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Facilitated Oxygen Diffusion

THE ROLE OF LEGHEMOGLOBIN IN NITROGEN FIXATION BY BACTEROIDS ISOLATED FROM SOYBEAN ROOT NODULES

From the ¹ From the Department of Physiology, Albert Einstein College of Medicine, New York New York, 10461
² From the Division of Plant Industry, Commonwealth Scientific and Industrial Research Organization, Canberra, A.C.T. 2601, Australia

Oxyleghemoglobin, added to well stirred suspensions of bacteroids isolated from soybean root nodules, enhances the rates of oxygen consumption and of reduction of acetylene to ethylene, a measure of the activity of the enzyme complex, nitrogenase. The rate of incorporation of labeled nitrogen from ¹⁵N₂ into NH₃, another measure of nitrogenase activity, is also augmented. Ferric leghemoglobin, as well as ferric horseradish peroxidase and ferric cytochrome c peroxidase, were ineffective. Superoxide dismutase and catalase did not significantly affect the rates of oxygen uptake or acetylene reduction. Each of 12 oxygen-binding proteins (including two non-heme proteins) tested augmented both oxygen up-take and acetylene reduction by bacteroid suspensions; the magnitude of the increments was different for different proteins. We deduce that oxyleghemoglobin and other oxygenbinding proteins exert their effects by facilitating the diffusion of oxygen through a thin layer of solution, the "unstirred layer," surrounding the bacteroids.

In the absence of oxyleghemoglobin, suspensions of bacteroids maintain a substantial oxygen uptake, which is largely ineffective in supporting nitrogenase activity. The addition of oxyleghemoglobin brings about an increment of oxygen consumption (the "effective oxygen uptake") and promotes nitrogenase activity, which increases roughly 10-fold. The ratio of the increment in oxygen consumption to the increment of acetylene reduction is about 2, corresponding to a calculated P:O ratio for bacteroid oxidative phosphorylation of 1:3.

A convenient explanation of our results is that in the absence of leghemoglobin, the ineffective oxygen uptake maintains a very low oxygen pressure at the bacteroid surface. In the presence of leghemoglobin we suggest that the operation of leghemoglobin-facilitated oxygen diffusion increases the po₂ at the bacteroid surface. This results in the "effective" oxygen uptake which supports nitrogenase activity. The possible mode of operation of this control system is discussed.

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