Enzyme amount regulation

Ribonucleotide reductases are discussed in Chapter 16. Some are iron-tyrosinate enzymes while others depend upon vitamin B12, and reduction is at the nucleoside triphosphate level. Mammalian ribonucleotide reductase, which may be similar to that of E. coli, is regarded as an appropriate target for anticancer drugs. The enzyme is regulated by a complex set of feedback mechanisms, which apparently ensure that DNA precursors are synthesized only in amounts needed for DNA synthesis.273 Because an excess of one deoxyribonucleotide can inhibit reduction of all... 

IP3 diffuses to specialized regions of the endoplasmic reticulum and induces it to release a small amount of stored Ca " (Golovina and Blaustein, 1997). The conse-tfuent increase in cytoplasmic levels stimulates a rtumber of events in the cytoplasm, including the further activation of protein kinase C. Calcium ions directly bind to and activate calmodulin, protein kinase C, phospholipase A2, proteins of muscle fibers (troponin, caldesmon), and proteins of the cytoskeleton (geisolin, villin). One might hesitate to cali these proteins Ca-metalloenzymes. It is more accurate to say that these enzymes are regulated by calcium. 

Mammalian acetyl-CoA carboxylase also seems to be subject to regulation by phosphorylation/ dephosphorylation. Phosphorylation with one mol of phosphate/mol of rat liver carboxylase subunit causes complete inactivation (Lent and Kim, 1980). The reactions have also been studied using preparations from the rat epididymal fat pad (Krakower and Kim, 1981). Long-term regulation of the carboxylase is caused by diet, thyroxine and insulin. Total enzyme amounts also change during cell differentiation and development (reviewed by Volpe and Vagelos, 1976). 

At physiological concentrations (10 M), flavonoids may either stimulate or inhibit lAA oxidase (an enzyme which regulates the amount of the plant-growth-regulating hormone indole acetic acid (lAA or auxin) activity in peas... 

Regulation of enzyme amount. Enzyme synthesis is governed by such regulatory mechanisms as induction, repression, and catabolite repression. These mechanisms, which may have drastic effects on cellular composition, have time constants on the order of minutes to hours, and hence result in a much slower adaption than the mechanisms treated before. Drastic... 

It is, however, not the only mechanism regulating enzyme amount. Experiments with actinomycin D (D 8.4.1), an inhibitor of transcription, revealed a considerable gap between the onset of transcription and the actual beginning of secondary product formation in dipicolinic acid biosynthesis (D 18) during bacterial sporulation. This suggests control of enzyme synthesis on the transcriptional and on one of the posttranscriptional levels. The bacteria probably have stable mRNA species whose translation offers a second level of control of the overall process. 

In addition to regulation of enzyme amount, secondary metabolism may also be controlled by regulation of the activity of the enzymes involved. Evidence for this comes from the large discrepancies found between the relatively high enzyme activities in vitro and the much lower activities in the living cell. 

The data discussed here so far provides a basis for proposing a mechanism (1 ) that may regulate the amount of unesterified cholesterol present in the cell (Fig. 8). In the liver cell, the utilization of cholesterol is regulated by ACAT and cholesterol 7a-hydroxylase. Both of these enzymes appear to be active in the phosphorylated state. The enzyme which regulates cholesterol synthesis, that is, HMG-CoA reductase, is active when dephosphorylated. Therefore, synthesis and utilization are oppositely regulated by phosphorylation/dephosphorylation. 

The numerator T, contains the kinetic constants and concentrations. In Eq. (6.135) the enzyme amount and a factorfor enzyme regulation are introduced. The denominator Dr is a polynomial of scaled concentrations. The latter are obtained by dividing the concentrations with so-called reactant constants obtained in the spirit of the MichaeUs-Menten approach. Terms arising from the specific enzyme regulation are separately listed in the term As an example, a generic reaction Si + S2 = >2S3 will be considered below. 

Hundreds of metabohc reac tions take place simultaneously in cells. There are branched and parallel pathways, and a single biochemical may participate in sever distinct reactions. Through mass action, concentration changes caused by one reac tion may effect the kinetics and equilibrium concentrations of another. In order to prevent accumulation of too much of a biochemical, the product or an intermediate in the pathway may slow the production of an enzyme or may inhibit the ac tivation of enzymes regulating the pathway. This is termed feedback control and is shown in Fig. 24-1. More complicated examples are known where two biochemicals ac t in concert to inhibit an enzyme. As accumulation of excessive amounts of a certain biochemical may be the key to economic success, creating mutant cultures with defective metabolic controls has great value to the produc tion of a given produc t. 

Regulation of enzyme activity is achieved in a variety of ways, ranging from controls over the amount of enzyme protein produced by the cell to more rapid, reversible interactions of the enzyme with metabolic inhibitors and activators. Chapter 15 is devoted to discussions of enzyme regulation. Because most enzymes are proteins, we can anticipate that the functional attributes of enzymes are due to the remarkable versatility found in protein structures. 

Production of phenylalanine starts after depletion of tyrosine at about 6 hours. This is logical since the micro-oiganism needs a certain amount of tyrosine, for example to synthesise key enzymes, but synthesis of L-phenylalanine is feedback regulated if tyrosine is present. 

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