FIG. 1.

Proposed bicyclic pathway for autotrophic CO₂ fixation in C. aurantiacus. (Left) 3-Hydroxypropionate cycle for CO₂ fixation affording glyoxylate as first net CO₂ fixation product. (Right) Proposed glyoxylate assimilation cycle. 1, acetyl-CoA carboxylase; 2, malonyl-CoA reductase; 3, propionyl-CoA synthase; 4, propionyl-CoA carboxylase; 5, epimerase; 6, methylmalonyl-CoA mutase; 7, citrate cycle enzymes (succinate dehydrogenase, fumarate hydratase); 8, succinyl-CoA:l-malate CoA transferase; 9, l-malyl-CoA lyase; 10, erythro-β-methylmalyl-CoA lyase; 11, postulated β-methylmalyl-CoA dehydratase; 12, postulated mesaconyl-CoA hydratase; 13, postulated citramalate activation by succinyl-CoA:l-citramalate CoA transferase; 14, postulated citramalyl-CoA lyase; 15, phosphoenolpyruvate carboxylase; 16, gluconeogenesis enzymes. erythro-β-methylmalyl-CoA, mesaconyl-CoA, and citramalate were identified as putative intermediates of the proposed glyoxylate assimilation cycle (18).