2-Methylisocitrate lyases from the bacterium Escherichia coli and the filamentous fungus Aspergillus nidulans: characterization and comparison of both enzymes.
In Escherichia coli and Aspergillus nidulans, propionate is oxidized to pyruvate via the methylcitrate cycle. The last step of this cycle, the cleavage of 2-methylisocitrate to succinate and pyruvate is catalysed by 2-methylisocitrate lyase. The enzymes from both organisms were assayed with chemically synthesized threo-2-methylisocitrate; the erythro-diastereomer was not active. 2-Methylisocitrate lyase from E. coli corresponds to the PrpB protein of the prp operon involved in propionate oxidation. The purified enzyme has a molecular mass of approximately 32 kDa per subunit, which is lower than those of isocitrate lyases from bacterial sources ( approximately 48 kDa). 2-Methylisocitrate lyase from A. nidulans shows an apparent molecular mass of 66 kDa per subunit, almost equal to that of isocitrate lyase of the same organism. Both 2-methylisocitrate lyases have a native homotetrameric structure as identified by size-exclusion chromatography. The enzymes show no measurable activity with isocitrate. Starting from 250 mM pyruvate, 150 mM succinate and 10 microM PrpB, the enzymatically active stereoisomer could be synthesized in 1% yield. As revealed by chiral HPLC, the product consisted of a single enantiomer. This isomer is cleaved by 2-methylisocitrate lyases from A. nidulans and E. coli. The PrpB protein reacted with stoichiometric amounts of 3-bromopyruvate whereby the activity was lost and one amino-acid residue per subunit became modified, most likely a cysteine as shown for isocitrate lyase of E. coli. PrpB exhibits 34% sequence identity with carboxyphosphoenolpyruvate phosphonomutase from Streptomyces hygroscopicus, in which the essential cysteine residue is conserved.
Brock
M
M
Laboratorium für Mikrobiologie, Fachbreich Biologie, Philipps-Universität, Marburg, Germany.
Darley
D
D
Textor
S
S
Buckel
W
W
D000595:Amino Acid Sequence; D001233:Aspergillus nidulans / Q000201:enzymology*; D019755:Carbon-Carbon Lyases / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification*; D003001:Cloning, Molecular; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000201:enzymology*; D008969:Molecular Sequence Data; D017386:Sequence Homology, Amino Acid; D013237:Stereoisomerism
D019755:Carbon-Carbon Lyases; C417277:methylisocitrate lyase