FIG. 5.

Pyruvate-based redox shuttles transfer matrix NADH equivalents to elevate cytosolic NADPH. (A) The pyruvate/malate redox shuttle (green arrows) and pyruvate/citrate shuttle (violet arrows). The pyruvate/malate redox shuttle bypasses PDH and the concomitant entry of the resulting acetyl-CoA into the Krebs cycle via the CS. This bypass exists due to the PC reaction producing OAA. Conditions upon glucose intake into pancreatic β-cells allow the reversed reaction of the matrix MDH2 that produces malate from oxaloacetate at the expense of NADH, which is converted to NAD⁺. That is why redox equivalents of NADH are transferred into cytosolic NADPH. The transfer is achieved by malate export via the “2-OGC” (SLC25A11) (183), where it is exchanged for 2OG, which is then imported to the matrix. The exported malate increases the cytosolic malate pool, which can be consumed by the ME1 reaction, driven by NADP⁺ and thus increasing the cytosolic NAPDH pool (82, 195). This reaction direction is driven by an instant return of pyruvate to the mitochondrial matrix ensured by the pyruvate carrier (MPC1 and MPC2, providing pyruvate-H⁺ symport). In this way, the cycle is achieved. The pyruvate/citrate shuttle is enabled by the citrate export from the matrix after the CS reaction (54). This truncated Krebs cycle has been confirmed using ¹³C-tracing, demonstrating that high amounts of the cytosolic citrate originate from glucose-derived acetyl-CoA (146). Citrate is exported by the citrate carrier (“Cit C”; SLC25A1), enabling citrate antiport with malate. Together with the pyruvate/malate redox shuttle, malate cycling occurs. The exported citrate is split in the cytosol by the ACL with CoA, yielding oxaloacetate and acetyl-CoA. The cytosolic isoform of MDH1 then converts oxaloacetate into malate, which is again used by ME1 to produce NADPH and pyruvate, which is finally imported back to the matrix. The ACL reaction and hence shuttle operation is minor compared with the acetoacetate pathway operating upon GSIS (52). Under low glucose conditions, levels of short-chain acyl-CoA are preserved. (B) The pyruvate/isocitrate shuttle (orange arrows) exists when the reductive carboxylation reaction of matrix IDH2 takes place. Unlike IDH3, which is the regular Krebs cycle enzyme providing NADH, IDH2 in a “forward” oxidative decarboxylation mode uses NADP⁺ plus citrate and produces NADPH and 2OG in the matrix. At high [glucose], such conditions are established instead to facilitate the reverse IDH2 reaction, which is the NADPH-driven reductive carboxylation of 2OG in the presence of CO₂ (193). This is also facilitated by the Krebs cycle truncation, leaving a slow aconitase reaction and isocitrate formation so that the reverse IDH2 reaction occurs. The citrate carrier finally exports isocitrate to the cytosol, exchanging it for imported malate. The enhanced isocitrate pool in the β-cell cytosol is concomitantly consumed by the cytosolic IDH1, ensuring the NADP⁺-driven oxidative decarboxylation of isocitrate to 2OG, yielding NADPH (82). As a result, IDH1 within this shuttle contributes to another portion of the cytosolic NADPH increase upon GSIS (209). 2OG, as with the pyruvate/malate shuttle, is imported to the matrix, being exchanged for malate again by the oxoglutarate carrier. 2OG thus contributes to the mitochondrial matrix 2OG pool, consumed massively by the 2OGDH complex within the Krebs cycle. However, a portion of the matrix 2OG pool is used for another cycle of this shuttle, that is, for IDH2-mediated reductive carboxylation. 2OG, 2-oxoglutarate; 2OGC, 2-oxoglutarate carrier, mitochondrial; 2OGDH, 2-oxoglutarate dehydrogenase; ACL, ATP citrate lyase; ACO, aconitase; Cit C, citrate carrier, mitochondrial; CoA, coenzyme-A; CS, citrate synthase; F6P, fructose-6-phosphate; FASN, fatty acid synthase; FH, fumarate hydratase; IDH1, isocitrate dehydrogenase 1, cytosolic NADP⁺ dependent; IDH2, isocitrate dehydrogenase 2, mitochondrial NADP⁺ dependent; IDH3, isocitrate dehydrogenase 3, cytosolic NAD⁺ dependent; MDH, malate dehydrogenase; ME1, malic enzyme 1, cytosolic; OAA, oxaloacetate; PC, pyruvate carboxylase; PDH, pyruvate dehydrogenase; PyrC, pyruvate carrier, mitochondrial.