Serine acetate

The enzyme cysteine synthase (EC 4.2.99.8) catalyzes the last step in the biosynthesis of cysteine, converting serine acetate 97 to cysteine 134. Floss et al. (84) studied this -replacement reaction using stereospecifically tritiated serine acetates prepared from the labeled serines 60 synthesized as in Scheme 18 (Section IV). Assessment of chirality of the cysteine produced by degradation to serine and use of tryptophan synthase showed that the j3-replacement reaction 97a134 had proceeded with retention of configuration (84) (Scheme 45),... 

Figure 6.1 Synthesis and metabolism of acetylcholine. Choline is acetylated by reacting with acetyl-CoA in the presence of choline acetyltransferase to form acetylcholine (1). The acetylcholine binds to the anionic site of cholinesterase and reacts with the hydroxy group of serine on the esteratic site of the enzyme (2). The cholinesterase thus becomes acetylated and choline splits off to be taken back into the nerve terminal for further ACh synthesis (3). The acetylated enzyme is then rapidly hydrolised back to its active state with the formation of acetic acid (4)...

The disaccharide derivatives 196 and 197 of N-tosyl-L-serine, of interest for the study of D-galactose-binding lectins, were prepared by Kaifu and Osawa148 from 194 and 195, respectively, by protection of 0-4 and -6 with a benzylidene group, condensation of the acetal with tetra-O-acetyl-a-D-galactopyranosyl bromide (110) under Koenigs-Knorr conditions, and deprotection. 

On intuition, a minute amount of water was added to the solvent (ethyl acetate) in the first crystallization experiment containing a molar excess of imidazole corresponding to 1, Regularly shaped crystals were formed within one hour. Such a crystal, subjected to X-ray analysis, has the structure as shown in Fig. 41 U1). Apart from the formation of the expected salt-type associate (carboxylate-imidazolium ion pair, cf. Sect. 4.2.2), two water molecules are present in the asymmetric unit of the crystal structure. This fact called our attention again to the family of serine protease enzymes, where water molecules are reported as being located in the close vicinity of the active sites 115-120),... 

Ring cleavage of the bicyclic dihydro-oxazolo [3,4-c]oxazol-3-one 111 derived from D-serine was realized by action of boron trifluoride-acetic acid complex leading to the oxazolidinone 112, a useful building block for the synthesis of... 

Column, octadecyl-bonded silica gel, 15 cm x 4.6 mm i.d. eluent, 0.01 M sodium acetate buffer containing 0.4 M copper acetate and 1.0 mM sodium alkanesulfonate (pH 5.6) flow rate, 1 ml min-1 detection, UV 230 nm. Counter-ion C6, sodium hexanesulfonate Cl, sodium heptanesulfonate and C8, sodium octane sulfonate. Compounds Ser, serine Gly, glycine Glu, glutamic acid and Asp, aspartic acid. 

Serine peptidases can hydrolyze both esters and amides, but there are marked differences in the kinetics of hydrolysis of the two types of substrates as monitored in vitro. Thus, the hydrolysis of 4-nitrophenyl acetate by a-chy-motrypsin occurs in two distinct phases [7] [22-24]. When large amounts of enzyme are used, there is an initial rapid burst in the production of 4-nitro-phenol, followed by its formation at a much slower steady-state rate (Fig. 3.7). It was shown that the initial burst of 4-nitrophenol corresponds to the formation of the acyl-enzyme complex (acylation step). The slower steady-state production of 4-nitrophenol corresponds to the hydrolysis of the acetyl-enzyme complex, regenerating the free enzyme. This second step, called deacylation, is much slower than the first, so that it determines the overall rate of ester hydrolysis. The rate of the deacylation step in ester hydrolysis is pH-dependent and can be slowed to such an extent that, at low pH, the acyl-enzyme complex can be isolated. 

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