A Direct Cross-talk Between Malic Enzyme And The Pentose Phosphate Pathway Via Structural Interactions

PPP is a major glucose catabolic pathway that directs glucose metabolic flux into macromolecular biosynthesis by providing precursors such as ribose 5-phosphate and NADPH.Ribose 5-phosphate is a key precursor for nucleotides biosynthesis,whereas NADPH acts as an electron donor for reductive biosynthesis including nucleotides and lipid biosynthesis.Besides,NADPH is an essential component of antioxidant system.In PPP,NADPH is generated by two reactions catalyzed by G6 PD and 6PGD separately.Specifically,PPP(G6PD and 6PGD reactions)is the only source of reductant to reduce glutathione and prevent damage from reactive oxygen species in red blood cells.Despite the key role in biosynthesis and redox control,PPP is significantly reprogrammed in many types of proliferating cells.Recent studies have revealed that the oxidative pentose phosphate pathway(PPP),malic enzyme(ME),and folate metabolism are the three major routes for generating cellular NADPH,a key cofactor involved in redox control and reductive biosynthesis.Many tumor cells exhibit altered NADPH metabolism to fuel their rapid proliferation.However,little is known about how NADPH metabolism is coordinated in tumor cells.Here we report that ME1 increases the PPP flux by forming physiological complexes with 6-phosphogluconate dehydrogenase(6PGD).We found that ME1 and 6PGD form a hetero-oligomer that increases the capability of 6PGD to bind its substrate 6-phosphogluconate.However,the enzymatic activity of ME1 itself could not be influenced,suggesting the specificity that ME1 promotes 6PGD enzymatic activity.Through activating 6PGD,ME1 enhances NADPH generation,PPP flux,and tumor cell growth.Interestingly,although ME1 could bind either the dimer-defect mutant 6PGD(K294R)or the NADP-binding defect 6PGD mutants,only 6PGD(K294R)activity was induced by ME1.Thus,ME1/6PGD hetero-complexes may mimic the active oligomer form of 6PGD.Together,these findings uncover a direct cross-talk mechanism between ME1 and PPP,may reveal an alternative model for signaling transduction via protein conformational simulation,and pave the way for better understanding how metabolic pathways are coordinated in cancer.