Heme Synthesis

Heme, a porphyrin, is a co-factor in haemoglobin, myoglobin, cytochrome, catalase, heme peroxidase, and endothelial nitric oxide synthase. It has a complex structure with four pyrrole rings with a ferrous iron in the centre that allows it to carry oxygen. The synthesis of heme takes place from glycine and succinyl CoA in eight steps and is extensively studied. Mutations in genes encoding for enzymes involved in heme synthesis result in porphyrias.

Steps in Heme synthesis

About 85% of the heme is synthesised in the developing erythroid cells and almost all the remaining is synthesised in the liver. The control of synthesis differs in erythroid and non-erythroid cells reflecting the exceedingly high heme requirement of the former for haemoglobin synthesis. Heme synthesis takes place in the mitochondria as well as cytosol. The first step, formation of δ-aminolevulenic acid, takes place in the mitochondrial matrix. The next few steps take place in the cytosol. The heme precursor, corpoprophyrinogen III, returns to the mitochondria, is converted to protoporphyrin IX and iron incorporated. The steps in heme synthesis are as follows

  1. Synthesis of δ-aminoleuvelinic acid: Synthesis of δ-aminoleuvelinic acid (ALA) from glycine and succnyl CoA catalysed by ALA synthase (ALAS) is the first step in the synthesis of heme. This is a rate limiting step. ALA synthase is encoded by two genes ALAS1 (OMIM 125290) and ALAS2 (OMIM 301300). ALAS2 codes for the erythroid ALAS and ALAS1 for the non-erythroid (housekeeping) ALAS. The gene ALAS1 is located on chromosome 3p21.1. The product has 12 exons and undergoes is alternate splicing to yield two distinct forms, isoform 1 (640 amino acids) and isoform 2 (657 amino acids). The erythroid specific gene (ALAS2) on X chromosome at Xp11.21. It has 12 exons and also undergo alternate splicing to yield two forms, isoform b (587 amino acids), isoform c (574 amino acids). ALAS is synthesised in the cytosol and transported to the mitochondria. It has a short half life. Heme synthesis is consoled by regulating levels and activity of ALAS (discussed below).
  2. Synthesis of prophobilinogen: ALA moves to the cytosol and is dimerised to prophobilinogen by the action of prophobilinogen synthase (ALA dehydratase). The enzyme is a homo-octomer (made of eight similar units) and needs zinc. The gene (gene ALAD, OMIM 125270) encoding the enzyme is located at 9q32. It has 15 exons. Four isoforms from alternate splicing 361 amino acid, 344 amino acid, 321 amino acid and 304 amino acid are known.
  3. Synthesis of hydroxymethylbilane: Prophobilinogen is converted to hydroxymethylbilane by the action of hydroxymethylbilane synthase. This enzyme is also known as propohbilinogen deaminase. The gene (HMBS OMIM 609806) is located at 11q23.3, has 15 exons. Four alternately spliced forms with 361, 344, 321 and 304 amino acids are known.
  4. Synthesis of uroporphyrinogen: Hydroxymethylbilane is converted to enzymatically to uroporphyrinogen III as well as non-enzymatically to uroporphyrinogen I. The enzymatic conversion is catalysed by the enzyme uroporphyrinogen III synthase. Uroporphyrinogen III synthatase is encoded by a gene (UROS, OMIM, 606938) on 10q25.2-q26 that has 16 exons and encodes for a 265 amino acid protein.
  5. Synthesis of corpoporphyrinogen III: Uroporphyrinogen III is decrboxylated to corpoporphyrinogen III by uroporphyrinogen decarboxylase. The gene (UROD, OMIM 613521) for thes enzyme is at 1p34. It has 10 exons and encodes for a protein 367 amino acid long. This is the last step in the cytosol.
  6. Synthesis of protoporphrinogern IX: Coproporphyrinogen III is converted to propoporphyrinogen IX by a reaction catalysed by corpoporphyrinogen oxidase  in the mitochondria in an oxygen dependent reaction. The gene for corpoporphyrinogen oxidase (COPX, OMIM 612732) is at 3q11.2-q12.1 8 exons. The product has 454 amino acids.
  7. Synthesis of protoporphyrin IX: Propoporphyrin is the final product of the pathway into which iron is incorporated. Protoporphyrin IX is synthesised by the action of protoporphyrinogen oxidase. The gene (PPOX, OMIM 600923) for this enzyme is located at 1q22  and has14 exon. It encodes for a 477 amino acid enzyme.
  8. Synthesis of heme: Ferrochelatse (protoporphyrin ferrochelatase) catalysed the incorporation of iron into protoporphyrin IX. The gene (FECH, OMIM 612386) for ferrochelatse is located at 18q21.31 and has 11 exons. It encodes for a 477 amino acid enzyme.

