Regulatory behavior

The GDH s that have been studied extensively are regulated either by induction-repression effects (Section III) or by allosteric effects. The effectors can be divided into three general classes purine nucleoside di-and triphosphates, substrates and coenzymes, and certain steroid hormones and lipids. 

Source Ref. i Glutamate a-Keto- glutarate NH 4+ NAD NADH NADP NADPH  

Price and Radda (338) found that N-acetylimidazole could acetylate up to six tyrosine residues without loss of activity or alteration of Km for substrate however, reaction of about one tyrosine per subunit results in desensitization toward GTP, but the response to ADP is not abolished even by extensive 0-acetylation. Essentially the same results are observed upon nitration with tetranitromethane (TNM). Acetylation does not grossly alter the molecular weight, as measured by sedimentation velocity, or the conformation, as determined by ORD. The GTP site is not protected by NADH alone, but is partially protected (25-50%) by GTP and is at least 75% protected by inclusion of both GTP and NADH in the reaction mixture. Piszkiewicz et al. (339) confirmed these findings by modification with TNM. The reaction is biphasic with initial rapid formation of one residue of 3-nitrotyrosine per subunit. The primary site of reaction is tyrosine-406 in the linear sequence (340). Later (338) the same effect was obtained with chicken GDH with both enzymes there is no influence on activation by ADP. Further, the pH optima of the enzymes are not influenced by the degree of nitration or the inhibition by GTP or activation by ADP (338). 

DiPrisco (341) reported that under certain conditions reaction of GDH with l-fluoro-2,4-dinitrobenzene led to desensitization to both GTP inhibition and ADP activation by modification of three residues of tyrosine per hexamer. Attempts to confirm these results in this laboratory (343) have been unsuccessful. 

Malcolm and Radda (394) have reported that reaction of GDH with lASA results in desensitization toward GTP concurrent with alkylation of a single cysteine residue per subunit. In agreement with this, Nishida and Yielding (343) have found that reaction of the enzyme with methyl-mercuric iodide also causes allosteric desensitization. LeJohn and Jackson 

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Such observations led us to the hypothesis that there is likely a diversity of growth promoters and inhibitors to be found in marine algae and invertebrates. We thus began an examination of selected extracts for plant growth regulatory behavior. 

So far, only very few of these models incorporate complex regulatory behavior [534,551,552], Specifically, no mathematical models have been developed to characterize the complex time behavior of the hypothermic response in a strict quantitative manner, and neither attempts to link existing temperature regulation models [553] to pharmacokinetic models describing the time course of the drug concentration in the body. 

In the plant itself, however, another pattern of control is established. In cotyledons and leaves of Petroselinum hortense seedlings the regulatory behavior of PAL and several group II enzymes... 

Figure 3.23a and b demonstrate the regulatory behavior of this system. To meet the required conditions for this case, we chose K = 10, Ha = 100, and Hb = 0.001. Note that, initially, adding B to the system causes a small change in the binding curve (Fig. 3.23a) the spacing between the curves increases with Xb- This is also shown by the (inverted) S-shaped regulatory curve in Fig. 3.23b. 

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