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Root nodules are complex structures that occur on roots of many agronomically important plants such as soybeans. Root nodules are a symbiosis between the plant and soil-borne bacteria. Once formed, and functional, root nodules can fix atmospheric nitrogen and provide the plant with a pollution-free source of nitrogen for seed production. A basic component of functional and efficient root nodules are the heme proteins called leghemoglobins. Leghemoglobins can bind to, and transport molecular oxygen and thereby maintain a low oxygen environment within the root nodule, permitting the microaerobic bacteria to convert atmospheric dinitrogen into ammonia, which is then assimilated by the plant for growth. Thus understanding the oxygen-binding characteristics of leghemoglobin are of major importance from both a basic and applied point of view. In this paper we have shown that specific amino acid residues are critical for allowing the orderly binding and transfer of oxygen from leghemoglobins. Although several hypotheses have been advanced to account for the biochemical properties of leghemoglobins, this work is the first that clearly demonstrates the molecular consequences of amino acids that surround the heme-pocket. Using engineered mutant proteins and spectroscopy, the role of Tyr (B10) has been investigated in detail for several vertebrate and plant hemoglobins.