The Universal Respiratory Pigment
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Histol Histopath (1989) 4: 509-51 4 Histology and Histopathology Invited Re vie W The phylogenetic odyssey of the erythrocyte. I Hemoglobin: the universal respiratory pigment Chester A. Glomski and Judith Tamburlin Department of Anatomical Sciences, State University of New York at Buffalo, New York, USA Summary. Hemoglobin is a molecular entity that is Discussion capable of reversibly binding and releasing oxygen in either extra- or intracellular milieus. It is present in The hemoglobin molecule is truly ancient and has scattered invertebrates in physical solution or in cellular been extant for millenia. It has a long evolutionary sites while in vertebrates it is universally located in history and in fact can be viewed as a molecular clock. It circulating erythrocytes. These cells serve as the vehicle has been used to date the separation of the vertebrates for and otherwise foster the optimum utilization hemo- and invertebrates some 1000 million years ago. The globin. Hernoglobin's variable sphere of respiratory emergence of a vertebrate hemoglobin molecule with activities can be viewed as reflecting the specific differing alpha and beta globin chains occurred 500-600 requirements for each organism in which it is observed. million years later. Hemoglobin has persisted since those Once these concepts have been established and the primordial events and has become established as the advantages and limitations of its cytologic packaging single, dominant, essentially universal respiratory recognized, the study of the erythrocyte as expressed in pigment of the vertebrates. By definition respiratory its dimensions, colligative aspects, geometry, internal pigments, of which hemoglobin is the prime represen- morphology and pathologic variations can be approached tative, are inherently pigmented metallo-proteins having in a purposeful manner. the unique ability of reversible binding with oxygen. Although other phylogenetically transitory respiratory Key words: Hemoglobin, Erythrocyte, Comparative molecules have appeared in the invertebrates (e.g. hematology, Respiratory pigments hemocyanin, chlorocruorin) none have approached the importance of hernoglobin. It has become increasingly apparent that hemoglobin enjoys a remarkably broad Introduction biological distribution. It is identifiable in some bacteria, occasional molds, some single celled animals (e.g. the The erythrocyte, by far the most numerous cell in the ciliated protozoans Paramecium caudatum and circulating blood, is the major carrier of oxygen in man Tetrahymena pyriformis) , scattered invertebrates, and all other vertebrates. This cell is uniquely adapted essentially all vertebrates and most surprisingly in the for this function by its high content of an iron-bearing, plant kingdom. This molecule has been identified in the oxygen binding protein, hemoglobin. An understanding nitrogen fixing nodules on the roots of some legumunous of the red cell's functional and pathologic activities in plants as soy bean plants and alfalfa as well as in the non- man (as well as in other vertebrates) can be approached nodulating plant Trema tomentosa. Indeed, recent by first assessing the phylogenetic expression and isolation of the hemoglobin gene from plant root cells has variation in activity of its respiratory pigment. This suggested that hemoglobin genes, inherited from an establishes a basis for an enhanced, continued study and ancestor common to plants and animals, might be deeper understanding of this cell. Hemoglobin is central present in all plants. All aerobic organisms are capable of to the biology of the erythrocyte. synthesizing catalysts such as the heme-containing cytochromes. Consequently they can be viewed as potential carriers of the heme moiety of hemoglobin. Offprint requests to: Dr. Chester A. Glomski, M.D., Ph.D., Department The limiting factor in the appearance of hemoglobin of Anatomical Sciences, State University of New York at Buffalo, apparently lies in the evolved ability to synthesize the Buffalo, New York 14214, U.S.A. highly specific proteins (globins) which when linked with Erythrocyte odyssey into a tetramer composed of two alpha and beta protein units. Thi\ developmental process is believed to have occurred after the evolution ol the jawless fish (e.g. lamprey ancl hagfish) which do not demonstratc heterogeneous hemoglobins composecl of pairs of differing globin chains but before the cartilaginous fish (sharks) and all other vertebrates which do manifest this progression. The evolutionary ascendancc to polymeric hemoglobins can bc appreciated when viewed in light of Alfalfa, a the benefits that are obtainable by legumumus polymeric as opposed to monomeric plant forms. The oxygenation of one heme in