Elucidation of the complete Azorhizobium nicotinate catabolism pathway.
A complete pathway for Azorhizobium caulinodans nicotinate catabolism has been determined from mutant phenotype analyses, isolation of metabolic intermediates, and structural studies. Nicotinate serves as a respiratory electron donor to O2 via a membrane-bound hydroxylase and a specific c-type cytochrome oxidase. The resulting oxidized product, 6-hydroxynicotinate, is next reduced to 1,4,5,6-tetrahydro-6-oxonicotinate. Hydrolytic ring breakage follows, with release of pyridine N as ammonium. Decarboxylation then releases the nicotinate C-7 carboxyl group as CO2, and the remaining C skeleton is then oxidized to yield glutarate. Transthioesterification with succinyl coenzyme A (succinyl-CoA) yields glutaryl-CoA, which is then oxidatively decarboxylated to yield crotonyl-CoA. As with general acyl beta oxidation, L-beta-hydroxybutyryl-CoA, acetoacetyl-CoA, and finally two molecules of acetyl-CoA are produced. In sum, nicotinate is catabolized to yield two CO2 molecules, two acetyl-CoA molecules, and ammonium. Nicotinate catabolism stimulates Azorhizobium N2 fixation rates in culture. Nicotinate catabolism mutants still able to liberate pyridine N as ammonium retain this capability, whereas mutants so blocked do not. From, mutant analyses and additional physiological tests, N2 fixation stimulation is indirect. In N-limited culture, nicotinate catabolism augments anabolic N pools and, as a consequence, yields N2-fixing cells with higher dinitrogenase content.
Kitts
C L
CL
Department of Biology, Sinsheimer Laboratories, University of California, Santa Cruz 95064.
Lapointe
J P
JP
Lam
V T
VT
Ludwig
R A
RA
D000214:Acyl Coenzyme A / Q000378:metabolism; D001673:Biodegradation, Environmental; D005977:Glutarates / Q000378:metabolism; D009525:Niacin / Q000378:metabolism*; D009539:Nicotinic Acids / Q000378:metabolism; D009586:Nitrogen Fixation / Q000502:physiology; D011725:Pyridines / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D012230:Rhizobiaceae / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*
D000214:Acyl Coenzyme A; D005977:Glutarates; D009539:Nicotinic Acids; D011725:Pyridines; D000644:Quaternary Ammonium Compounds; D009525:Niacin; C030372:3-hydroxybutyryl-coenzyme A; C015901:glutaryl-coenzyme A; C078389:1,4,5,6-tetrahydro-6-oxonicotinic acid; C046300:6-hydroxynicotinic acid; C010701:crotonyl-coenzyme A; C035736:glutaric acid; C023666:pyridine