Enzyme Synthesis and Degradation

Since the mechanisms which govern the rate of enzyme synthesis and degradation in mammalian tissue will be considered elsewhere in this volime, we will have little to remark here, save to note that, both from the point of view of energy expenditure and of time, such mechanisms are more costly forms of regulation than those we have thus far considered. Synthesis, in particular, must operate over a longer time frame than other forms of enzyme modulation and thus cannot be responsible for the rapid or fine tuning response required by interme- 

Up to this point we have considered the nature of the mechanisms which may be responsible for modulating enzymic activity, having chosen examples of mechanisms of presumed physiologic relevance. In this section we inquire into methods by which the site(s) of rate control in a given metabolic pathway can be identified. 

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Manners, D. J., Enzymic Synthesis and Degradation of Starch and Glycogen, 17, 371-430... 

Cells regulate their metabolic activities by controlling rates of enzyme synthesis and degradation and by adjusting the activities of specific enzymes. Enzyme activities vary in response to changes in pH, temperature, and the concentrations of substrates or products, but also can be controlled by covalent modifications of the protein or by interactions with activators or inhibitors. 

The rate of cholesterol biosynthesis appears to be regulated primarily by the activity of HMG-CoA reductase. This key enzyme is controlled by the rate of enzyme synthesis and degradation and by phosphorylation-dephosphorylation reactions. Synthesis of the mRNA for the reductase is inhibited by cholesterol delivered to cells by means of low-density lipoproteins (LDLs). 

M. Rechcigl, in Enzyme Synthesis and Degradation in Mammalian Systems (M. Rechcigl, Jr., ed.), p. 273. Univ. Park Press, Baltimore, Maryland, 1971. 

The continuing interest of Bourne in the chemistry of polysaccharides and associated enzymes originated from the work of Haworth and Peat directed towards the enzymic synthesis and degradation of starch. The impetus for this work was given by the discovery, made by C. S. Hanes in 1940, that a phosphorylase isolated from the potato and pea effects the synthesis, from D-glucosyl phosphate, of starch, later shown (by Haworth, Heath, and Peat) to be amylose. In his first paper (with Haworth and Peat) in 1944, Bourne described the isolation of the Q-enzyme which, in conjunction with phosphorylase, effects the conversion of D-glucosyl phosphate into the major component of whole starch, namely, amylopectin. He had discovered the Q-enzyme in a fraction discarded by previous workers. Already, the quintessence of his mind was revealed in this work meticulous attention to detail, and perception of essentials. 

The Enzymic Synthesis and Degradation of Starch. Part VIII, The Use of Mixtures of P- and Q-Enzymes in the Synthesis of Starch-type Polysaccharides, S. A. Barker,... 

With the recent identification of the anomalous linkages in amylose as a-D-(l 6)-glucosidic inter-chain linkages, the metabolism of starch and glycogen can be adequately considered in terms of the enzymic synthesis and degradation of only two types of n-glucosidic linkage, namely, a-D-(l— 4) and a-n-(l- ). 

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