Figure 3. Nitrogen metabolism in the gut.

(a) Stickland metabolism couples reduction of one amino acid (left) with oxidation of a second amino acid (right) via electron carriers indicated by [H]. Deamination of the amino acids results in production of carboxylic acid intermediates and ammonium. Stoichiometric equivalents are indicated by n. (b) Deamination of urea by ureases also produces ammonia. (c) Nitrogen and fiber metabolism vary along the length of the colon. Whereas ample dietary fiber in the proximal colon leads to high concentrations of SCFA, amino acid fermentation is more common in the distal colon. Ammonium and SCFA concentrations shown are estimated from human samples⁷⁴.