Amino group, covalent modification

Although the hydrophobic surface of nylon membranes may be improved by hydrolysis or chemical modification, composite polyamide membranes are preferred in chromatographic applications. Klein et al. [42] prepared polyamide microporous membranes by modification of terminal amino groups. Covalent binding of a polyhydroxyl-containing material to the polyamide membrane increases the density... 

FIGURE 15.2 Enzymes regulated by covalent modification are called interconvertible enzymes. The enzymes protein kinase and protein phosphatase, in the example shown here) catalyzing the conversion of the interconvertible enzyme between its two forms are called converter enzymes. In this example, the free enzyme form is catalytically active, whereas the phosphoryl-enzyme form represents an inactive state. The —OH on the interconvertible enzyme represents an —OH group on a specific amino acid side chain in the protein (for example, a particular Ser residue) capable of accepting the phosphoryl group. 

Pyruvate kinase possesses allosteric sites for numerous effectors. It is activated by AMP and fructose-1,6-bisphosphate and inhibited by ATP, acetyl-CoA, and alanine. (Note that alanine is the a-amino acid counterpart of the a-keto acid, pyruvate.) Furthermore, liver pyruvate kinase is regulated by covalent modification. Flormones such as glucagon activate a cAMP-dependent protein kinase, which transfers a phosphoryl group from ATP to the enzyme. The phos-phorylated form of pyruvate kinase is more strongly inhibited by ATP and alanine and has a higher for PEP, so that, in the presence of physiological levels of PEP, the enzyme is inactive. Then PEP is used as a substrate for glucose synthesis in the pathway (to be described in Chapter 23), instead... 

Histone acetylation is a reversible and covalent modification of histone proteins introduced at the e-amino groups of lysine residues. Histones and DNA form a complex - chromatin - which condenses DNA and controls gene activity. Current models interpret histone acetylation as a means to regulate chromatin activity. 

Evolution has provided the cell with a repertoire of 20 amino acids to build proteins. The diversity of amino acid side chain properties is enormous, yet many additional functional groups have been selectively chosen to be covalently attached to side chains and this further increases the unique properties of proteins. Diese additional groups play a regulatory role allowing the cell to respond to changing cellular conditions and events. Known covalent modifications of proteins now include phosphorylation, methylation, acetylation, ubi-quitylation, hydroxylation, uridylylation and glycosyl-ation, among many others. Intense study in this field has shown the addition of a phosphate moiety to a protein... 

Chromatin-modifying complexes are classified into two major groups (1) enzymes that conttol covalent modifications of the amino-terminal tails of histones (acetylation, methylation, phosphorylation, ubiquitinylation) (see Sections 1.3 and... 

Since the discovery that glycolate was an alternate substrate for pyruvate kinase ( ), several other o-hydroxy acids have also been found to be substrates for this enzyme ( ). This class of alternate substrates provides a new approach the problem of substrate specificity for pyruvate kinase. 3-Nitrolactate is one such alternate substrate. Interestingly, the phosphorylated product of this reaction inactivates the enzyme (86). However, 3-nitrolac-tate does not behave as a straightforward affinity label since covalent modification occurs nonspecifically. It is hoped that this new Information may lead to the design of an affinity label of this enzyme, further serving to pinpoint amino acid groups at the active site. 

The major drawback of this reaction system is the high energy and equipment costs due to the use of high pressures. In addition, the use of supercritical carbon dioxide can have adverse effects on enzymes, for example, by decreasing the pH of the microenvironment of the enzyme, by the formation of carbamates owing to covalent modification of free amino groups at the surface of the protein and by deactivation during pressurisation-depressurisation cycles [4]. 

Other regulatory enzymes are modulated by covalent modification of a specific functional group necessary for activity. The phosphorylation of specific amino acid residues is a particularly common way to regulate enzyme activity. 

Covalent modification by phosphorylation and dephosphorylation of hydroxyl groups on amino acid side chains. 

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