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G L Turner

Publications and source records attributed to G L Turner.

30 records · Page 2Linked to original sources
PubMed1975

Involvement of oxyleghaemoglobin and cytochrome P-450 in an efficient oxidative phosphorylation pathway which supports nitrogen fixation in Rhizobium.

Cellular ATP level, ATP/ADP ratio and nitrogenase activity rise when oxyleghaemoglobin is added to respiring suspensions of Rhizobium japonicum bacteroids from soybean root nodules. Increased gaseous O2 tension is much less efficient than oxyleghaemoglobin in stimulation of bacteroid ATP production. Studies with the inhibitor carbonyl cyanide m-chlorophenylhydrazone show this ATP to be generated as a consequence of oxidative phosphorylation. N-Phenylimidazole, a specific cytochrome P-450 inhibitor, also lowers the efficiency of bacteroid oxidative phosphorylation. An approximately linear relationship is observed between ATP/ADP ratio and nitrogenase activity as N-phenylimidazole concentration is lowered. It is suggested that cytochrome P-450 is a component of the leghaemoglobin-facilitated respiration pathway and that it may act as intracellular O2 carrier rather than terminal oxidase. A less efficient oxidase appears to function when cytochrome P-450 is inhibited.

Adenosine Diphosphate↗
PubMed1975

Leghaemoglobin and the supply of O2 to nitrogen-fixing root nodule bacteroids: presence of two oxidase systems and ATP production at low free O2 concentration.

Studies of rates of consumption of dissolved O2 by suspensions of bacteroids (Rhizobium japonicum, strain CB1809) from soybean root nodules showed the presence of two different terminal oxidase systems. A high-affinity system, sensitive to inhibition by N-phenylimidazole and by carbon monoxide, was most active when the dissolved O2 was between 0-01 and 0-1 muM. At 1 muM-O2 or higher, this oxidase system had little activity and O2 was consumed largely by a low-affinity system insensitive to these inhibitors. At low concentrations of dissolved O2, bacteroid respiration rates appeared to be diffusion-limited. When purified oxyleghaemoglobin was added to such systems, this restriction was relieved and respiration was maintained to much lower concentrations of free dissolved O2, where nitrogenase activity was greatest. Analysis of reactions which were terminated at various stages during the depletion of O2 from oxyleghaemoglobin showed that at low free O2 concentration, the high-affinity pathway produced up to five times greater bacteroid ATP concentrations than the low-affinity oxidase pathway operating about 1 muM free O2 in the absence of leghaemoglobin. At intermediate free O2 concentrations, occurring during the later stages of deoxygenation of oxymyoglobin, intermediate concentrations of ATP were found in the bacteroids.

Adenosine Triphosphate↗
PubMed1973

Kinetic studies of nitrogenase from soya-bean root-nodule bacteroids.

The apparent Michaelis constants [K'(N(2)) and K'(C(2)H(2))] and the corresponding apparent maximum velocity values (V') for soya-bean bacteroid nitrogenase increased concomitantly in response to increases in nitrogenase Fe-protein concentration and ATP concentration in cell-free assays and in response to O(2) pressure in intact nodules and bacteroid suspensions. K'(C(2)H(2)) in cell-free assays was also affected by pH and by Na(2)S(2)O(4) concentration. Nitrogenase Fe-protein behaved as a catalytic effector reacting at interacting sites on the nitrogenase Fe-Mo-protein. The results indicated that the Fe-Mo-protein probably bears the catalytic sites for N(2) and C(2)H(2) reduction. It is concluded that reduction of N(2) or C(2)H(2) by this nitrogenase involves a reaction mechanism with a sequence of unknown order. The sequence in which substrate, enzyme, effector, ATP and reductant react determines which of the various rate-constants are involved in the apparent Michaelis constant, whose true kinetic meaning was thus unresolved.

Adenosine Triphosphate↗
PubMed1969

The relationship between nitrogen fixation and the production of HD from D2 by cell-free extracts of soya-bean nodule bacteroids.

1. Cell-free extracts prepared from soya-bean nodule bacteroids produced HD from D(2) in the presence of dithionite, an ATP-generating system and nitrogen. 2. Crude extracts of bacteroids or of Azotobacter vinelandii showed some background D(2) exchange when any one of these was omitted. 3. Partial purification of bacteroid extracts diminished this background activity and gave increased D(2) exchange and nitrogen fixation. 4. Although increasing pN(2) stimulated both reactions, the apparent K(m) (N(2)) for nitrogen fixation was much higher than the apparent K(m) (N(2)) for D(2) exchange when partially purified bacteroid extracts were used. 5. Carbon monoxide was a competitive inhibitor of nitrogen fixation by partially purified bacteroid extracts, but D(2) exchange was inhibited in a non-competitive fashion. 6. These results are discussed in relation to the possible existence of enzyme-bound intermediates of nitrogen fixation.

Adenosine Triphosphate↗