Meta-Biol

• Pathways

5-Phospho-alpha-D-ribose 1-diphosphate (PRPP) is a key intermediate in both the de novo and salvage pathways of purine and pyrimidine synthesis. PRPP and the enzymatic activity responsible for its synthesis were first described by Kornberg et al. (1955). The enzyme, phosphoribosyl pyrophosphate synthetase 1, has been purified from human erythrocytes and characterized biochemically. The purified enzyme readily forms multimers; its smallest active form appears to be a dimer and for simplicity it is annotated as a dimer here. It specifically catalyzes the transfer of pyrophosphate from ATP or dATP to D-ribose 5-phosphate, and has an absolute requirement for Mg++ and orthophosphate (Fox and Kelley 1971; Roth et al. 1974). The significance of the reaction with dATP in vivo is unclear, as the concentration of cytosolic dATP is normally much lower than that of ATP. The importance of this enzyme for purine synthesis in vivo has been established by demonstrating excess phosphoribosyl pyrophosphate synthetase activity, correlated with elevated enzyme levels or altered enzyme properties, in individuals whose rates of uric acid production are constitutively abnormally high (Becker and Kim 1987; Roessler et al. 1993).

Molecular cloning studies have revealed the existence of two additional genes that encode phosphoribosyl pyrophosphate synthetase-like proteins, one widely expressed (phosphoribosyl pyrophosphate synthetase 2) and one whose expression appears to be confined to the testis (phosphoribosyl pyrophosphate synthetase 1-like 1) (Taira et al. 1989; 1991). Neither of these proteins has been purified and characterized enzymatically, nor have variations in the abundance or sequence of either protein been associated with alterations in human nucleotide metabolism (Roessler et al. 1993; Becker et al. 1996), so their dimerization and ability to catalyze the synthesis of PRPP from D-ribose 5-phosphate are inferred here on the basis of their predicted amino acid sequence similarity to phosphoribosyl pyrophosphate synthetase 1.

Starches and sugars are major constituents of the human diet and the catabolism of monosaccharides, notably glucose, derived from them is an essential part of human energy metabolism (Dashty 2013). Glucose can be catabolized to pyruvate (glycolysis) and pyruvate synthesized from diverse sources can be metabolized to form glucose (gluconeogenesis). Glucose can be polymerized to form glycogen under conditions of glucose excess (glycogen synthesis), and glycogen can be broken down to glucose in response to stress or starvation (glycogenolysis). Other monosaccharides prominent in the diet, fructose and galactose, can be converted to glucose. The disaccharide lactose, the major carbohydrate in breast milk, is synthesized in the lactating mammary gland. The pentose phosphate pathway allows the synthesis of diverse monosaccharides from glucose including the pentose ribose-5-phosphate and the regulatory molecule xylulose-5-phosphate, as well as the generation of reducing equivalents for biosynthetic processes. Glycosaminoglycan metabolism and xylulose-5-phosphate synthesis from glucuronate are also annotated as parts of carbohydrate metabolism.

The digestion of dietary starch and sugars and the uptake of the resulting monosaccharides into the circulation from the small intestine are annotated as parts of the “Digestion and absorption” pathway.

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.