Enzyme-assisted hydrolysis

The first P-chiral hydroxy phosphoryl compounds that were enzymatically resolved into enantiomers were o-hydroxyaryl phosphines and their oxides 75. The resolution was achieved via enzyme-assisted hydrolysis of their O-acetyl derivatives 74, the most effective enzymes being CE and Upase from C. rugosa (CRL) (Equation 35). The highest enanfioselectivity was observed in the case of naphthyl derivatives (Equation 36), having a P=0 moiety. ... 

Scheme 10. Possible mechanisms for conversion ofDMPS to the silanol by NDO. B) Oxidation by molecular oxygen B) Enzyme-assisted hydrolysis of the hydrosilane function.

Because sucrose is hydrolyzed by enzymes that specifically assist hydrolysis of both a glycosides (such as yeast a-glucosidase) and /3-fructosides (such as invertase), it is inferred that the glucose residue is present as an a glucoside and the fructose residue as a /3 fructoside. If so, the remaining uncertainty in the structure of sucrose is the size of the rings in the glucose and fructose residues. 

These results are indeed reminiscent of enzyme-catalyzed reactions where basic organic groups, being part of the enzyme, assist in proton delivery, e. g., in hydrolysis reactions. 

The molecular weights of all of the peptides in a mixture obtained by the enzyme-catalyzed hydrolysis of a protein are determined simultaneously by mass spectrometry using matrix-assisted laser desorption ionization (MALDI). 

Isoprene units all originate by the same biochemical route through isopentenyl pyrophosphate. This latter compound is formed from mevalonic acid in a series of enzyme-assisted steps using energy transfer from ATP ADP hydrolysis. Mevalonic acid (mevalonate) is obtained by condensation of acetyl-CoA with acetoacetyl-CoA. 

This illustrates the fact that the proper positioning of a group (electrophilic or nucleophilic) may accelerate the rate of a reaction. There is thus an analogy to be made with the active site of an enzyme such as lysozyme. Of course the nature of the leaving group is also important in describing the properties. Furthermore, solvation effects can be of paramount importance for the course of the transformation especially in the transition state. Reactions of this type are called assisted hydrolysis and occur by an intramolecular displacement mechanism steric factors may retard the reactions. 

The fact that ) -D-galactosidase from Escherichia coli is inactivated more rapidly in the absence of Mg than in its presence can be taken as evidence that the activation of the triazene 38, that is, formation of )5-D-galactosyl-methyldiazonium ion, proceeds without acid catalysis, because Mg is required for the proton-assisted catalysis of yS-D-galactoside hydrolysis by this enzyme.Additional evidence for the absence of acid catalysis in the de-... 

Attention has been drawn to the potential of phosphoric acid anhydrides of nucleoside 5 -carboxylic acids (14) as specific reagents for investigating the binding sites of enzymes. For example, (14 B = adenosine) inactivates adenylosuccinate lyase from E. coli almost completely, but has little effect on rabbit muscle AMP deaminase. The rate of hydrolysis of (14) is considerably faster than that of acetyl phosphate, suggesting intramolecular assistance by the 3 -hydroxyl group or the 3-nitrogen atom. 

Glucose-6-phosphate dehydrogenase is a dimer with a molecular mass of about 135 000. Up to eight electrophoretically separable isoenzymes for this enzyme are known. A specific feature of the above reaction is the formation of NADP Hr The reaction equilibrium is strongly shifted to the right, since the lactone formed is liable to hydrolysis, which is spontaneous or lactonase-assisted. 

The implications of the above observations may be important, especially if similar trends are observed in pyranose anomers. For example, with respect to the mechanism of acid-catalyzed hydrolysis of pyranosides, endocyclic C-0 bond cleavage (preceeded by 05 protonation) may be assisted in P-anomers in which the Cl-01 bond is equatorial, since the 04-Cl bond may already be extended in these anomers. By a similar argument, exocyclic C-0 scission (preceeded by 01 protonation) may be assisted in the hydrolysis of a-pyranosides in which the Cl-01 is axial and extended, thus resembling the transition state. Post and Karplus have recently suggested that enzyme-catalyzed glycoside hydrolysis of P-pyranosides may indeed take place by ring oxygen protonation, followed by endocyclic C-0 bond scission. 

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