Enzyme synthesis, genetic control

Three mechanisms of cellular control over enzyme activity exist. One method involves the synthesis of enzyme precursors called zymogens, which are activated when needed by the ceU. The second mechanism relies on the binding of small molecules (modulators), which increase or decrease enzyme activity. Genetic control of enzyme synthesis, the third method, regulates the amount of enzyme available. 

As noted earlier, the velocity of any enzyme-catalyzed reaction is dependent upon the amount of effective enzyme present. Enzyme biosynthesis, like that of all proteins, is under genetic control, a long-term process. Biosynthesis of enzymes may be increased or decreased at the genome level. Various hormones can activate or repress the mechanisms controlling gene expression. Enzyme levels are the result of the balance between synthesis and degradation. This enzyme turnover may be altered by diverse physiological conditions, by hormone effects, and by the level of metabolites. 

Moreover, this enzyme is the most extensively studied example of induced enzyme-formation, and it has been employed for investigating the mechanism of specific protein synthesis and the genetic control thereof. An understanding of the constitution of the active center of this enzyme might help in identifying, in the bacterial cell, the apparatus which controls its production. 

B. Genetic Control of Enzyme Synthesis 546 Box 11-A Cholera Toxin and Other Dangerous... 

In addition to nutritional inadequacy, vitamin deficiency may result from malabsorption, effects of pharmacological agents, and abnormalities of vitamin metabolism or utilization. Thus, in biliary obstruction or pancreatic disease, the fat-soluble vitamins are poorly absorbed despite adequate dietary intake because of steatorrhea. Absorption, transport, activation, and utilization of vitamins require the participation of enzymes or other proteins whose synthesis is under genetic control. Dysfunction or absence of one of these proteins can produce a disease that is clinically indistinguishable from one caused by dietary deficiency. In vitamin-dependent or vitamin-responsive... 

Genetic control The amount of enzyme present is increased by protein synthesis. (Longer-term control than the other mechanisms listed here.) Induction of 3-galactosidase (Section 11.2)... 

Quantity of enzyme The enzyme concentration in cells is regulated at the synthetic level by genetic control, which may occur positively or negatively. Alternatively, the control can be exerted by the specihc degradative pathways. The genetic control of enzyme (protein) synthesis and specific protein degradation will be considered in the Chapters 12 and 13. 

Microbial enzymes produced by fermentation under controlled conditions constitute now the most relevant option for enzyme synthesis. Microbial strains can produce not only the enzyme proteins coded by their own genetic information, but also those produced by the expression of foreign genes as recombinant proteins. As analyzed before, microorganisms are ideal hosts for enzyme synthesis and only those glycoenzymes which cannot be properly produced in microbial hosts are to be produced in plant (Ma et al. 2003) or animal cell culture (Altamirano et al. 2004 Wurm 2004). 

This large and important group of glycolipids is derived from lactosylceramide and the pathways involved are, perhaps, the best understood of any concerned in the synthesis of complex saccharides. Many of these transfers are also found in glycoproteins, where they appear to be brought about by the same enzymes that act on the glycolipids — or, at least, enzymes that are under almost exactly the same genetic control (see Section 5B above). 

One way to increase production from an enzyme-catalyzed reaction, given a sufficient supply of substrate, is for a cell to increase the number of enzyme molecules present. The synthesis of all proteins, including enzymes, is under genetic control by nucleic acids (Chapter 11). An example of the genetic control of enzyme activity involves enzyme induction, the synthesis of enzymes in response to a temporary need of the cell. 

Genetic control of enzyme activity means that a cell synthesizes an enzyme when conditions temporarily require it. Such a controlled synthesis is called enzyme induction. 

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