Systems involving Phosphate Groups

Systems involving Phosphate Groups.— The binding between Mn i and inosine 5 -triphosphate (ITP) has been studied by and n.m.r. techniques and it has 

Mildvan and Grisham have reviewed the role of bivalent cations in the mechanism of enzyme-catalysed phosphoryl and nucleotidyl transfer reactions. 

Mn -ATP (from a folded chelate to an extended outer-sphere complex) when the nucleotide binds to pyruvate kinase. It has also been established that the substitution-inert complex Cr iL-ATP binds at the ATP binding site of the pyruvate kinase-M + complex, and studies with this magnetic probe have led to the construction of molecular models for composite complexes of this important enzyme. Steady-state kinetic studies on the Mn +-, Ni +-, and Co +-activated systems suggest that the substrates of pyruvate kinase are PEP, uncomplexed ADP, and free bivalent cations. Magnesium-complexed ADP and ATP bind at the same site on yeast phosphoglycerate kinase, as do the uncomplexed nucleotides. 

The binding of Co + to apo-alkaline phosphatase has been studied by the stopped-flow technique and the process appears to be complex. Three reaction steps were detected, in the millisecond, second, and minute ranges none of these was a bimolecular process but the authors suggest that they might all be associated with enzyme isomerization. The spontaneous hydrolysis of (5) is catalysed by bivalent metal ions and it seems that optimal behaviour depends almost entirely on the cationic radius. 

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Other enzymes present in myelin include those involved in phosphoinositide metabolism phosphatidylinositol kinase, diphosphoinositide kinase, the corresponding phosphatases and diglyceride kinases. These are of interest because of the high concentration of polyphosphoinositides of myelin and the rapid turnover of their phosphate groups. This area of research has expanded towards characterization of signal transduction system(s), with evidence of G proteins and phospholipases C and D in myelin. 

Why are there two pyridine nucleotides, NAD+ and NADP+, differing only in the presence or absence of an extra phosphate group One important answer is that they are members of two different oxidation-reduction systems, both based on nicotinamide but functionally independent. The experimentally measured ratio [NAD+] / [NADH] is much higher than the ratio [NADP+] / [NADPH]. Thus, these two coenzyme systems also can operate within a cell at different redox potentials. A related generalization that holds much of the time is that NAD+ is usually involved in pathways of catabolism, where it functions as an oxidant, while NADPH is more often used as a reducing agent in biosynthetic processes. See Chapter 17, Section I for further discussion. 

Structurally, NADP differs from NAD only by a phosphate group esterified at the 2 C of the adenosine ribose, a difference which is reflected in the enzymatic roles NAD-dependent dehydrogenases are mostly involved in catabolic reactions, while NADP-specific enzymes are usually confined to biosynthetic pathways (1). The marked specificities displayed by dehydrogenases towards NAD and NADP have provided attractive model systems to understand the process of molecular recognition by protein engineering. 

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