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Enzymatic and nonenzymatic mechanisms for ferric leghemoglobin reduction in legume root nodules

AuthorsBecana Ausejo, Manuel ; Klucas, R. V.
KeywordsPhysiological reductants
Nitrogen fixation
Ferric leghemoglobin reductase
Issue Date1990
PublisherNational Academy of Sciences (U.S.)
CitationProceedings of the National Academy of Sciences of the United States of America 87 (18): 7295-7299 (1990)
AbstractEvidence is presented for the operation in nodules of at least four systems for restoring functional ferrous leghemoglobin (Lb2+) from its inactive, ferric form. (i) Reduction of ferric leghemoglobin (Lb3+) by a reductase. The enzyme is a flavoprotein of 100 kDa with two equally sized subunits and exhibits aK(m) of 9 ¿M for soybean Lb3+ component a and aK(m) of 51 ¿M for NADH. NADPH is only 30% (initial velocities) as effective as NADH. Lb3+ reductase converts 215 nmol of Lb3+ to Lb2+·CO (or Lb2+·O2) per mg of protein per min and does not require an exogenous electron carrier. The enzyme shows similar affinity for soybean, bean, and cowpea Lb3+, but different V(max) values. The reductase is inactive when Lb3+ is bound to nicotinate or NO2-. (ii) Direct reduction of Lb3+ by NAD(P)H, ascorbate, and cysteine. Reduction by NAD(P)H is greatly stimulated by trace amounts of metals such as Mn2+. (iii) Reduction of Lb3+ by the flow of electrons from NAD(P)H to free flavins to Lb3+. The reaction does not occur via O2-/· or H2O2, and thus NAD(P)H-reduced flavins can directly reduce Lb3+. The efficiency of the reaction follows the order riboflavin > FMN > FAD. (iv) Reduction of Lb3+ by an unknown compound, B, of nodules. B has a molecular mass < 1 kDa and is heat-stable. The reaction mediated by B differs from those mediated by flavins and metals in several ways, requires NAD(P)H, and generates O2-/·.
Publisher version (URL)http://www.pnas.org/content/87/18/7295
Identifiersdoi: 10.1073/pnas.87.18.7295
issn: 0027-8424
Appears in Collections:(EEAD) Artículos
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