D-Fructose 1,6-diphosphatase

D-Fructose 1,6-diphosphatase (FDPase) (EC 3.1.3.11) catalyzes the irreversible reaction ... 

D-Fructose 1,6-diphosphatase utilizes both D-fructose 1,6-bisphos-phate and sedoheptulose 1,7-bisphosphate as substrate, but the affinity for D-fructose 1,6-bisphosphate is the greater. However, D-fructose 1,6-diphosphatase isolated from Candida utilis380 or spinach chloroplast379 is specific for D-fructose 1,6-bisphosphate, and does not attack sedoheptulose 1,7-bisphosphate.379 380 The enzyme from C. utilis has an alkaline pH optimum (9.0-9.5), and a requirement for Mg2+ or Mn2+. At pH 7.5-8.0, the enzyme is inactive, unless EDTA is added.380... 

D-Fructose 1,6-diphosphatases from rabbit liver and muscle are similar in their cation-requirement profile, molecular weight, substrate affinity, and substrate inhibition, but have different amino acid compositions, and the muscle enzyme does not cross-react with antibody to the purified liver-enzyme.361,364 The muscle enzyme is more sensitive to AMP than the enzyme from liver or kidney.385... 

Both by proteolytic enzymes and sulfhydryl reagents, D-fructose 1,6-diphosphatase can be modified to act as an alkaline enzyme with optimal activity391,392 at pH 9 or as a neutral enzyme with pH optimum at or near neutrality.393 Upon conversion of native (neutral) enzyme into the altered (alkaline) enzyme, at least three general, enzymic properties are significantly changed, including (I) a shift in the pH... 

D-Fructose 1,6-diphosphatase is a regulatory enzyme playing a key role in the control of gluconeogenesis. A number of mechanisms have been proposed for the regulation of D-fructose 1,6-diphosphatase activity, including allosteric control by AMP,396 inhibition by D-fructose 1,6-bisphosphate, ADP, or ATP, and modification of the sulfhydryl groups of the enzyme.397,398... 

The specific inhibition of D-fructose 1,6-diphosphatase by AMP decreases if the pH of the solution moves399 to above 9. Inhibition by AMP and catalytic activity can be lost by acetylation of the tyrosine residues with 1-acetylimidazole. The presence of substrate or allosteric effectors protects the tyrosine from acetylation.400 Pyridoxal phosphate can also desensitize the enzyme by forming a Schiff base with L-lysinyl residues401 this indicates some participation of L-lysinyl residues in allosteric regulation.401,402... 

Taketa and coworkers398 reported that ATP and ADP inhibit rabbit-liver D-fructose 1,6-diphosphatase, and that the inhibition results from a conversion of the enzyme into a conformer having low activity. Skeletal-muscle enzyme is inhibited by ADP. 

Regulation of liver D-fructose 1,6-diphosphatase could involve modification of the sulfhydryl group of the enzyme.397 Cystamine (2,2 -dithiobisethylamine) or homocysteine undergoes a disulfide exchange-reaction with two reactive cysteine residues. This exchange leads to a four-fold increase in catalytic activity.405 Other sulfhydryl reagents, such as l-fluoro-2,4-dinitrobenzene (FDNB), p-mercuri-benzoate (PMB), and 2-iodoacetamide, also activate liver D-fructose... 

Fructose 1,6-diphosphatase hydrolyzes D-fructose 1,6-diphosphate to give D-fructose 6-phosphate and PO . It is a key enzyme in the gluconeo-genesis pathway. Two divalent metal ions (Mg2+, Mn2+, Zn2+, and Co2+) are involved in catalysis. In the enzyme isolated from pork kidney the metal-metal distance accounts to 3.7 A [12]. A reaction mechanism similar to that of protein phosphatase 1 was proposed, but leaving group stabilization by metal coordination of the ester oxygen atom appears to be absent (Figure 6) [12]. 

Second, GSH functions, presumably nonenzymically, in the reduction of protein thiols which have become oxidized to mixed disulfides (803). In this latter function GSH in some cases converts inactive enzymes to active ones, or vice versa, and may thus serve as a means of metabolic control. Examples of this important possibility are glycogen synthetase D (EC 2.4.1.11) and fructose-1,6-diphosphatase (EC 3.1.3.11). The D form of glycogen synthetase is dependent for activity upon the presence of glucose 6-phosphate. The enzyme is inactivated by GSSG and reactivated by GSH (204). Mixed disulfide formation between thiols of the enzyme and GSSG leads to a decrease in affinity of the enzyme for its activator (205). 

The binding of AMP, D-fructose 1,6-bisphosphate, and ATP to the kidney 1,6-diphosphatase markedly influences the binding of each of the others. ATP and AMP both increase the affinity of the enzyme for D-fructose 1,6-bisphosphate. In contrast, a decrease in the affinity of the enzyme for ATP is observed when AMP or D-fructose 1,6-bisphosphate is present. ATP decreases the affinity of the enzyme for AMP, and D-fructose 1,6-bisphosphate increases the affinity of the enzyme for AMP. These cooperative interactions between the AMP, ATP, and D-fructose 1,6-bisphosphate binding-sites suggest that the structural alterations that occur in the enzyme upon binding of any one compound leads to changes in the affinity of the enzyme for other compounds. 403,404... 

Fig. 15.2. Pathways of fructose metabolism. 15.4, Fructose-1-phosphate aldolase 15.5, fructose-1,6-diphosphatase 15.7, D-glycerate kinase. F-l-P, Fructose-1-phosphate F-6-P, fructose-6-phosphate F-l,6-DiP, fructose-1,6-diphosphate Glc-6-F, glucose-6-phosphate ATP, adenosine triphosphate ADP, adenosine diphosphate...

Further reading Froesch, E.R. (1978). Essential fructosuria, hereditary fructose intolerance and fructose-1,6-diphosphatase deficiency. In Stanbury, J.B., Wyngaarden, J.B. and Fredrickson, D.S. (eds.) The Metabolic Basis of Inherited Disease. 4th Ed., p. 121. (New York McGraw-Hill)... 

Rallison, M.L., Meikle, A.W. and Zigrong, W.D. (1979), Hypoglycaemia and lactic aciduria asssociated with fructose-1,6-diphosphatase deficiency. J. Pediatr., 94,933. 

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