Meta-Biol

• Pathways

In humans, most catabolism of L-lysine normally proceeds via a sequence of seven reactions which feeds into the pathway for fatty acid catabolism. In the first two reactions, catalyzed by a single enzyme complex, lysine is combined with alpha-ketoglutarate to form saccharopine, which in turn is cleaved and oxidized to yield glutamate and alpha-ketoadipic semialdehyde. The latter molecule is further oxidized to alpha-ketoadipate. Alpha-ketoadipate is oxidatively decarboxylated by the alpha-ketoglutarate dehydrogenase complex (the same enzyme complex responsible for the conversion of alpha-ketoglutarate to succinyl-CoA in the citric acid cycle), yielding glutaryl-CoA. Glutaryl-CoA is converted to crotonyl-CoA, crotonyl-CoA is converted to beta-hydroxybutyryl-CoA, and beta-hydroxybutyryl-CoA is converted to acetoacetyl-CoA. The products of lysine catabolism are thus exclusively ketogenic; i.e., under starvation conditions they can be used for the synthesis of ketone bodies, beta-hydroxybutyrate and acetoacetate, but not for the net synthesis of glucose (Cox 2001; Goodman and Freeman 2001).

Cellular metabolism of amino acids and related molecules includes the pathways for the catabolism of amino acids, the biosynthesis of the nonessential amino acids (alanine, arginine, aspartate, asparagine, cysteine, glutamate, glutamine, glycine, proline, and serine) and selenocysteine, the synthesis of urea, and the metabolism of carnitine, creatine, choline, polyamides, melanin, and amine-derived hormones. The metabolism of amino acids provides a balanced supply of amino acids for protein synthesis. In the fasting state, the catabolism of amino acids derived from breakdown of skeletal muscle protein and other sources is coupled to the processes of gluconeogenesis and ketogenesis to meet the body’s energy needs in the absence of dietary energy sources. These metabolic processes also provide the nitrogen atoms for the biosynthesis of nucleotides and heme, annotated as separate metabolic processes (Felig 1975; Häussinger 1990; Owen et al. 1979).

Transport of these molecuels across lipid bilayer membranes is annotated separately as part of the module on "transmembrane transport of small molecules".

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.