Isoleucine deamination

Deamination 17 Examples of deamination and decarboxylation include conversion of amino acids to fusel oil (leucine to isoamyl alcohol, isoleucine to amyl alcohol, and phenylalanine to phenyl ethanol). Fusel oil formation is a normal function of all yeast fermentations (in alcoholic beverages, levels range from trace to 2200 parts per million). Deamination Glutamic acid to gamma-OH-butyric acid (S. cerevisiae). 

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

Liver plays a major role, since it can oxidize all amino acids except leucine, isoleucine, and valine (see Chapter 22). It also produces the nonessential amino acids from the appropriate carbon precursors. Ammonia formed in the gastrointestinal tract or from various deaminations in the liver is converted to urea and excreted in urine (discussed later). 

The syntheses of valine, leucine, and isoleucine from pyruvate are illustrated in Figure 14.9. Valine and isoleucine are synthesized in parallel pathways with the same four enzymes. Valine synthesis begins with the condensation of pyruvate with hydroxyethyl-TPP (a decarboxylation product of a pyruvate-thiamine pyrophosphate intermediate) catalyzed by acetohydroxy acid synthase. The a-acetolactate product is then reduced to form a,/3-dihydroxyisovalerate followed by a dehydration to a-ketoisovalerate. Valine is produced in a subsequent transamination reaction. (a-Ketoisovalerate is also a precursor of leucine.) Isoleucine synthesis also involves hydroxyethyl-TPP, which condenses with a-ketobutyrate to form a-aceto-a-hydroxybutyrate. (a-Ketobutyrate is derived from L-threonine in a deamination reaction catalyzed by threonine deaminase.) a,/3-Dihydroxy-/3-methylvalerate, the reduced product of a-aceto-a-hydroxybutyrate, subsequently loses an HzO molecule, thus forming a-keto-/kmethylvalerate. Isoleucine is then produced during a transamination reaction. In the first step of leucine biosynthesis from a-ketoisovalerate, acetyl-CoA donates a two-carbon unit. Leucine is formed after isomerization, reduction, and transamination. 

Changing the cytosine to uracil, such as by deamination, in these codes gives AUC, AUA and AUU, the codes which specify isoleucine. This mutation is therefore a C— TJ... 

Amino acid catabohsm is particularly important dining starvation. Because of the mass of muscle, amino acid catabohsm is particularly important in this tissue which, in starvation, supplies the liver with most of its gluconeogenic precursors (see also Fig. 13-11). Amino acids resulting from proteolysis during starvation are interconverted in the muscle so that 60% of the amino acid mass that leaves the muscle is either glutamine or alanine. The branched-chain amino acids valine, leucine, and isoleucine, which are aU essential amino acids, are deaminated in muscle by a specific aminotransferase, and the corresponding 2-oxoacids are transported to the liver for further metabohsm via branched-chain 2-oxoacid dehydrogenase (BCOADH). The aminotransferase is inactive in the hver, and this ensures that the peripheral tissues are supphed with valine, leucine, and isoleucine. 

In the oxidative deamination reaction, the enzyme was active toward N-[l-D-(carboxyl)ethyl]-L-methionine, N-[l-D-(carboxyl)ethyl]-L-phenylalanine, etc. The substrate specificity for amino donors of ODH in the reductive secondary amine-forming reaction was examined with pyruvate as a fixed amino acceptor [15,24]. The enzyme utilized L-norvaline, L-2-aminobutyric acid, L-norleucine, P-chloro-L-alanine, o-acetyl-L-serine, L-methionine, L-isoleucine, L-valine, L-phenylalanine, L-homophenylalanine, L-leucine, L-alanine, etc. 3-Aminobutyric acid and L-phenylalaninol also acted as substrates for the enzyme. Other amino compounds, such as P-amino acids, amino acid esters and amides, amino alcohols, organic amines, hydroxylamines, and hydrazines, were inactive as substrates. Pyruvate, oxaloacetate, glyoxylate, and a-ketobutyrate were good amino acceptors. We named the enzyme as opine... 

Amino acids are used by the body to form proteins, hormones, and enzymes. Transamination reactions can convert one amino acid into another to meet immediate needs. However, just as there are essential fatty acids, there are also essential amino acids. These amino acids cannot be synthesized in the body and must come from external sources. Humans require phenylalanine, valine, tryptophan, threonine, lysine, leucine, isoleucine, and methionine as essential amino acids. All other amino acids in the body can be synthesized at rates sufficient to meet body needs. If any one of the amino acids necessary to synthesize a particular protein is not available, then the other amino acids that would have gone into the protein are deaminated, and their excess nitrogen is excreted as urea (Ganong, 1963). 

A fermentation study [18] using Streptomyces avermitilis culture 5192 and [1- C] acetate and [1- C] propionate precursors, followed by NMR analysis of the products, established the incorporation of seven acetate and five propionate units into the macrocycle. Carbon 25 and its substituents were shown to be derived from L-isoleucine and l-valine following deamination and conversion into the analogs carboxylic acids. That the C-25 substituent is derived from a... 

Fusel oil unpleasant-tasting side product of alco-hohc fermentation, consisting mainly of amyl, isoamyl, isobutyl and propyl alcohols. The compounds are formed from amino acids, in particular leucine, isoleucine and tyrosine by deamination and decarboxylation. Tyrosol, which is formed from tyrosine, is a component of beer. 

The degradation of leucine, isoleucine and valine operates by oxidative deamination and then decarboxylation of the corresponding keto acids. 

Studies on banana tissue slices have shown that valine and leucine concentrations increase about threefold following the climacteric rise in respiration [10]. Radioactive labeling studies have shown that valine and leucine are transformed into branched chain flavor compounds that are essential to banana flavor (2-methyl propyl esters and 3-methyl butyl esters, respectively). As can be seen in Figure 4.6, the initial step is deamination of the amino acid followed by decarboxylation. Various reductions and esterifications then lead to a number of volatiles that are significant to fruit flavor (acids, alcohols, and esters). Recent work has shown that amino acids play a role in apple flavor as well. For example, isoleucine is the precursor of 2-methyl butyl and 2-methyl butenyl esters in apples [24,25]. An unusual flavor compound, 2-isobutylthiazole, has been found to be important to the flavor of tomato. It is hypothesized that this compound is formed from the reaction of 3-methyl-l-butanal (from leucine) with cysteamine. 

The butyric acid derivative (19) has been synthesized by thermal rearrangement of the isoxazolidine (18) obtained by 1,3-dipolar addition of a nitronic ester to methyl acrylate. Nitrous acid deamination of (25,35)-isoleucine has been found to give predominantly (95 %) (25, 35)-2-hydroxy-3-methylvaleric acid (20). 

Some amino acids enter the pathway of fatty acid catabolism after transamination or oxidative deamination. Since they give rise to branched-chain fatty acids, they undergo several changes (cf. Chapt. XII-5, and the right side of the fold-out chart). Leucine is broken down via hydroxymethylglutarate. If the shortening of the carbon chain yields propionyl-CoA, as in the case of isoleucine, then succinate can be obtained by carboxylation. 

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