HS-Hydrolysates

The results given in Figure 6.19 for the fractionation samples (NOM concentrate, fractionated at 20 mM NaCI) show three distinct components of NOM in the samples humics, HS-hydrolysates and low molecular mass acids. In the feed sample some polysaccharides are present which is typical for a surface water source. 

Only the outside ester—on the same side as the ring junction Hs—is hydrolysed. In the mechanism for ester hydrolysis, the rate-determining step is the attack by the hydroxide ion so the functional group increases in size in the vital step. This will be much easier for the free outside C02Bt group than for the one inside the half-open book. 

A kinetic analysis of recombinant hs-PLA has shown that this enzyme strongly prefers phosphatidic acid (PA) as a substrate over other phospholipids found in the mammalian plasma membrane including PS, PC and PE (70). The order of preference is PA PE approximately PS > PC. It appears that Lys-69 is at least partially involved in the PA specificity of hs-PLAj and Glu-56 in the distinction between PE and PC. These studies suggest that hs-PLA can rapidly hydrolyse PA molecules exposed to the outer layer of cell-derived microvesicles and thereby produce LPA (70). 

The formation of alkali-metal and alkaline-earth-metal sulphides and polysulphides from the elements in liquid ammonia has been extensively studied in the past, but the reactions between the metals and hydrogen sulphide in liquid ammonia have drawn detailed attention only recently. It has been suggested that the equilibrium of H2S in this solvent to give the solvated hydrosulphide ion accounts for the formation of KSH even with an excess of metal. With the alkaline-earth metals, effective preparative methods have been developed for the sulphides from H2S in liquid ammonia but anhydrous hydrosulphides have not been obtained. Now, hydrosulphides have been prepared of the form M(SH)2,xNH3 (M = Ca, Sr, or Ba x — 4, 6, or 0, respectively) from the metals with H2S in ammonia, but the compounds are stable only at low temperatures. Those of Ca and Sr are stable at —45 °C but decompose to the monosulphides at room temperature. Ba(HS)2 decomposes to BaS at 100 °C with evolution of a mole of H2S. For M (SH) (M = Rb or Cs), thermal decomposition gives polysulphides. The hydrosulphides of Rb, Cs, Sr, and Ba hydrolyse rapidly in moist air.74... 

Petersen G, Hansen HS (1999) N-acylphosphatidylethanolamine-hydrolysing phospholipase D lacks the ability to transphosphatidylate. FEBS Lett 455 41-44... 

Reactivity of esters towards base-catalysed hydrolysis was studied by Stirling and coworkers. They have shown that well-packed monolayers of aliphatic esters with the carbonyl group buried deeply below the surface [HS(CH2)ioOCO(CH2)8CH3] are very resistant towards hydrolysis. At the same time, esters which have the carbonyl function close to the monolayer surface [HS(CH2)ioOCOCH3] hydrolyse more rapidly400. In such... 

Acyl CoA synthetase synthesises, in the presence of HS-CoA, acyl-CoA from a saturated fatty acid. Acyl CoA is transformed by acyl-CoA dehydrogenase into 2,3-dehydroacyl-CoA, and isomerism leads to trhydroxyacyl dehydrogenase the latter compound is hydrolysed by 3-ketoacyl-CoA thiolase to HS-CoA and 3-oxoacid. This acid eliminates carbon dioxide and yields methylketone, with catalysis of decarboxylase. Reductase can reduce methylketone to a secondary alcohol. 

Olsen HS, Adler-Nissen J. 1979. Industrial production and application of soluble enzymatic hydrolysate of soy protein. Process Biochemistry, 14 7-10. 

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