Cysteine essentiality

This is followed by removal of the glutamic acid and the glycine residues, which is followed by acetylation of the remaining cysteine. Essential amino acids are required for the synthesis of the proteins involved, pantothenic acid for coenzyme A synthesis, and phosphorus for synthesis of the ATP needed for glutathione synthesis. Similar scenarios can be developed for glucuronide and sulfate formation, acetylation, and other phase II reaction systems. 

Although molybdenum is an essential element, excess levels can have deleterious effects. The LD q and TLV values of the most common Mo compounds are Hsted in Table 3 (63,64). In general the toxicity of Mo compounds is considered to be low. For example, M0S2 has been found to be virtually nontoxic even at high levels. Certain Mo compounds such as MoCl and Mo(CO), have higher toxicity because of the chemical nature and reactivity of these compounds rather than the Mo content. Supplementary dietary Cu ", thiosulfate, methionine, and cysteine have been shown to be effective in alleviating Mo toxicity in animals. 

In terms of amino acids bacterial protein is similar to fish protein. The yeast s protein is almost identical to soya protein fungal protein is lower than yeast protein. In addition, SCP is deficient in amino acids with a sulphur bridge, such as cystine, cysteine and methionine. SCP as a food may require supplements of cysteine and methionine whereas they have high levels of lysine vitamins and other amino acids. The vitamins of microorganisms are primarily of the B type. Vitamin B12 occurs mostly hi bacteria, whereas algae are usually rich in vitamin A. The most common vitamins in SCP are thiamine, riboflavin, niacin, pyridoxine, pantothenic acid, choline, folic acid, inositol, biotin, B12 and P-aminobenzoic acid. Table 14.4 shows the essential amino acid analysis of SCP compared with several sources of protein. 

For many serine and cysteine peptidases catalysis first involves formation of a complex known as an acyl intermediate. An essential residue is required to stabilize this intermediate by helping to form the oxyanion hole. In cathepsin B a glutamine performs this role and sometimes a catalytic tetrad (Gin, Cys, His, Asn) is referred too. In chymotrypsin, a glycine is essential for stabilizing the oxyanion hole. 

AVP and OT are cyclic nonapeptides with a disulphide bridge between the cysteine residues 1 and 6, resulting in a six-amino acid ring and a COOH-terminal a-amidated three-residue tail. OT differs only in two amino acids from AVP lie in position 3, which is essential for OT recqrtor (OTR) stimulation and Leu in position 8. AVP has a Phe in position 3 and an Arg in position 8. Arg 8 is essential for acting upon vasopressin receptors (Fig. 1). Lysipressin, found in pigs and some marsupials, has a Lys in position 8 [1]. 

A comparison of the structures of penicillin and Dalanyl-Dalanine (cf. structures 41 and 42) shows that there is a great deal of similarity between the two molecules. Penicillin is essentially an acylated cyclic dipeptide of Dcysteine and Dvaline (84). As such, it contains a peptide bond, that of the /3-lactam ring, that can acylate the enzyme. Labeling studies of the peptidoglycan transpeptidase of Bacillus subtilis indicate that radioactive penicillin reacts with a sulfhydryl group of a cysteine residue of the enzyme (86). 

Early mutational studies of the Rieske protein from 6ci complexes have been performed with the intention of identifying the ligands of the Rieske cluster. These studies have shown that the four conserved cysteine residues as well as the two conserved histidine residues are essential for the insertion of the [2Fe-2S] cluster (44, 45). Small amounts of a Rieske cluster with altered properties were obtained in Rhodobacter capsulatus when the second cysteine in the cluster binding loop II (Cys 155, corresponding to Cys 160 in the bovine ISF) was replaced by serine (45). The fact that all four cysteine residues are essential in Rieske clusters from be complexes, but that only two cysteines are conserved in Rieske-type clusters, led to the suggestion that the Rieske protein may contain a disulfide bridge the disulfide bridge was finally shown to exist in the X-ray structure (9). 

As mentioned in the Introduction, in iron—sulfur proteins, the hyperfine shifts of the nuclei of the coordinating cysteines are essentially contact in origin (21, 22). In the case of [Fe4S4l (17) and [FegS4] (112) cluster, it has been shown that the hyperfine shift of the cysteinyl H/3 and Ca nuclei can be related to the value of the Fe-Sy-C/S-H/S/Ca dihedral angle (6) through a Karplus-type relationship of the form... 

