Catabolism pathways

Tryptophan is a precursor for a series of metabolic reactions. Two tryptophan catabolizing pathways are well characterized (i) tryptophan converts to serotonin (ii) tryptophan is also converted to kynurenine. 

Nicotinamide is an essential part of two important coenzymes nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ) (Figure 18.19). The reduced forms of these coenzymes are NADH and NADPH. The nieotinamide eoenzymes (also known as pyridine nucleotides) are electron carriers. They play vital roles in a variety of enzyme-catalyzed oxidation-reduction reactions. (NAD is an electron acceptor in oxidative (catabolic) pathways and NADPH is an electron donor in reductive (biosynthetic) pathways.) These reactions involve direct transfer of hydride anion either to NAD(P) or from NAD(P)H. The enzymes that facilitate such... 

The next three steps—reduction of the /3-carbonyl group to form a /3-alcohol, followed by dehydration and reduction to saturate the chain (Figure 25.7) — look very similar to the fatty acid degradation pathway in reverse. However, there are two crucial differences between fatty acid biosynthesis and fatty acid oxidation (besides the fact that different enzymes are involved) First, the alcohol formed in the first step has the D configuration rather than the L form seen in catabolism, and, second, the reducing coenzyme is NADPH, although NAD and FAD are the oxidants in the catabolic pathway. 

Figure 29.1 An overview of catabolic pathways for the degradation of food and the production of biochemical energy. The ultimate products of food catabolism are C02 and H2O, with the energy released in the citric acid cycle used to drive the endergonic synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) plus phosphate ion, HOPO32-.

As a rule, the anabolic pathway by which a substance is made is not the reverse of the catabolic pathway by which the same substance is degraded. The two paths must differ in some respects for both to be energetically favorable. Thus, the y3-oxidation pathway for converting fatty acids into acetyl CoA and the biosynthesis of fatty acids from acetyl CoA are related but are not exact opposites. Differences include the identity of the acvl-group carrier, the stereochemistry of the / -hydroxyacyl reaction intermediate, and the identity of the redox coenzyme. FAD is used to introduce a double bond in jS-oxidalion, while NADPH is used to reduce the double bond in fatty-acid biosynthesis. 

The energy released in catabolic pathways is used in the electron-transport chain to make molecules of adenosine triphosphate, ATP. ATP, the final result of food catabolism, couples to and drives many otherwise unfavorable reactions. 

The nitrogen source in the medium is the amino add glutamate. There are several cations K Mn2, Cn2, Zn2, Mg2, Co2, Fe2, Ca2 Mo6. Phosphate (POi") is the major anionic component. Fumaric add is a TCA cycle intermediate and may improve metabolic balance through the catabolic pathways and oxidation through the TCA cyde. Peptone may improve growth through the provision of growth factors (amino acids, vitamins, nudeotides). 

Reactions involve several enzymes, which have to follow in sequence for lactic acid and alcohol fermentation. This is known as the glucose catabolism pathway, with emphasis on energetic and energy carrier molecules such as ATP, ADP, NAD+ and NADH. In this pathway the six-carbon substrate yields two three-carbon intermediates, each of which passes through a sequence of reactions to the stable end product of pyruvic acid. 

Once activated, the AMPK system switches on catabolic pathways that generate ATP (upper entries in Table 2), such as the uptake and oxidation of fatty... 

In eukaryotes, anabolic and catabolic pathways that interconvert common products may take place in specific subcellular compartments. For example, many of the enzymes that degrade proteins and polysaccharides reside inside organelles called lysosomes. Similarly, fatty acid biosynthesis occurs in the cytosol, whereas fatty... 

Figure 16-2. The citric acid cycle the major catabolic pathway for acetyl-CoA in aerobic organisms. Acetyl-CoA, the product of carbohydrate, protein, and lipid catabolism, is taken into the cycle, together with HjO, and oxidized to CO2 with the release of reducing equivalents (2H). Subsequent oxidation of 2H in the respiratory chain leads to coupled phosphorylation of ADP to ATP. For one turn of the cycle, 11 are generated via oxidative phosphorylation and one arises at substrate level from the conversion of succinyl-CoA to succinate.

