Amino acids converting enzyme

Fernandez De Palencia, P., de la Plaza, A., Amarita, F., et al. (2006) Diversity of amino acid converting enzymes in wild lactic acid bacteria. Enz Microbial Technol 38, 88—93. 

The reversal of this process could potentially occur with reprotonation from either face of the C=N double bond, and a mixture of aldimines would result, leading to generation of a racemic amino acid. This accounts for the mode of action of PLP-dependent amino acid racemase enzymes. Of course, the enzyme controls removal and supply of protons this is not a random event. One important example of this reaction is alanine racemase, employed by bacteria to convert L-alanine into o-alanine for cell-wall synthesis (see Box 13.12). 

Modification of Ring a.—Some of the modifications mentioned above (Scheme 11) are common to many organisms.The complete structure of A -3-keto-steroid isomerase (EC 5.3.3.1) has been determined. It has three sub-units, each of 125 amino-acids. This enzyme transfers the 4/9-hydrogen atom to the 6j3-position as the A -double bond is moved into conjugation. Starfish convert cholesterol into 5a-cholest-7-enol by a similar process. The A -double bond is introduced only after reduction of the A -3-one. ... 

While all proteases convert proteins to amino acids, each enzyme acts on a particnlar section of the protein. Endopeptidases hydrolyze peptide bonds that are adjacent to specific amino acid side chains, thns converting long protein chains into many shorter peptides. The exopeptidases hydrolyze peptide bonds at either the carboxyl- or amino-terminal ends of these peptides. The endopeptidases, therefore, prodnce more snbstrates for the exopeptidases so that the rate of protein digestion accelerates almost like a chain reaction and then declines as the hydrolytic processes approach completion. 

Before collagen molecules can cross-link, the primary amino groups of the lysine residues of collagen must be converted to aldehyde groups. (Lysine is an amino acid.) The enzyme that catalyzes this reaction is called lysyl oxidase. An aldol condensation between two aldehyde groups results in a cross-linked protein. 

Once the mRNA is synthesized, it migrates out of the nucleus into the cytoplasm to the ribosomes. In the translation process, tRNA molecules, amino acids, and enzymes convert the codons on mRNA to build a protein. 

A. L-Amino Acid Oxidase. Snake venom amino acid oxidase is different from microorganism (bacteria and fungi) enzymes in that the former acts of levorotary-amino acids, and the latter work on dextrorotary-amino acids. The enzyme converts free amino acids into a-keto acids. 

Endothelin. The endothelin (ET) peptide family (50) comprises thiee peptides ET-1 (133), ET-2 (134), and ET-3 (135). ET-1, the most abundant, is a 21-amino acid peptide. A 203-amino acid peptide piecuisoi, piepioET, is cleaved after translation by endopeptidases to form a 38-amino acid proET which is converted to active ET by a putative endothelin-converting enzyme (ECE). ET-3 differs from ET-1 and ET-2 by sis amino acids. 

Enzymatic Process. Chemically synthesized substrates can be converted to the corresponding amino acids by the catalytic action of an enzyme or the microbial cells as an enzyme source, t - Alanine production from L-aspartic acid, L-aspartic acid production from fumaric acid, L-cysteine production from DL-2-aminothiazoline-4-catboxyhc acid, D-phenylglycine (and D-/> -hydtoxyphenylglycine) production from DL-phenyUiydantoin (and DL-/)-hydroxyphenylhydantoin), and L-tryptophan production from indole and DL-serine have been in operation as commercial processes. Some of the other processes shown in Table 10 are at a technical level high enough to be useful for commercial production (24). Representative chemical reactions used ia the enzymatic process are shown ia Figure 6. 

FIGURE 15.2 Enzymes regulated by covalent modification are called interconvertible enzymes. The enzymes protein kinase and protein phosphatase, in the example shown here) catalyzing the conversion of the interconvertible enzyme between its two forms are called converter enzymes. In this example, the free enzyme form is catalytically active, whereas the phosphoryl-enzyme form represents an inactive state. The —OH on the interconvertible enzyme represents an —OH group on a specific amino acid side chain in the protein (for example, a particular Ser residue) capable of accepting the phosphoryl group. 

Fatty acids with odd numbers of carbon atoms are rare in mammals, but fairly common in plants and marine organisms. Humans and animals whose diets include these food sources metabolize odd-carbon fatty acids via the /3-oxida-tion pathway. The final product of /3-oxidation in this case is the 3-carbon pro-pionyl-CoA instead of acetyl-CoA. Three specialized enzymes then carry out the reactions that convert propionyl-CoA to succinyl-CoA, a TCA cycle intermediate. (Because propionyl-CoA is a degradation product of methionine, valine, and isoleucine, this sequence of reactions is also important in amino acid catabolism, as we shall see in Chapter 26.) The pathway involves an initial carboxylation at the a-carbon of propionyl-CoA to produce D-methylmalonyl-CoA (Figure 24.19). The reaction is catalyzed by a biotin-dependent enzyme, propionyl-CoA carboxylase. The mechanism involves ATP-driven carboxylation of biotin at Nj, followed by nucleophilic attack by the a-carbanion of propi-onyl-CoA in a stereo-specific manner. 

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

Together with dopamine, adrenaline and noradrenaline belong to the endogenous catecholamines that are synthesized from the precursor amino acid tyrosine (Fig. 1). In the first biosynthetic step, tyrosine hydroxylase generates l-DOPA which is further converted to dopamine by the aromatic L-amino acid decarboxylase ( Dopa decarboxylase). Dopamine is transported from the cytosol into synaptic vesicles by a vesicular monoamine transporter. In sympathetic nerves, vesicular dopamine (3-hydroxylase generates the neurotransmitter noradrenaline. In chromaffin cells of the adrenal medulla, approximately 80% of the noradrenaline is further converted into adrenaline by the enzyme phenylethanolamine-A-methyltransferase. 

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