Major Catabolic Pathways

Similarly to other rhizosphere symbiotic bacteria, P. putida KT2440 lacks the enzyme 6-phophofructokinase (Pfk) [18] and therefore it exhibits a nonfunctional glycolytic Embden-Meyerhof-Parnas (EMP) pathway (Eigure 8.1). Accordingly, 

TktA (PP4965) 6-P-2 Keto-3-Tai (PP2168) deoxygluconate RpiA(PP5165) j 

I Embden-Meyerhof-Parnas pathw I Pentose phosphate pathway I Entner-Doudoroff pathw  

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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.

We begin Part II with a discussion of the basic energetic principles that govern all metabolism (Chapter 13). We then consider the major catabolic pathways by which cells obtain energy from the oxidation of various fuels (Chapters 14 through 19). Chapter 19 is the pivotal point of our discussion of metabolism it concerns... 

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... 

Fig. 1. Pathways of glucose metabolism in eubacteria and eukaryotes. The three major catabolic pathways are the Embden-Meyerhof glycolytic sequence (solid lines), the Entner-Doudoroff pathway (heavy solid lines) and the pentose phosphate pathway (dashed lines). The sequence from glyceraldehyde 3-phosphate to pyruvate is common to all three pathways.

In the major catabolic pathway, serotonin is deaminated and oxidized to form 5-hydroxyin-dole-3-acetaldehyde. The latter molecule is then further oxidized to form 5-hydroxyindole-3-acetate. 

Hydrolysis of pyridoxal phosphate to pyridoxal followed by oxidation to 4-pyridoxic acid is the major catabolic pathway for vitamin B6 in most mammalian species. In cats, however, the major urinary metabolites are pyridoxine 3-sulfate and N-methylpyridoxine (Cobum and Mahuren, 1987). Also, in humans receiving very large vitamin B6 intakes excretion of 5-pyridoxic acid may become significant (Mahuren et aL, 1991). 

Fig. 20.1 Major catabolic pathway for phenylalanine and tyrosine. The loci of known enzymatic defects are indicated by dashed lines. Note that hereditary tyrosinemia is now believed to be due to fumarylacetoacetate hydrolase, the final step in the pathway. (Redrawn with modifications from Mazur A, Harrow B Textbook of Biochemistry. WB Saunders, Philadelphia, 1971)...

Nitroaromatics are slowly degraded by microorganisms both under aerobic and anaerobic conditions, and the metabolic steps involved in the degradation have been poorly documented until now. Two major catabolic pathways are involved in the degradation of nitroaromatics (Figure 1) [23]. In the first pathway, tiie nitro group is reduced to an aniline intermediate, which is further degraded to ammonimn ion and catechol [5,24-28]. 

The major catabolic pathway of exogenous lAA in Vida seedlings is lAA lAA-Asp DIA-Asp- Glc-DIA-Asp. 

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