Control of heme sythesis

The rate limiting enzyme of heme synthesis is the synthesis of ALA. ALA synthase has a short half life. Heme synthesis is regulated  by controlling the levels and activity of ALA synthase.

  1. Inhibition of ALA synthase: ALA synthase is subject to feedback inhibition by heme and and it’s oxidation product hemin. ALA synthase is synthesised in the cytosol and transported to the mitochondrial matrix. In addition to being an inhibitor of ALA synthase hemin also inhibits the metochondrial transport of the enzyme.
  2. Promotion of ALA synthase activity: Cellular iron and factors promoting erythroid differentiation increase the synthesis of ALAS-2, the enzyme responsible ALA synthesis in erythroid cells. Erythroid specific factors like GATA-1 promote the transcription of the ALAS-2 gene. Untranslated portions of the ALAS-2 mRNA have iron responsive elements (IRE) that promote translation. The activity of ASLS in iron deficient cells is low.


Porphyrias are inherited diseases resulting from a mutation of genes involved in heme synthesis. With one exception, X-linked porphyria that results from a gain of function mutation of ALAS synthase 2, porphyrias result from a partial deficiency of the enzymes involved in heme synthesis. Enzyme deficiency results in accumulation of substrates for the reaction catalysed by the enzyme encoded by the gene. Symptoms of porphyrias may be intermittant and/or chronic. The symptoms are diverse and include skin changes, photosensitivity, abdominal pain, muscle weakness, CNS disturbance, seizures, hyponatremia, discolouration of urine. Enzyme deficiencies associated with porphyrias as as follows:

  1. ALA synthatase 2: Gains of function mutation in X linked protoporphyria
  2. ALA dehydratase: ALA dehydrate deficient porphyria (ADP). Lead displaces zinc from binding sites inhibiting the function of the  with enzyme. In patients with tyrosinaemia type 1 Succinylacetone (4,6-dioxoheptanoic acid) accumulates in tyrosinaemia type I. It is structurally similar to ALA and a potent inhibitor of ALA dehydratase.
  3. PBG Deaminase deficiency results in acute intermittent porphyria
  4. Uroporphyrin III synthatase deficiency results in congenital erythrocytic porphyria
  5. Uroporphyrin decarboxylase deficiency results in porphyria cutanea tarde. All patients with porphyria cutanea trade do not have a mutation. Only type II has gene mutations. Types I and III are due to mulifactorial effects on the gene.
  6. Coproporphyrin III oxidase deficiency results in hereditary coproporphyria
  7. Protoporphyrin oxidase results in varigate porphyria
  8. Ferrochalase results in erythropoietic porphyria

Further Reading

Porphyrin and Heme Metabolism
Erythroid Heme Biosynthesis and Its Disorders (doi:  10.1101/cshperspect.a011676)


The Erythrocyte Membrane

The erythrocyte membrane is subject a great deal of physical stress in circulation. It needs to withstand the high sheering stress in the arteries, it needs to squeeze past capillaries that ma be as small as 7.5µm and need to withstand the ionic changes. It has a well-developed network of proteins known as the erythrocyte cytoskeleton below the lipid bilayer of the plasma membrane to meet these needs. Inherited defects in these proteins have been associated with disorders of erythrocyte shape including hereditary spherocytosis, hereditary elliptocytosis, hereditary pyropoikilocytosis, Southeast Asian stomatocytosis and hereditary acanthocytosis (see table below).

Protein Gene Chromosome Disorders
α-Spectrin SPTA1 1q22-q23 Hereditary ElliptocytosisHereditary PyropoikilocytosisHereditary Spherocytosis
β-Spectrin SPTB 14q23-q24.1 Hereditary ElliptocytosisHereditary PyropoikilocytosisHereditary Spherocytosis
Ankyrin-1 ANK1 8p11.2 Hereditary Spherocytosis
Band 3 SLC4A1 17q21 Hereditary SpherocytosisHereditary AcanthocytosisSoutheast Asian OvalocytosisHereditary Stomatocytosis
Protein 4.1R EPB41 1p33-p34.2 Hereditary Spherocytosis
Protein 4.2 EPB42 15q15-q21 Hereditary Spherocytosis
Stomatin STOM 9q33.1 Hereditary Stomatocytocytosis
Glycophorin C GYPC 2q14-q21 Hereditary Elliptocytosis
Glycophorin D GYPD 2q14-q21 Hereditary Elliptocytosis

Organization of the Erythrocyte Membrane

The erythrocyte membrane consists of a lipid layer on protein scaffolding known as the cytoskeleton. The relationship between erythrocyte membrane proteins and lipid membrane bilayer is shown in the figure below. The main component of cytoskeleton is spectrin. Spectrin is tethered to the cell membrane by vertical interactions with band 3 proteins via ankyrin and protein 4.2. Spectrin also has horizontal interactions with protein 4.1R, actin, tropomodulin, tropomyosin and adducin. Protein 4.1R interacts with glycophorin C, a trans membrane protein.

