Meta-Biol

• Pathways

The two stereoisomers of 2-hydroxyglutarate are normally converted to 2-oxoglutarate in the mitochondrial matrix, and can then be metabolized by the citric acid cycle. The physiological sources of 2-hydroxyglutarate have not been established although plausible hypotheses are that it is generated by lysine breakdown or as a byproduct of delta-aminolevulinate metabolism. The stereoisomers are oxidized to 2-oxoglutarate in FAD-dependent reactions catalyzed by the enzymes D2HGDH (specific for R(-)-2-hydroxyglutarate) and L2HGDH (specific for S(-)-2-hydroxyglutarate). An inherited deficiency in either enzyme is associated with accumulation of 2-hydroxyglutarate and variable neurological symptoms. R(-)-2-hydroxyglutarate also reacts reversibly with succinate semialdehyde to form 4-hydroxybutyrate and 2-oxoglutarate, catalyzed by ADHFE1. No deficiencies of this enzyme have been found in patients with elevated 2-hydroxyglutarate levels (Struys 2006).

The metabolism of pyruvate provides one source of acetyl-CoA which enters the citric acid (TCA, tricarboxylic acid) cycle to generate energy and the reducing equivalent NADH. These reducing equivalents are re-oxidized back to NAD+ in the electron transport chain (ETC), coupling this process with the export of protons across the inner mitochondrial membrane. The chemiosmotic gradient created is used to drive ATP synthesis.

Metabolic processes in human cells generate energy through the oxidation of molecules consumed in the diet and mediate the synthesis of diverse essential molecules not taken in the diet as well as the inactivation and elimination of toxic ones generated endogenously or present in the extracellular environment. The processes of energy metabolism can be classified into two groups according to whether they involve carbohydrate-derived or lipid-derived molecules, and within each group it is useful to distinguish processes that mediate the breakdown and oxidation of these molecules to yield energy from ones that mediate their synthesis and storage as internal energy reserves. Synthetic reactions are conveniently grouped by the chemical nature of the end products, such as nucleotides, amino acids and related molecules, and porphyrins. Detoxification reactions (biological oxidations) are likewise conveniently classified by the chemical nature of the toxin.

At the same time, all of these processes are tightly integrated. Intermediates in reactions of energy generation are starting materials for biosyntheses of amino acids and other compounds, broad-specificity oxidoreductase enzymes can be involved in both detoxification reactions and biosyntheses, and hormone-mediated signaling processes function to coordinate the operation of energy-generating and energy-storing reactions and to couple these to other biosynthetic processes.