The magnesium-protoporphyrin IX (oxidative) cyclase system. Studies on the mechanism and specificity of the reaction sequence.
Mg-protoporphyrin IX monomethyl ester cyclase activity was assayed in isolated developing cucumber (Cucumis sativus L. var. Beit Alpha) chloroplasts [Chereskin, Wong & Castelfranco (1982) Plant Physiol. 70, 987-993]. The presence of both 6- and 7-methyl esterase activities was detected, which permitted the use of diester porphyrins in a substrate-specificity study. It was found that: (1) the 6-methyl acrylate derivative of Mg-protoporphyrin monomethyl ester was inactive as a substrate for cyclization; (2) only one of the two enantiomers of 6-beta-hydroxy-Mg-protoporphyrin dimethyl ester had detectable activity as a substrate for the cyclase; (3) the 2-vinyl-4-ethyl-6-beta-oxopropionate derivatives of Mg-protoporphyrin mono- or di-methyl ester were approx. 4 times more active as substrates for cyclization than the corresponding divinyl forms; (4) at the level of Mg-protoporphyrin there was no difference in cyclase activity between the 4-vinyl and 4-ethyl substrates; (5) reduction of the side chain of Mg-protoporphyrin in the 2-position from a vinyl to an ethyl resulted in a partial loss of cyclase activity. This work suggests that the original scheme for cyclization proposed by Granick [(1950) Harvey Lect. 44, 220-245] should now be modified by the omission of the 6-methyl acrylate derivative of Mg-protoporphyrin monomethyl ester and the introduction of stereo-specificity at the level of the hydroxylated intermediate.
Walker
C J
CJ
Department of Botany, University of California, Davis 95616.
Mansfield
K E
KE
Rezzano
I N
IN
Hanamoto
C M
CM
Smith
K M
KM
Castelfranco
P A
PA
D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts; D002851:Chromatography, High Pressure Liquid; D003905:Deuteroporphyrins / Q000378:metabolism; D010105:Oxygenases / Q000378:metabolism*; D010944:Plants; D011166:Porphyrins / Q000378:metabolism; D011521:Protochlorophyllide / Q000031:analogs & derivatives / Q000378:metabolism; D012436:S-Adenosylmethionine / Q000378:metabolism; D013053:Spectrophotometry; D013379:Substrate Specificity
D003905:Deuteroporphyrins; D011166:Porphyrins; D002734:Chlorophyll; D011521:Protochlorophyllide; D012436:S-Adenosylmethionine; D010105:Oxygenases; C053161:magnesium protoporphyrin monomethyl ester oxidative cyclase