Porphyrin biosynthesis in soybean callus. V. The porphobilinogen deaminase-uroporphyrinogen cosynthetase system. Kinetic studies

Elena B.C Llambías,Alcira M Del C Batlle

doi:10.1016/0005-2744(70)90285-8

Elena B.C Llambías, Alcira M Del C Batlle

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https://doi.org/10.1016/0005-2744(70)90285-8

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Journal: BBA - Enzymology	Publication Date: Dec 1, 1970
Citations: 10	License type: cc-by

Affiliation: Fundación Ciencias Exactas y Naturales

Abstract

Kinetic studies were carried out using purified porphobilinogenase and deaminase preparations in the presence and absence of ammonium ions. It has been found in plots of v versus [ S] that a deviation from the Michaelis-Menten hyperbola occurs with both enzymes; double-reciprocal plots were concave downward; R s values were greater than 81; and in some cases the Hill coefficient was less than 1, indicating negative homotropic kinetics. Evidence also suggested that porphobilinogenase contains at least two substrate-binding sites per molecule of enzyme. It has also been found that ammonium ions act competitively on the first reaction of the porphobilinogenase.

Highlights

Kinetic studies were carried out using tmrified porphobilinogenase and &'aminase pretmrat ions in the presence an
There are several examples in the literature of the kind of kinetics exhibited bysoybean callus porphot)ilinogenase and deaminasen "", a n d it has been noted b y LI-VITZKY AND I~OSHI.ANI) '~'a, t h a t the sequential model for subunit interactions '~,2a,'a is ideally suited for the conq)lex phenomena observed for these enzymes. It has been observed by the same authors'-'" that these results are understood if we consider the types of curves that one would expect from various types of cooperativity; when these criteria are applied to soybean t)(wph
No changes in the kinetic constants of porphobilinogenase fl)r the second reaction were obtained, in the presence and absence of ammonium ions, it can be seen from the saturation curves of Fig. I B that while in the absence of inhibitor, porphotfilinogenase did not appear to become saturated with porphobilinogen up to lh.thm~, t¢i(,phys..4(ta. 2z(>(197o) 552 55()