AChE is typically synthesized in nerve, muscle, and certain hematopoietic cells. AChE (EC 3.1.1.7) is distinguished from butyrylcholinesterase (BChE EC 3.1.1.8) by its catalytic selectivity for acetylcholine over butyrylcholine hydrolysis. Rapidity of catalysis of released acetylcholine in a submillisecond time frame is essential in the skeletal neuromuscular junction to allow the next volley of released acetylcholine to trigger a postsynaptic excitatory potential. The turnover number of acetylcholinesterase (AChE) approaches 1.5 × 10 4 s −1, making it one of the most efficient enzymes known. Radić, in Encyclopedia of Neuroscience, 2009 Introduction Most enzymes, however, have specificity constants orders of magnitude below this value. Triose phosphate isomerase (EC 5.3.1.1, its enzyme commission number), an enzyme of the glycolytic pathway, is an enzyme that has this attribute. Enzymes that have ratios of k cat/ K m near 10 8 to 10 9 M − 1 s − 1 (close to the maximum allowed by the rate of diffusion) have achieved catalytic perfection.
This rate is proportional to the substrate concentration and is therefore designated first order. The product of k cat/ K m and the substrate concentration (at subsaturating levels) yields the rate of the enzyme-catalyzed reaction. The ratio of k cat/ K m is a first-order rate constant. The diffusional limiting rate for a bimolecular reaction is 10 8 to 10 9 M − 1 s − 1. The rate of any reaction is limited by the rate at which reactant molecules collide. That substrate with the highest value is the best substrate for the enzyme, accounting for the name specificity constant. The k cat/ K m value, or specificity constant, of the various substrates can be compared. The turnover number of an enzyme ( k cat or catalytic rate constant) is the maximal number of molecules of substrate converted to product per active site per unit time of several different substrates to different products. Robert Roskoski, in Reference Module in Biomedical Sciences, 2015 The Specificity Constant
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