Plant Hemoglobins. In: Mangum CP, editor. Blood and Tissue Oxygen Carriers. Berlin: Springer Berlin Heidelberg; 1992. pp. 23-58. [DOI: 10.1007/978-3-642-76418-9_2]

Plant hemoglobins: what we know six decades after their discovery

This review describes contributions to the study of plant hemoglobins (Hbs) from a historical perspective with emphasis on non-symbiotic Hbs (nsHbs). Plant Hbs were first identified in soybean root nodules, are known as leghemoglobins (Lbs) and have been characterized in detail. It is widely accepted that a function of Lbs in nodules is to facilitate the diffusion of O(2) to bacteroids. For many years Hbs could not be identified in plants other than N(2)-fixing legumes, however in the 1980s a Hb was isolated from the nodules of the non-legume dicot plant Parasponia, a hb gene was cloned from the non-nodulating Trema, and Hbs were detected in nodules of actinorhizal plants. Gene expression analysis showed that Trema Hb transcripts exist in non-symbiotic roots. In the 1990s nsHb sequences were also identified in monocot and primitive (bryophyte) plants. In addition to Lbs and nsHbs, Hb sequences that are similar to microbial truncated (2/2) Hbs were also detected in plants. Plant nsHbs have been characterized in detail. These proteins have very high O(2)-affinities because of an extremely low O(2)-dissociation constant. Analysis of rice Hb1 showed that distal His coordinates heme Fe and stabilizes bound O(2); this means that O(2) is not released easily from oxygenated nsHbs. Non-symbiotic hb genes are expressed in specific plant tissues, and overexpress in organs of stressed plants. These observations suggest that nsHbs have functions additional to O(2)-transport, such as to modulate levels of ATP and NO.

Nonvertebrate hemoglobins: functions and molecular adaptations

Hemoglobin (Hb) occurs in all the kingdoms of living organisms. Its distribution is episodic among the nonvertebrate groups in contrast to vertebrates. Nonvertebrate Hbs range from single-chain globins found in bacteria, algae, protozoa, and plants to large, multisubunit, multidomain Hbs found in nematodes, molluscs and crustaceans, and the giant annelid and vestimentiferan Hbs comprised of globin and nonglobin subunits. Chimeric hemoglobins have been found recently in bacteria and fungi. Hb occurs intracellularly in specific tissues and in circulating red blood cells (RBCs) and freely dissolved in various body fluids. In addition to transporting and storing O(2) and facilitating its diffusion, several novel Hb functions have emerged, including control of nitric oxide (NO) levels in microorganisms, use of NO to control the level of O(2) in nematodes, binding and transport of sulfide in endosymbiont-harboring species and protection against sulfide, scavenging of O(2 )in symbiotic leguminous plants, O(2 )sensing in bacteria and archaebacteria, and dehaloperoxidase activity useful in detoxification of chlorinated materials. This review focuses on the extensive variation in the functional properties of nonvertebrate Hbs, their O(2 )binding affinities, their homotropic interactions (cooperativity), and the sensitivities of these parameters to temperature and heterotropic effectors such as protons and cations. Whenever possible, it attempts to relate the ligand binding properties to the known molecular structures. The divergent and convergent evolutionary trends evident in the structures and functions of nonvertebrate Hbs appear to be adaptive in extending the inhabitable environment available to Hb-containing organisms.