Enzymes Whose keat/Km Approaches the Diffusion-Controlled Rate of Association with Substrate kcat Km kcat/Km Enzyme Substrate (sec-1) (M0 (sec1M厂1) Acetylcholinesterase Acetylcholine 1.4×10 9×10-5 1.6×108 Carbonic C02 1×105 0.012 8.3×107 anhydrase HCO; 4×105 0.026 1.5×107 Catalase H202 4×107 1.1 4×107 Crotonase Crotonyl-CoA 5.7×103 2×10-5 2.8×108 Fumarase Fumarate 800 5×10-6 1.6×108 Malate 900 2.5×10-5 3.6×107 Triosephosphate Glyceraldehyde- 4.3×103 1.8×10-5 2.4×108 isomerase 3-phosphate* B-Lactamase Benzylpenicillin 2×103 2×10-5 1×108