Sulfur amino acids, catabolism

Sulfite oxidase catalyzes one of the final stops in the oxidation of the sulfur amino acids. The catabolism of methionine can result in the appearance of its sulfur atom in cysteine, as shown in Chapter 8. Cysteine can be oxidized to cysteine sulfonate, as shciwn in the section on taurine in Chapter 2, and then degraded to pyruvate. Daily, an average of 25 mmol of sulfite is produced in the body. This amount is large compared with the dally intake of fo< sulfite, which is about 2.5 mmol- The point at which sulfite oxidase occurs in the cysteine catabolic pathway is shown in Figure 10,53, Sulfate (SO ") is required for the synthesis of su I fated polypeptides and polysaccharides. It is thought that sulfate is not required in the dict-... 

MAZELIS, M. Catabolism of sulfur-containing amino acids. In Sulfiir Nutrition and Assimilation in Higher Plants (L. De Kok, I. Stulen, H. Rennenberg, C. Brunold and W. Rauser, eds.), SPB Academic Publishing, The Hague. 1993, pp. 95-108. 

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... 

Taurine was discovered in 1827 in ox hUe, where it is conjugated with the bile acids. It was later shown to be a major excretory product of the sulfur amino acids methionine and cysteine. Until about 1976, it was assumed that it was a metabolic end-product whose only function was the conjugation of bile acids. In the rat, taurine synthesis accounts for 70% to 85% of total cysteine catabolism. 

The mechanism by which dietary protein induces an increase in urinary calcium is not clear, The effect has been attributed, in part, to the catabolism of sulfur-containing amino acids to yield sulfate. Elevated levels of plasma sulfate can form a complex with calcium. The complex passes into the renal tubule, where it is poorly reabsorbed, resulting in its excrehon in the urine. The mechanism by which phosphate reverses the hypercalciuric effect of protein is also not dear. 

Free sulfate occurs in the plasma at concentrations of 1 to 2 mM. The sulfate in the plasma and glomerular filtrate has been a concern for those interested in calcium status. Consumption of high-protein diets leads to increases in urinary calcium levels. This effect has been attributed, in part, to the catabolism of sulfur amino acids to yield free sulfate. The sulfate forms a complex with the calcium in... 

The second metabolic pathway which we have chosen to describe is the tricarboxylic acid cycle, often referred to as the Krebs cycle. This represents the biochemical hub of intermediary metabolism, not only in the oxidative catabolism of carbohydrates, lipids, and amino acids in aerobic eukaryotes and prokaryotes, but also as a source of numerous biosynthetic precursors. Pyruvate, formed in the cytosol by glycolysis, is transported into the matrix of the mitochondria where it is converted to acetyl CoA by the multi-enzyme complex, pyruvate dehydrogenase. Acetyl CoA is also produced by the mitochondrial S-oxidation of fatty acids and by the oxidative metabolism of a number of amino acids. The first reaction of the cycle (Figure 5.12) involves the condensation of acetyl Co and oxaloacetate to form citrate (1), a Claisen ester condensation. Citrate is then converted to the more easily oxidised secondary alcohol, isocitrate (2), by the iron-sulfur centre of the enzyme aconitase (described in Chapter 13). This reaction involves successive dehydration of citrate, producing enzyme-bound cis-aconitate, followed by rehydration, to give isocitrate. In this reaction, the enzyme distinguishes between the two external carboxyl groups... 

Normal blood pH is 7.35-7.45 (corresponding to 35-45 nmol of H+ per liter). Values below 6.80 (160 nmol of H+ per liter) or above 7.70 (20 nmol of H+per liter) are seldom compatible with life. A large amount of acid produced is a byproduct of metabolism. The lungs remove 14,000 mEq of CO2 per day. From a diet that supplies 1 -2 g of protein per kilogram per day, the kidneys remove 40-70 mEq of acid per day as sulfate (from oxidation of sulfur-containing amino acids), phosphate (from phospholipid, phosphoprotein, and nucleic acid catabolism), and organic acids (e.g., lactic, )3-hydroxybutyric, and ace-toacetic). These organic acids are produced by incomplete oxidation of carbohydrate and fats, and in some conditions (e.g., ketosis see Chapter 18) considerable amounts may be produced. 

One of the pathways to propanoyl-CoA is from catabolism of the amino acid threonine (Chapter 12). Thus, threonine (threonine dehydratase, EC 4.3.1.19, cofactor pyridoxal phosphate) undergoes deamination to give 2-oxobutanoate (a-ketobutyrate) as shown below. Then, 2-oxobutanoate (a-ketobutyrate) undergoes decarboxylation (perhaps as shown in Scheme 11.30) with formation of propanoyl dihydro-lipoamide in a (cofactor) thiamine diphosphate mediated step. Finally, as in Scheme 11.31, propanoyl-CoA is formed. An alternative pathway uses aferrodoxin to effect the decarboxylation of 2-oxobutanoate (a-ketobutyrate) Ferredoxins are small proteins containing iron and sulfur atoms in iron-sulfur clusters. 

The carbon chain of cysteine and cystine is derived from serine by a mechanism discussed in the chapter, Metabolism of Sulfur-Containing Compounds. The sole source of tyrosine for the vertebrates is phenylalanine, as is explained in the chapter. Carbon Catabolism of Amino Acids. 

Molybdenum is considered an ultra-trace element with an approximate amount of 5 mg in the adult human body. It is a cofactor for at least three enzymes in humans (sulfite oxidase, xanthine oxidase, and aldehyde oxidase) and is involved in the catabolism of sulfur-containing amino acids, purine, and pyrimidine. A better understanding of human molybdenum metabolism is needed in order to give evidence-based recommendations regarding optimal nutrition, although molybdenum deficiency and associated pathological symptoms have not yet been observed in humans [74]. 

Sulfur compounds are generated by sulfur amino acid catabolism and are potent odorants that contribute flavor to many fermented foods. Methionine catabolism produces various volatile sulfur compounds (VSCs) such as H S, methanethiol, dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and dimethyl trisulfide (DMTS) (Fernandez et al. 2000). The enzymes in LAB strains from raw goats milk cheeses crucial for VSC formation from L-methionine have very diverse enzyme capabilities. 

Methionine metabolism Sulfur compounds, responsible for aroma in wine and typically related to the grape variety, are released by yeast during the AF. In addition, the metabolism of the sulfur-containing amino acid methionine has an impact on wine aroma. Lact. brevis, Lact. plantarum, and O. oeni strains, using a pathway similar to dairy LAB, catabolize methionine producing light volatile sulfur molecules such as methanethiol and dimethyldisulfide, and heavy volatile molecules such as 3-(methylsulphanyl) propan-l-ol and 3-(methylsulphanyl) propionic acid (Pripis-Nicolau et al. 2004 Weimer et al. 1999 VaUet et al. 2008). In wine, O. oeni strains produce more heavy compounds, mainly 3-(methylsulphanyl) propionic acid, than lactobacilli. In water 3-(methyl-sulphanyl) propionic acid descriptors are chocolate and roasted but these notes are not found in wine where they are replaced by red fruit and earthy odors probably because of interactions with other wine components (Pripis-Nicolau et al. 2004). 

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