Amino acids synthesis, regulation

Although uptake and accumulation of most amino acids from the external medium seems to be irreversible, amino acids are excreted into the medium whenever they are overproduced above a given threshold by yeast cells [6], This can occur under a number of specific conditions, namely in mutants with impaired regulation of amino acid biosynthesis, or in the presence of mutations preventing substrate catabolism, or when growth occurs in the presence of metabolic intermediates. It can even occur when growth is arrested under conditions where amino acid synthesis can continue. 

Both the overall rate of protein synthesis and the translation of certain specific mRNAs are controlled by agents such as hormones, growth factors, and other extracellular stimuli. As precursors for protein assembly, amino acids also regulate the translational machinery. Because protein synthesis consumes a high proportion of cellular metabolic energy, the energy status of the cell also modulates translation factors. 

Insulin is a polypeptide hormone that consists of two peptide chains bonded by two disulfide bonds. The two chains are designated A and B. The A chain consists of 21 amino acids with a third internal disulfide bond, and the chain contains the remaining 30 amino acids. All vertebrates produce insulin and the structure is similar in these species. For example, the insulin produced in humans and porcine species differs by only one amino acid, and humans and bovine insulin differ by three amino acids. Insulin plays a crucial role in several physiological processes. These include the regulation of sugar in the body, fatty acid synthesis, formation of triglycerides, and amino acid synthesis. 

The most responsive regulation of amino acid synthesis takes place through feedback inhibition of the first reaction in a sequence by the end product of the pathway. This first reaction is usually irreversible and catalyzed by an allosteric enzyme. As an example, Figure 22-21 shows the allosteric regulation of isoleucine synthesis from threonine (detailed in Fig. 22-15). The end product, isoleucine, is an allosteric inhibitor of the first reaction in the sequence. In bacteria, such allosteric modulation of amino acid synthesis occurs as a minute-to-minute response. 

Sen, A.K. W. Liu. 1990. Dynamic analysis of genetic control and regulation of amino acid synthesis The tryptophan operon in Escherichia coli. Biotechnol. Bioeng. 35 185-94. 

The actions of mRNA and ribosome are similar to a tape playing instructions into a parts-assembly machine. The protein or polypeptide is assembled one amino acid at a time, and, once produced, can act as an enzyme, for example, to create the amino acids required for the next set of proteins. In this way, the sequence of DNA bases is translated into the amino acid sequence of a protein. The feedback loop comprising protein formation, enzymes, amino acid synthesis, and ribosome action indicates that the process can be regulated so that the amount of an enzyme produced depends on the metabolic needs of the cells. 

However, formation of the specific repressor by the regulator gene is evidently only one of the possible methods of repression of transcription of operons. Other methods of repression of the structural genes in bacteria have recently been found (Richmond, 1967 Bretscher, 1968) and the repressor function of aminoacyl-transfer RNAs in repression of the enzymes of amino acid synthesis has been studied in more detail (Freundlich, 1967). 

The role of the end products of a metabolic pathway in regulating their own biosynthesis was first demonstrated by Roberts et al. (1955). Working with E. co/z, they showed that amino acid synthesis from glucose is inhibited by the addition of amino acids to the incubation medium. Umbarger (1956) demonstrated that end products may inhibit the activity of enzymes mediating end-product synthesis. Often this inhibition is exerted on the first enzyme of the metabolic sequence. End products may also inhibit enzyme synthesis itself, as is frequently observed in anabolic pathways for amino acids, purines, and pyrimidines. This latter mode of metabolic regulation is termed repression and may occur independently of feedback inhibition. Both mechanisms may be involved in regulation of the same biosynthetic pathway. However, unlike feedback inhibition, which provides very rapid control, repression is a relatively slow process which permits adjustment of metabolism over an extended period of time. 

In this scheme, F symbolizes an essential metabolite, such as an amino acid or a nucleotide. In such systems, F, the essential end product, inhibits enzyme 1, xAie first step in the pathway. Therefore, when sufficient F is synthesized, it blocks further synthesis of itself. This phenomenon is called feedback inhibition or feedback regulation. 

Pyruvate kinase possesses allosteric sites for numerous effectors. It is activated by AMP and fructose-1,6-bisphosphate and inhibited by ATP, acetyl-CoA, and alanine. (Note that alanine is the a-amino acid counterpart of the a-keto acid, pyruvate.) Furthermore, liver pyruvate kinase is regulated by covalent modification. Flormones such as glucagon activate a cAMP-dependent protein kinase, which transfers a phosphoryl group from ATP to the enzyme. The phos-phorylated form of pyruvate kinase is more strongly inhibited by ATP and alanine and has a higher for PEP, so that, in the presence of physiological levels of PEP, the enzyme is inactive. Then PEP is used as a substrate for glucose synthesis in the pathway (to be described in Chapter 23), instead... 

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