Hemoglobin biosynthesis in animals
Hemoglobin (Hb) is synthesized in a complex series of steps. The heme part is synthesized in a series of steps in the mitochondria and the cytosol of immature red blood cells, while the globin protein parts are synthesized by ribosomes in the cytosol.
Production of Hb continues in the cell throughout its early development from the proerythroblast to the reticulocyte in the bone marrow. At this point, the nucleus is lost in mammalian red blood cells, but not in birds and many other species. Even after the loss of the nucleus in mammals, residual ribosomal RNA allows further synthesis of Hb until the reticulocyte loses its RNA soon after entering the vasculature.
Heme synthesis
Heme is synthesized in a complex series of steps involving enzymes in the mitochondrion and in the cytosol of the cell. The first step in heme synthesis takes place in the mitochondrion, with the condensation of succinyl CoA and glycine to form delta-amino levulinate. This molecule is transported to the cytosol where a series of reactions produce a ring structure called protoporphyrin IX.
Many enzymes concerned with heme synthesis are intra-mitochondrial, limited to erythroid precursors including reticulocytes. The ALA synthetase is the rate-limiting enzyme of the Hb synthesis, present within the mitochondria.
Globin synthesis
After heme is synthesized within the mitochondria, 4 heme molecules combine with 4 globin polypeptides to form one molecule of haemoglobin. The globin molecule of haemoglobin differs among the species, whereas there is no difference in the heme portion.
FACTORS INFLUENCING HEMOGLOBIN SYNTHESIS
The amount of Hb in the blood is influenced by age, sex, muscular activity, season, excitement etc. Erythropoietin stimulates RNA and DNA synthesis, the cell division, heme synthesis and hemoglobin production.
At an oxygen pressure (PO2) of 100 mm of Hg in the lung, the Hb forms loose and reversible combination with oxygen, the oxyhemoglobin, but at low oxygen pressure of 40 mm of Hg at tissue level, it readily releases oxygen to the tissues for complex metabolic process.
ABSORPTION SPECTRA
When white light is passed through a solution of hemoglobin or one of its derivatives, certain wavelengths are absorbed. The resulting spectrum is termed as absorption spectra; the region of absorption is known as absorption bands.
They can be seen by examining the solution with a spectroscope.
When white light is examined spectroscopically, a series of colours known as spectrum (VIBGYOR) is obtained.
When sun light is examined, certain black vertical lines called as Fraunhofer’s lines are found at definite places in the spectrum; these lines are designated as A, B, C, D, E, etc. In lamp light, these lines are not seen.
When hemoglobin solution or its derivatives are examined in certain concentrations spectroscopically, absorption bands of definite size, appearance, and position are noticed.
Hence, spectroscopic examination helps to identify these pigments in solution. E.g. Dilute oxy-Hb solution shows two absorption bands between line D and E; adding a reducing agent (produces reduced-Hb) gives one band at line D.
Carboxy-Hb shows two bands but adding a reducing agent does not produce a single band. Met-Hb shows a band between line C and D.