catalyst or reagent, resulting in an enantioenriched sample of the less reactive enantiomer. [1]