Meta-Biol

• Pathways

Extracellular PM20D1 (N-fatty-acyl-amino acid synthase/hydrolase PM20D1) catalyzes the reversible condensation of L-phenylalanine (L-phe) and oleate ((9Z)-octadecenoate) to form oleoyl-phe (N-(9Z-octadecenoyl)-L-phenylalanine) and water. In addition to the condensation of phe with oleate ((9Z)-octadecenoate) annotated here, purified human PM20D1 protein in vitro can catalyze the condensation of leucine and isoleucine with oleate and with other long-chain unsaturated fatty acids including arachidonate, with lower efficiencies. Although the reverse (hydrolysis) direction of this reaction is thermodynamically favored, expression of PM20D1 protein in mice or in cultured cells was associated with elevated levels of oleoyl-phe in serum and culture media, respectively. Treatment of cultured mouse brown adipose tissue adipocytes and of isolated mitochondria with oleoyl-phe induced uncoupled respiration independently of UCP1 (uncoupling protein 1). Photolabeling studies of isolated mitochondria identified the ADP/ATP symporters SLC25A4 and SLC25A5 as possible targets of oleoyl-phe. Consistent with these observations, expression of PM20D1 and elevated blood levels of oleoyl-phe in mice were associated with increased energy expenditure and improved glucose homeostasis. These results suggest a physiological role for PM20D1 and its condensation reaction product in thermogenesis and raise the possibility that oleoyl-phe and related molecules might have a clinical role in treatment of obesity (Long et al. 2016).

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.