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Non-symbiotic hemoglobins in the life of seeds

Matilla, Angel J., Rodríguez-Gacio, María del Carmen
Phytochemistry 2013 v.87 pp. 7-15
NAD (coenzyme), adenosine triphosphate, ancestry, crystal structure, dissociation, evolution, germination, hemoglobin, imbibition, metabolism, nitric oxide, oxygen, seeds
Non-symbiotic hemoglobins (nsHbs), ancestors of symbiotic-Hbs, are hexacoordinated dimeric proteins, for which the crystal structure is well described. According to the extent of hexacoordination, nsHbs are classified as belonging to class-1 (nsHbs1) or class-2 (nsHbs2). The nsHbs1 show weak hexacoordination, moderate rates of O2-binding, very small rates of O2 dissociation, and a remarkably high affinity for O2, all suggesting a function involving O2 scavenging. In contrast, the nsHbs2 exhibit strong hexacoordination, low rates of O2-binding and moderately low O2 dissociation and affinity, suggesting a sensing role for sustained low (μM) levels of O2. The existence of spatial and specific expression of nsHbs1 suggests that nsHbs play tissue-specific rather than housekeeping functions. The permeation of O2 into seeds is usually prevented during the desiccation phase and early imbibition, generating an internal hypoxic environment that leads to ATP limitation. During evolution, the seed has acquired mechanisms to prevent or reduce this hypoxic stress. The nsHbs1/NO cycle appear to be involved in modulating the redox state in the seed and in maintaining an active metabolism. Under O2 deficit, NADH and NO are synthesized in the seed and nsHbs1 scavenges O2, which is used to transform NO into NO3− with concomitant formation of Fe3+-nsHbs1. Expression of nsHbs1 is not detectable in dry viable seeds. However, in the seeds cross-talk occurs between nsHbs1 and NO during germination. This review considers the current status of our knowledge of seed nsHbs and considers key issues of further work to better understand their role in seed physiology.