Meta-Biol

• Pathways

Gluconeogenesis converts mitochondrial pyruvate to cytosolic glucose 6 phosphate which in turn can be hydrolyzed to glucose and exported from the cell. Gluconeogenesis is confined to cells of the liver and kidney and enables glucose synthesis from molecules such as lactate and alanine and other amino acids when exogenous glucose is not available (reviewed, e.g., by Chourpiliadis & Mohiuddin 2022). Gluconeogenesis occurs in two parts: a network of reactions converts mitochondrial pyruvate to cytosolic phosphoenolpyruvate; then phosphoenolpyruvate is converted to glucose 6 phosphate in a single sequence of cytosolic reactions.

Three variants of the first part of the process are physiologically important. 1) A series of transport and transamination reactions convert mitochondrial oxaloacetate to cytosolic oxaloacetate which is converted to phosphoenolpyruvate by a hormonally regulated, cytosolic isoform of phosphoenolpyruvate carboxykinase. This variant allows regulated glucose synthesis from lactate. 2) Mitochondrial oxaloacetate is reduced to malate, which is exported to the cytosol and re oxidized to oxaloacetate. This variant provides reducing equivalents to the cytosol, needed for glucose synthesis from amino acids such as alanine and glutamine. 3) Constitutively expressed mitochondrial phosphoenolpyruvate carboxykinase catalyzes the conversion of mitochondrial oxaloacetate to phosphoenolpyruvate which may then be transported to the cytosol. The exact path followed by any one molecule of pyruvate through this reaction network is determined by the tissue in which the reactions are occurring, the source of the pyruvate, and the physiological stress that triggered gluconeogenesis.

In the second part of gluconeogenesis, cytosolic phosphoenolpyruvate, however derived, is converted to fructose 1,6 bisphosphate by reactions that are the reverse of steps of glycolysis. Hydrolysis of fructose 1,6 bisphosphate to fructose 6 phosphate is catalyzed by fructose 1,6 bisphosphatase, and fructose 6 phosphate is reversibly isomerized to glucose 6 phosphate.

In all cases, the synthesis of glucose from two molecules of pyruvate requires the generation and consumption of two reducing equivalents as cytosolic NADH + H+. For pyruvate derived from lactate (variants 1 and 3), NADH + H+ is generated with the oxidation of lactate to pyruvate in the cytosol (a reaction of pyruvate metabolism not shown in the diagram). For pyruvate derived from amino acids (variant 2), mitochondrial NADH + H+ generated by glutamate dehydrogenase (a reaction of amino acid metabolism, not shown) is used to reduce oxaloacetate to malate, which is transported to the cytosol and re oxidized, generating cytosolic NADH + H+. The synthesis of glucose from pyruvate also requires the consumption of six high energy phosphates, four from ATP and two from GTP.

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.