The influence of hydrogen-ion concentration upon the equilibrium state in phosphorylase systems

Abstract

Observations have been reported in earlier communications [Hanes, 1940, 1, 2, 3] on the reversible transformation of glucose-l-phosphate into starch and inorganic orthophosphate, catalysed by an enzyme (phosphorylase) which is present in many higher plants. The reaction may be represented as follows:Starch + inorganic phosphate = glucose-I-phosphate.(free-P) (ester-P) Experiments with purified phosphorylase from potatoes showed that the equilibrium state, defined by the values of the ratio free-P/(free-P + ester-P), or by the ratio free-P/ ester-P, was not significantly affected by alterations within wide limits in the gross concentration of starch or in the concentration of total phosphate (free-P + ester-P). It was observed, however, that the equilibrium was markedly affected by alterations in the concentration of hydrogen ions, larger proportions of the reactants existing in the form of starch and inorganic phosphate under more acid conditions. Thus, as the pH value was varied from 5.0 to 7 0, the values of the ratio free-P/ester-P decreased progressively from about 10-8 to 3-1 [Hanes, 1940, 1, 2].It is ofinterest that a similar effect has been reported recentlybyCori & Cori [1940] in connexion with the action of the corresponding enzyme (glycogen-phosphorylase) from animal tissues (brain, liver, and muscle). The average values which they give for the equilibrium ratio free-P/ester-P are 5-7 at pH 60 and 2-9 at pH 7*55. Also they show that this ratio is unaffected by wide variations in the concentration of polysaccharide (glycogen) present.The purpose of the present note is to compare the effects of H+ concentration upon the equilibria attained in these reversible reactions catalysed by the plant and animal phosphorylases and at the same time to extend and. modify the interpretation of this effect which has already been advanced. It was concluded earlier [Hanes, 1940, 3] that alterations in the hydrogen-ion concentration affected the equilibrium owing to the differential effects upon the dissociation of orthophosphoric acid and glucose-l-phosphoric acid. Whereas the values of the ratio free-P/ester-P, i.e. total orthophosphate/total glucose-i-phosphate, decreased from about 10-8 to 3-1 over the range of pH from 5 0 to 7-0, calculations showed that the ratio of the divalent ions of the two acids remained approximately constant. This ratio [HOPO%]/[ROPO=] (R representing the glucose molecule substituted at carbon atom 1) had a value of about 2*2. It was thus clear that an equilibrium, independent of H+, existed between similarly charged ions of the opposing reactants. This preliminary analysis was, however, incomplete, and misleading to the extent that emphasis was placed. on the divalent ions to the exclusion of the other species of the two reactants. It should have been made clear that a constant ratio for the divalent ions implies a constant ratio also for the undissociated acids and for the monovalent ions. For, given the two acids HOPO3H2 (with dissociation constants k1 = 1i07 x 10-2 and k2= 1i51 x 1...

Post-mortem glycolysis in skeletal muscle. The extent of glycolysis in diluted preparation of mammalian muscle

“…The ultimate pH of the ox-muscle mince containing the smallest amount of residual glycogen was 5-78 and of that containing the largest amount 5-72. The mean ultimate pH (5-68) of minced ox sternomandibularis (22 muscles) was appreciably lower than that reported by Howard & Lawrie (1957) for ox sternocephalicus stored for 12-14 weeks at -10°(5-93). The mean ultimate pH (5.46) of sheep longissimus dorsi (ten preparations) was almost identical with that reported by Howard & Lawrie (1957) for ox longissimus dorsi (5-48), and was lower than that of ox sternomandibularis muscles (P < 0.001).…”

“…Where these ranges overlap the two systems are closely similar in their pH relationships. Hanes & Maskell (1942) deduced, however, that the two systems were probably not identical in all respects.…”

“…However, in some muscles the ultimate pH is appreciably higher than this (Lawrie, 1955;Howard & Lawrie, 1957; Lawrie, Manners & Wright, 1959). Excessively low values (below 5.1) have also been noted by Lawrie, Gatherum & Hale (1958) in pig muscle.…”

The Effect of Temperature on the Carbohydrate Metabolism of Potato Tubers

“…However, this hypothesis is consistent with the absence of detectable ADPG-pyrophosphorylase in potato tuber (Espada 1962), and synthesis of starch from ADPG or UDPG produced by direct reaction of sucrose with ADP or UDP appears a more probable pathway (equations 1 and 2). Since the concentration of orthophosphate is 2000 times the probable concentration of GIP, the synthesis of starch by amylose phosphorylase is unlikely (Hanes and Maskell 1942). Pressey (1969) assigns a possible role to sucrose synthetase (equation I) in starch synthesis in immature potato tubers.…”

Studies on the Chemistry of the Living Bark of the Black Locust in Relation to its Frost Hardiness. VIII. Possible Enzymatic Processes Involved in Starch-Sucrose Interconversions

“…The possibility that phosphorylase and glucose-i-phosphate may be involved in the production of starch can be tested by determining whether or not the ratio of inorganic phosphate to glucose-i-phosphate in the tissue at the time of starch synthesis is favorable to such synthesis by phosphorylase. The equilibrium point of the reaction catalyzed by the locust tree phosphorylase (reaction A) is, as in the case of other phosphorylases, independent of the concentration of polysaccharide (13,14,15 The two compounds may not diffuse with equal ease between the extracellular and intracellular fluids, or the protoplasm may have a mechanism for maintaining local concentrations of glucose-i-phosphate. Phosphorylase has been detected in many plant tissues (29,30) by infiltrating the starch-free tissues with solutions of glucose-i-phosphate and subsequently testing for starch.…”