Intact cytosolic pathway

Aromatic amino acids interface with a diverse and vast network of connecting secondary metabolism in the cytosol, but not in other major compartments such as the chloroplast. A strong rationale and emerging lines of experimental evidence support the probable existence of an intact cytosolic pathway of aromatic amino acid biosynthesis which links carbohydrate metabolism (via PEP and erythrose-4-P, or possibly glyceraldehyde-3-P) and secondary metabolism. 

It is generally accepted that chloroplasts possess an intact pathway of aromatic amino acid biosynthesis that is tightly regulated. In addition, the subcellular location of some aromatic-pathway isozymes has been shown to be in the cytosol, but whether an intact pathway exists in the cytosol has not yet been proven. The evidence bearing on aromatic amino acid compartmentation and regulation is reviewed, with particular emphasis given to the relationship between primary biosynthesis and secondary metabolism in the cytosol. 

There is a fifth bifunctional enzyme which catalyzes reactions 8 and 12 of the purine pathway (Fig. 15-16) but adenylosuccinate lyase has one active site with dual specificity, catalyzing both reactions (SA1CAR—> AICAR, sAMP—> AMP Fig. 15-16). All 14 enzymatic activities of Fig. 15-16 are cytosolic and there is a variety of evidence for association of subsets of these activities in vivo. The existence of a pathway particle or metabolon" for de novo purine biosynthesis in intact cells has been proposed. 

Fig. 11. Demonstration of proline - P5C cycling in reconstituted systems. Adapted from data published in refs. (35) and (71). (A) The generation of oxidizing potential to drive the pentose phosphate pathway. Intact rat kidney mitochondria were incubated with erythrocyte cytosol and COj production from [l- C]glucose (1 mAf) was measured. With NADPH (0.1 mM) and ADP (0.2) mM) in the incubation, the amount of glucose oxidized in the presence (A) and absence (O) of 10 mM proline is shown. (B) The transfer of reducing potential to proline. The reconstituted system was similar to that for A except intact rat liver mitochondria were incubated with erythrocyte cytosol. In the presence of [ 1 - H]glu-cose (0.8 mM), ADP (5 mM), and NADP (0.3 rnM), the formation of PH]proline in the presence ( ) and absence (O) of unlabeled proline (5 mM) occurs when P5C produced from proline is the recipient of the from NADPPH] generated from [l- H]glucose 6-phosphate by G-6-P dehydrogenase.

Considerable investigative effort has been directed to the role of metabolite ratios and multiple metabolites in enzymic control. The complex nature of such control is exemplified by studies of oscillatory responses in the glycolytic pathway in both intact yeast cells and bovine heart cytosol. This subject has been considered by Higgins (1967), Hess and Boiteux (1971), Walter (1972, 1974), and Goldbeter and Caplan (1976) and is discussed in Chapter 8 of this volume. 

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