Malate, synthesis

The biochemistry of jS-poly(L-malate) synthesis has been investigated for P. polycephalum but is far from... 

BRs increase C02 fixation (an indication of cytoplasmic malate synthesis via phosphoenol pyruvate carboxylase, PEP carboxylase this activity results in the accumulation of fraction-1 protein) and also in vivo C02 fixation by ribulose biphosphate action... 

Vickery s early experiments (1954 a, b) indicated that during the early phase of acidification, there was more malate produced than starch consumed. Hence, he inferred CO2 uptake. Finally, based on data of Beevers and Gibbs (1954) and Stiller (1959), suggesting both glycolytic and pentose phosphate activity in supplying substrate from starch for malate synthesis, Ranson and Thomas (1960) proposed the following hypothesis for malate synthesis ... 

Since virtually all of the acids of the citric acid cycle were labeled, the question was asked early to what extent the mitochondrial citric acid cycle was involved. In this regard. Brown (cited in Ranson and Thomas, 1960) showed continued malate synthesis by Bryophyllum when in the presence of malonate, an inhibitor decreasing oxygen consumption and blocking the continued cycle. 

It should be clear that as well as stored carbohydrate degradation to form P-enolpyruvate for malate synthesis in CAM, oxidative metabolism is proceeding concurrently and acetate must be generated for the citric acid cycle. However, the rate of malate synthesis may be five times greater than respiration. The enzymol-ogy of the citric acid cycle was studied in detail by Khan (1969). 

Fig. 3.16. The presumed C4 photosynthetic pathway showing some of the similarities and differences between C4 and CAM. Whereas in CAM malate is produced largely at night, in C4 photosynthesis malate synthesis is light-dependent and proceeds in cells different from malate or C4-acid decarboxylation. In C4, carboxylation and decarboxylation are separated spatially whereas in CAM they are separated in time. Also, in C4, PEP is generated from the C3 product of the C4-acid decarboxylation whereas in CAM it comes from stored carbohydrate synthesized during a previous light period...

The regulation of malate enzyme is poorly understood. Although the equilibrium constant favors malate formation, the CO2 release tends to drive the reaction toward decarboxylation (Harary et al., 1953). Hence the suggestion that the in vivo reaction is toward malate utilization rather than malate synthesis (Walker, 1962 Ting and Dugger, 1965). 

Brandon (1967) postulated that diurnally changing thermoperiods must be responsible for the control of CAM. This hypothesis is based on in vitro experiments which showed that the malate-synthesizing enzymes (PEP carboxylase plus MDH) reach their optimum at 35° C, the thermal optimum of malate enzyme which may initiate malic acid consumption during CAM, was not reached even at 53° C. Thus, with in vitro combinations of PEP-C, MDH, and malate enzyme, net malate synthesis was observed only at temperatures clearly below 20° C. Above that temperature, depletion of malate dominated. Brandon (1967) concluded from these results that functioning of CAM predominates at low night and high day temperatures. 

Furthermore, inhibitors (of respiration) such as cyanide, azide, and arsenite also inhibit malate synthesis (Brown, 1956 Kinraide and Behan, 1975). Despite the fact that respiration is known to proceed as well at 1% O2 as at 20%, these studies implicate aerobic respiration as the main oxygen-related factor. Brown (1956), however, showed that malonate reduced oxygen uptake by 50%, yet malate accumulation was largely unaffected. 

Bradbeer,J.W., Ranson,S.L., Stiller, M. Malate synthesis in Crassulacean leaves. I. The distribution of C in malate of leaves exposed in C02 in the dark. Plant Physiol. 33, 66-70 (1958)... 

Cockburn,W., McAulay,A. The pathway of malate synthesis in crassulacean acid metabolism. In Environmental and biological control of photosynthesis. Marcelle,R., Junk, Dr. W. b.v. Publ. The Hague 1975 a... 

Jacobsen,H. A handbook of succulent plants. London Bradford Press 1960 Jacoby, B., Laties,G.G. Bicarbonate fixation and malate synthesis in relation to salt induced stoichiometric synthesis of organic acids. Plant Physiol. 47,525-531 (1971)... 

Stiller, M.L. The mechanism of malate synthesis in Crassulacean leaves. Ph.D. Thesis, Purdue Univ., Lafayette, Ind. 1959... 

Walker,D. A. Malate synthesis in a cell free extract from a crassulacean plant. Nature (London) 173, 593-594 (1956 b)... 

---