Roca, M. and Minguez-Mosquera, M.I., Chlorophyll catabolism pathway in frnits of Capsicum annum (L.) stay-green versns red frnits, J. Agric. Food Chem., 54, 4035, 2006. 

Three regulators were identified by genetic analysis. The main repressor, KdgR, controls the transcription of pectinase genes, the intracellular catabolic pathway and the secretion machinery. The PecS repressor controls the production of pectate lyases and cellulases, the secretion machinery and the biosynthesis of a blue pigment. PecT acts as a repressor of the production of some pectate lyases. Other proteins are involved in the regulation of pectinase s5mthesis but their role is not well characterized. 

The best known catabolic pathways of nitrogenous compounds are those of arginine, proline, allantoin and 4-aminobutyrate (GABA) degradation. Each of these is inducible under specific conditions, and all are subject to nitrogen-catabo-lite repression (see [7,9] and section 6.3). 

In Saccharomyces cerevisiae, as in most eukaryotic cells, the plasma membrane is not freely permeable to nitrogenous compounds such as amino acids. Therefore, the first step in their utilization is their catalyzed transport across the plasma membrane. Most of the transported amino acids are accumulated inside the yeast cells against a concentration gradient. When amino acids are to be used as a general source of nitrogen, this concentration is crucial because most enzymes which catalyze the first step of catabolic pathways have a low affinity for their substrates. 

Ganesan B, P Dobrowski, BC Weimer (2006) Identification of the leucine-to-2-methylbutyric acid catabolic pathway of Lactococcus lactis. Appl Environ Microbiol 72 4264-4273. 

Arias-Barrau E, ER Olivera, JM Lnengo, C Eemandez, B Galan, JL Garcia, E Dfaz, B Minambres (2004) The homogentisate pathway a central catabolic pathway involved in the degradation of L-phenylalanine, L-tyrosine, and 3-hydroxyphenylacetate in Pseudomonas putida J Bacterial 186 5062-5077. 

Morawski B, RW Eaton, JT Rossiter, S Guoping, H Griengl, DW Ribbons (1997) 2-Naphthoate catabolic pathway in Burkholderia strain JT 1500. J Bacterial 179 115-121. 

Whited GM, DT Gibson (1991) Separation and partial characterization of the enzymes of the toluene-4-mono-oxygenase catabolic pathway in Pseudomonas mendocina KRl. J Bacterial 173 3017-3020. 

Austen RA, NW Dunn (1980) Regulation of the plasmid-specified naphthalene catabolic pathway of Pseudomonas putida. J Gen Microbiol 117 521-528. 

Duetz WA, S Marques, C de Jong, JL Ramos, J G van Andel (1994) Inducibility of the TOL catabolic pathway in Pseudomonas putida (pWWO) growing on succinate in continuous culture evidence of carbon catabolite repression control. J Bacteriol 176 2354-2361. 

Whyte LG, L Bourbonniere, CW Greer (1997) Biodegradation of petroleum hydrocarbons by psychotrophic Pseudomonas strains possessing both alkane (alk) and naphthalene (nah) catabolic pathways. Appl Environ Microbiol 63 3719-3723. 

Kosaka T et al. (2006) Reconstruction and regulation of the central catabolic pathway in the thermophilic propionate-oxidizing syntroph Pelotomaculum thermopropionicum. J Bacterial 188 202-210. 

Martin VJJ, WW Mohn (2000) Genetic investigation of the catabolic pathway for degradation of abietane diterpenoids by Pseudomonas abietaniphila BKME-9. J Bacteriol 182 3784-3793. 

Vannelli T, M Messmer, A Studer, S Vuilleumier, T Leisinger (1999) A corrinoid-dependent catabolic pathway for growth of a Methylobacterium strain with chloromethane. Proc Natl Acad Sci USA 96 4615-4620. 

Gescher J, W Eisenreich, J Wort, A Bacher, G Fuchs (2005) Aerobic benzoyl-CoA catabolic pathway inAzoarcus evansii studies on the non-oxygenolytic ring cleavage enzyme. Mol Microbiol 56 1586-1600. 

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