PYRUVATE KINASE: MOLECULAR DIFFERENCES BETWEEN HUMAN RED CELL AND LEUKOCYTE ENZYME.
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At 9 mM glucose, experimental results show that mitochondrial phosphate depletion (induced by glucose phosphorylation, catalyzed by mitochondrial hexokinase) reduces the activities of the respiratory chain, oxidative phosphorylation, and glutaminase. Consequently, the 14C-lactate oxidation to 14CO2 is lowered in the presence of glucose. The fall of ATP level triggers a high aerobic glycolysis by deinhibiting fructose-6-P kinase. NADH, generated by enhanced glyceraldehyde-3-P dehydrogenase activity, increases the reducing power. Moreover, the lactate dehydrogenase (LDH) system is shifted toward lactate formation, while NAD+ is regenerated and the oligomycin-inhibited ATP production is replaced by the iodoacetate-inhibited ATP production. From 14CO2 production and lactate accumulation it is calculated that about 60% of 14C-glucose which disappears is channelled into extraglycolytic reactions. On the contrary, 82% of glucose below l mM is metabolized through non-glycolytic reactions. The pyruvate kinase-M2 (PK-M2) inhibition does not limit the glycolytic flow from 9 mM glucose, but it may cause sustained gluconeogenesis.
This communication introduces a nuclear relaxation approach for an estimation of the distance between two paramagnetic metal ion sites on a metal-activated enzyme. The method is based on the existence of an exchange of unpaired electron spin magnetizations between the two metals via energy-conserving concerted mutual spin flips which arise from time-dependent dipolar interactions of the electronic magnetizations. This cross-relaxation of electronic magnetizations depends on the inverse sixth power of the intermetal distance and may, under suitable conditions, affect the longitudinal relaxation rate of inner sphere water protons by altering the electron-proton dipolar correlation time when the latter is dominated by electron spin relaxation. The technique is applied to estimate the distance of 5.2 +/- 0.9 A between Mn2+ and Cr3+ in the pyruvate kinase-Mn2+-ATPCr3+ complex and indicates the existence of a van der Waals contact between the hydration spheres of the enzyme- and nucleotide-bound metal ions.
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