Humans can synthesize 12 of the 20 common amino acids from the amphiboHc intermediates of glycolysis and of the citric acid cycle (Table 28-1). While nutritionally nonessenrial, these 12 amino acids are not nonessential. AH 20 amino acids are biologically essential. Of the 12 nutritionally nonessential amino acids, nine are formed from amphibolic intermediates and three (cysteine, tyrosine and hydroxylysine) from nutritionally essential amino acids. Identification of the twelve amino acids that humans can synthesize rested primarily on data derived from feeding diets in which purified amino acids replaced protein. This chapter considers only the biosynthesis of the twelve amino acids that are synthesized in human tissues, not the other eight that are synthesized by plants. 

Cysteine. Cysteine, while not nutritionally essential, is formed from methionine, which is nutritionally essential. Following conversion of methionine to ho-... 

Cysteine, tyrosine, and hydroxylysine are formed from nutritionally essential amino acids. Serine provides the carbon skeleton and homocysteine the sulfur for cysteine biosynthesis. Phenylalanine hydroxylase converts phenylalanine to tyrosine. 

Two amino acids—cysteine and tyrosine—can be synthesized in the body, but only from essential amino acid ptecutsots (cysteine from methionine and tyrosine from phenylalanine). The dietary intakes of cysteine and tytosine thus affect the requirements for methionine and phenylalanine. The remaining 11 amino acids in proteins are considered to be nonessential or dispensable, since they can be synthesized as long as there is enough total protein in the diet—ie, if one of these amino acids is omitted from the diet, nitrogen balance can stiU be maintained. Howevet, only three amino acids—alanine, aspartate, and glutamate—can be considered to be truly dispensable they ate synthesized from common metabolic intetmediates (pyruvate, ox-... 

Amino acid sequences of eleven homologous sea anemone polypeptides have been elucidated. All possess three disulfide bonds. The six half-cysteine residues always occur in the same positions (7,8). Initial studies concerning the toxin secondary and tertiary structures relied upon circular dichroism, laser Raman, and, to a lesser extent, fluorescence spectral measurements (15—18). The circular dichroism spectra of the four toxins so far examined are essentially superimpos-able and thus indicate a common secondary structure. The only peak observed, a negative ellipticity at 203 nm, largely results from a non-regular ("random")... 

The essential genetic material ofthe original vegetative bacterium is retained in the core or protoplast around this lies the thick cortex which contains the murein or peptidoglycan already encountered as a cell wall component (see Fig. 1.2). The outer coats which are protein in composition are distinguished by their high cysteine content. In this respect they resemble keratin, the protein of hair and horn. 

DNA sequence indicated that AMDase contains four cysteine residues located at 101, 148, 171 and 188 from amino terminal (Eig. 9). At least one of these four is estimated to play an essential role in the decarboxylation. The most effective way to determine which Cys is responsible to enzyme activity will be site-directed mutagenesis. To determine which amino acid should be introduced in place of active Cys, its role was estimated as illustrated in Eig. 13. One possibility is that... 

However, this is not so easy without the tertiary structure of the enzyme. The possible clues are the homology search with functionally resembling enzymes and computer simulation of the tert-structure of the enzyme. The characteristic features of AMDase are (i) the reaction proceeds via an enolate-type transition state, (ii) the cysteine residue plays an essential role and (iii) the reaction involves an inversion of configuration on the a-carbon of the carboxyl group. 

Insulin, a small protein of molecular mass 6000 daltons, is composed of two chains designated A and B. There are no reduced cysteine residues in insulin, but it contains three essential disulfide bonds two that crosslink the A and B chains, and one internal to the A chain to stabilize the overall tertiary stmcture. These disulfide bonds are cleaved in the presence of excess AuX4, leaving A and B chains that have cysteine residues that have become oxidized to sulfonic adds [119]. With smaller amounts of AuX4, a single disulfide bond will be attacked to form sulfinic acid [119]. The reaction is second order for AuCU while AuBr4 reacts too quickly for accurate monitoring. 

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