Discovery of a second citric acid cycle complex

Abstract

Together, Nobel Prize honoured work, mathematics, physics and the laws of nature have drawn a concept of clockwise cycling carboxylic acids in Krebs' Citric Acid Cycle. A Citric Acid Cycle complex is defined by specific substrate, product and regulation. Recently, the Citric Acid Cycle 1.1 complex was introduced as an NAD⁺-regulated cycle with the substrate, lactic acid and the product, malic acid. Here, we introduce the concept of the Citric Acid Cycle 2.1 complex as an FAD-regulated cycle with the substrate, malic acid and the products, succinic acid or citric acid. The function of the Citric Acid Cycle 2.1 complex is to balance stress situations within the cell. We propose that the biological function of Citric Acid Cycle 2.1 in muscles is to accelerate recovery of ATP; whereas in white tissue adipocytes our testing of the theoretical concept led to the storage of energy as lipids.

Keywords: Citric acid cycle; Proton-linked monocarboxylate transporter; Pyruvate dehydrogenase complex.

Fig. 1

Krebs' Original Citric Acid Cycle. This blueprint of Krebs' Citric Acid Cycle is taken from his Nobel Prize lecture, 1953 and was first published 1937 [5,6,8]. Importantly, this degradative unidirectional cycle contains carboxylic acids. ©Nobel Foundation.

Fig. 2

Regulation of the Citric Acid Cycle 2.1. (1) The regulation of the Citric Acid Cycle 2.1 starts with the ‘metabolic switch’ comprising glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and cytosolic glycerol-3-phosphate dehydrogenase (cGPDH). The enzyme complex ensures direct transfer of water-free NAD⁺/NADH-H⁺ converting glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate and dihydroxyacetone phosphate to glycerol-3-phosphate. (2) The FAD-dependent mitochondrial form of GPDH (mGPDH) catalyses the reverse reaction, the formation of dihydroxyacetone from glycerol-3-phosphate. This process is often referred to as the glycerol-3-phosphate shuttle. (3) FAD-dependent enzymes recover activity by reducing ubiquinone (Q) to ubiquinol (QH₂). The high levels of generated QH₂ blocks (red lines) and holds succinate dehydrogenase (SDH) in the reduced (FADH₂) form, creating a ‘metabolic traffic jam’ at succinic acid in the Citric Acid Cycle 2.1. At acyl-SCoA dehydrogenase (ASCoA-DH), QH₂ prevents β-oxidation as path for replenishing the mitochondrial acetyl-SCoA pool. Reduced activity at SDH stops cycling and holds malate dehydrogenase (MDH) in its active form. (4) The direct anchoring of the Citric Acid Cycle 2.1 to the mitochondrial dicarboxylate carrier (MDC, SLC25A10) facilitates the import of malic acid from mitochondrial pool of carboxylates. (5) When cycling reaches citric acid, citric acid is pushed out of the cycle likely due to the blockage of the cycle at IDH. The citric acid is then transported to the intermembrane space by the mitochondrial tricarboxylate transport protein (mTTP, SLC25A1), replenishing the mitochondrial pool of carboxylates. (6) The cytosol which has an imbalance of the lac⁻:pyr⁻ ratio in favour of lac-continues to provide lac⁻ as source of acetyl-SCoA via the actions of heart LDH (LDH-h), monocarboxylic acid transporter 1 (MCT1) and pyruvate dehydrogenase complex (PDHc). Dashed lines indicate free diffusion and OMM = outer mitochondrial membrane. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)