Extracellular control of erythrocyte metabolism mediated by a cytoplasmic tyrosine kinase

We wish to propose a new mechanism of metabolic regulation mediated by a cytoplasmic tyrosine kinase. Briefly, as Steck et al. have shown, we propose that glyceraldehyde-3-phosphate dehydrogenase (G3PDH) associates reversibly with the N-terminus of the cytoplasmic domain of band 3. Once the enzyme is bound, it is totally inhibited; however, upon release it is restored to full activity. We demonstrate that control of enzyme binding and consequently the glycolytic flux through this control point is executed by phosphorylation of Tyr 8 and Tyr 21 within the glycolytic enzyme binding site on band 3. This phosphorylation results in obstruction of enzyme binding, leading to G3PDH activation. Although not essential to the hypothesis, molecular modeling studies reveal that G3PDH interacts with band 3 like a "donut on a string" in a manner that is sterically prohibited by phosphorylation of band 3. The tyrosine kinase involved in band 3 phosphorylation is further demonstrated to be regulated by receptors located in the plasma membrane of the erythrocyte. Any agent which activates the tyrosine kinase is shown to coordinately activate red cell glycolysis. Conversely, any pharmaceutical which blocks tyrosine phosphorylation of band 3 also blocks stimulation of glucose metabolism. The change in profile of glycolytic intermediates resulting from stimulation of the kinase reveals a cross-over at the G3PDH reaction, confirming G3PDH as the site of this regulation. Thus, while steady state red cell metabolism may be regulated by conventional feedback inhibition, external modulation of the glycolytic flux is likely controlled by tyrosine kinase regulation of the inhibitory association of G3PDH with band 3.