an oligomeric hemoglobin molecule, for example, subsequently permits an adjacent fellow heme to bind oxygen with Partial Pressure 0, (mm Hg) greater facility. This alteration in binding affinity, 1 termed heme-heme interaction Fig. 1. Typ~cal s~gmo~dhemoglob~n oxygen d~ssoc~at~oncurve or cooperativity, is due to small (human hernoglobin). The Bohr effect is also ~llustrated.Note the changes in the three dimensional progressive shift of the dissociation curve to the right with a structure of the molecule. An decrease in pH. After VanSlyke and Peters. extension of this enhancement is the observation that all heme establish the constraints of its remarkable property hemoglobin molecules in a of reversible binding and release of oxygen. solution or within an erythrocyte The simplest molecular configuration of hemoglobin become equally oxygenated consists of one heme prosthetic group (a porphyrin ring when exposed to a given volume complexed with a single atom of iron) attached to one of oxygen; e.g. if a solution of globin chain approximately 150 amino acids in length. human (tetrameric) hemoglobin The molecular weight is approximately 17,000 daltons. is 50% saturated with oxygen all Hemoglobins of this order can be found in the free of the molecules will have two coelomocytes of some annelids (multisegmented Representative polychaete oxygenated ferrous atoms. This worms), the larva of the insect Chironomous and in some annelid (multisegmented characteristic also leads to sea cucumbers. The molecular weights of the primitive marine worm) the establishment of the well (invertebrate) hemoglobins that exist free in solution recognized sigmoid hemoglobin oxygen dissociation conversely, tend to be markedly greater and frequently curve which is ideally suited for taking up and release of approach three million daltons as a result of a high oxygen under different conditions (Fig. 1). Monomeric degree of polymerization. Their large molecular dimen- hemoglobins, in contradistinction, usually have hyperbolic sions serve to prevent them from diffusing out of the dissociation curves. The heme moiety of the molecule plasma into the tissues. Most intracellular invertebrate can be considered evolutionarily stable and responsible hemoglobins are monomers or dimers of the basic for the reversible binding of oxygen while the globin is molecular model although tetramers and larger forms structurally variable and through this variation determines can be represented. The characteristic the conditions under which oxygen binding and release tetrameric molecule of vertebrates will be favored (CO,, temperature, oxygen tension, (including man) most likely arose by organic phosphate and inorganic ion levels etc.). the joining of two dimers. The Hemoglobin molecules can therefore be viewed as vertebrates, with the exception of the cctailor made,, for specific species and temporal jawless fish (Agnatha. cyclostomes), all requisites. Man himself demonstrates at least four have a more complex, tetrameric different hemoglobins hemoglobin containing two pairs of during his existence. dissimilar (alpha and beta) globin the Gower I and 'k&~??%wd* chains; it has an average molecular I1 hemoglobins of .&+;4$$:$$;ifGf;ii%a$ weight of 68,000. It has been concluded ;4(r early embryonic life. ' .&%&2"@-4.,> that the original primitive vertebrate fetal and adult Parameoum hemoglobin consisted of four heme hemoglobins. Thc caudatum, a units and four identical polypeptide bRepresentative Sea Cucumber, chicken protozoan chains; it evolved by gene duplication multiple hemoglobins a holothurian Erythrocyte odyssey during its lifetime (hemoglobins P, E, D, A and H) while interpreted as the net result of its fetal and adult hemoglobins are commonplace among two major functions, the loading mammalians. The maturation of a tadpole to a frog is and unloading of oxygen. The similarly associated with a molecular change of its effects of ambient organismnl hemoglobin. Some fish concomitantly manifest two conditions upon hemoglobin at the types of hemoglobin, one highly sensitive to acid pH and oxygen-rich side of the cyelc j one that is acid-insensitive. Certain representatives of (interface with air or water at the, the annelids and molluscs (bivalves or clams) that have gills, lungs, etc.) and the physiologic erythrocytes demonstrate the simultaneous existence of circumstances at the oxygen-pool- mono- and polymeric intracellular hemoglobins. The (tissue) side have to be both taken arcid clam Noetia ponderosa (a colloquial .blood clamn) into balanced consideration. Thu\. possesses erythrocytes with two dimeric hemoglobins fish with both acid sensitive an(l with own ou\cen-binding properties. It can be acid independent hemoglobins (e.g. ~i'ilna, iiaina gi,~;,,,~ assumed that each rainbow trout, Salmo irideus,)