Brain hexokinase has no preexisting allosteric site for glucose 6-phosphate

Difference spectroscopic investigations on the interaction of brain hexokinase with glucose and glucose 6-phosphate (Glc-6-P) show that the binary complexes E-glucose and E-Glc-6-P give very similar UV difference spectra. However, the spectrum of the ternary E-glucose-Glc-6-P complex differs markedly from the spectra of the binary complexes, but resembles that produced by the E-glucose-Pi complex. Direct binding studies of the interaction of Glc-6-P with brain hexokinase detect only a single high-affinity binding site for Glc-6-P (KD = 2.8 μ M). In the ternary E-glucose-Glc-6-P complex, Glc-6-P has a much higher affinity for the enzyme (KD = 0.9 μ M) and a single binding site. Ribose 5-phosphate displaces Glc-6-P from E-glucose-Glc-6-P only, but not from E-Glc-6-P complex. It also fails to displace glucose from E-glucose and E-glucose-Glc-6-P complexes. Scatchard plots of the binding of glucose to brain hexokinase reveal only a single binding site but show distinct evidence of positive cooperativity, which is abolished by Glc-6-P and Pi. These ligands, as well as ribose 5-phosphate, substantially increase the binding affinity of glucose for the enzyme. The spectral evidence, as well as the interactive nature of the sites binding glucose and phosphate-bearing ligands, lead us to conclude that an allosteric site for Glc-6-P of physiological relevance occurs on the enzyme only in the presence of glucose, as a common locus where Glc-6-P, Pi, and ribose 5-phosphate bind. In the absence of glucose, Glc-6-P binds to the enzyme at its active site with high affinity. We also discuss the possibility that, in the absence of glucose, Glc-6-P may still bind to the allosteric site, but with very low affinity, as has been observed in studies on the reverse hexokinase reaction

Biophysical investigations on the active site of brain hexokinase

Replacement of Mg (II), the natural activator of brain hexokinase (EC 2.7.1.1) by paramagnetic Mn (II) without affecting the physiological properties of the enzyme, has rendered brain hexokinase accessible to investigations by magnetic resonance methods. Based on such studies, a site on the enzyme, where Mn (II) binds directly with high affinity has been identified and characterized in detail. Use of β,γ-bidentate Cr (III) ATP as an exchange-inert analogue for Mn (II) ATP has shown that Mn (II) binding directly to the enzyme has no catalytic role but another Mn (II) ion binding simultaneously and independently to the enzyme through the nucleotide bridge participates in enzyme function. However, using this direct binding Mn (II) ion and a covalently bound spin label as paramagnetic probes a beginning has been made in mapping the ligand binding sites of the enzyme. Ultra-violet difference spectroscopy has revealed the presence of at least two glucose 6-phosphate locations on the enzyme one of which presumably is the high affinity regulatory site modulated by substrate glucose. Elution behaviour of the enzyme on a phosphocellulose column suggests that glucose induces a specific phosphate site on the enzyme to which the phosphate bearing regulatory ligands of the enzyme may bind

Mitral Annular Plane Systolic Excursion: An Early Marker of Mortality in Severe COVID-19 Infection

Respiratory failure is a major cause of mortality among hospitalized patients with COVID-19. Previous studies have shown that right ventricular (RV) dilation and reduced RV longitudinal strain are markers of poor outcome in this disease. COVID-19 can cause direct myocardial injury resulting in left ventricular (LV) systolic dysfunction and heart failure suggesting that assessment of LV function might also have prognostic value. Reduction of longitudinal systolic function assessed by mitral annular plane systolic excursion (MAPSE) is an early indicator of myocardial disease in various cardiac disorders. In this retrospective study, we investigated the prognostic value of MAPSE in patients admitted with respiratory failure related to COVID-19 infection