Red Cell Membrane-600px

Erythrocyte Membrane Proteins


Spectrin has three functions

  1. Supporting the lipid layer
  2. Maintaining cell shape
  3. Regulating the lateral movement of integral membrane proteins.

It is made of two chains α and β that interwine to form dimers. Two dimers associate to form a tetramer which is the functional subunit. The α-chain is encoded by the gene SPTA1 at 1q22-q23 and the β-spectrin is encoded by the gene SPTB at 14q23-q24.1.


Ankyrins are ubiquitous adapter proteins thattarget diverse proteins to specialized membrane domains of smooth muscles and endoplasmic reticulum. The erythrocyte ankyrin, ankyrin-R is encoded by the gene ANK1 located at 8p11.

Band 3

Band 3 glycoprotein of the erythrocyte membrane that is coded by the gene is SLC4A1 at 17q21. The membrane domain transports anions across the cell membrane and the cytoplasmic domain provide binds the lipid membrane to spectrin via ankyrin.  major integal

Protein 4.2

Protein 4.2 regulates the interactiom of band 3 with ankyrin. It is encoded by the gene EPB42 at 1p33-p34.2.

Protein 4.1R

Protein 4.1R stabilized the spectrin-actin interactions. It is encoded by the EPB42 gene at 1p33-p34.2.

Disorders of Erythrocyte shape

Disruption in the cytoskeleton is the basis of in a viariey erythrocyte disorders charecterzid by alterations in erythrcoyte shape (see tabel and figure above). Disruption in vertical interactions results in instability of lipid layer resulting in loss of lipid layer and spherocytosis. Disruptions in horizontal interactions results in hereditary elliptocytosis.

Changes in the lipid layes also results in changes in erythrocyte shape. Unlike disorders ofthe cytskeleton, most of these disorders are accquired.

  1. Target Cells: Traget cells or codocytes are cells that have an appearance of a shooting target with a central bulls eye. Reletive increase in the membrane lipids results in the formation of target cells. This is seen in severe microcytis anaemias like severe iron deficiency, thalassaemia, haemoglobin C disease and haemoglobin E disease where the intracellular contents decrease. It may aslo bee seen in obstructive liver disease where the lipid and cholesterol content of the membrena increase.
  2. Stomatocytes: Stomatocytes are erythrocytes with a central elongated mouth-like area of pallor. Expansion of the inner layer results in stomatocytosis. This may be seen in alcoholism and with the use of vinca alkaloids.
  3. Echinocytes Ecchinocytes are cells that are no longer disc shaped and are covered by 10-30 short projections. The change is because of expansion of outer lipid layer. Ecchinocytes are seen in uraemia, pyruvate kinase deficiency or may be a fixing/staining artefact.
  4. Acanthocytes: Acanthocytes are cells with a few spiny projections on the surface (from acanthus, The Greek word for thorn). The result from accumulation of cholesterol (liver disease) or sphingomyelin (abetalipoproteinaemia) in the outer lipid layer results in acanthocytosis.

Erythrocyte Metabolism

GlucoseMetaboloisRBC As erythrocytes lack mitochondria they are not able to use fats or generate energy from Krebs cycle. Though they have enzymes to synthesize glycogen the balance between synthesis and breakdown favours breakdown. Normal erythrocytes do not have glycogen and depend on a continuous supply of glucose to meet their energy requirements.  Glucose enters the erythrocyte freely by facilitated diffusion. Insulin does not have any effect on the entry of glucose into the erythrocyte.

They erythrocyte metabolism needs ATP as a source of energy and NADH and NADPH cofactors. The erythrocyte does not synthesize nucleic acids but it has a small requirement for ribose to synthesize nucleosides for energy transfer The metabolic needs of erythrocytes are met by metabolism of glucose through three pathways glycolysis, the hexose monophosphate shunt and Rapport-Luebering glycolytic shunt.

Glycolysis in Erythrocytes
Glycolysis is a process in which one molecule of glucose is converted to two molecules of pyruvate with the a net formation of two ATP and two NADH molecules. The energy released in the process stored as ATP. The electron released in the conversion of glyceraldehyde-3-phosophate to 1,3-bisphosphoglycerate is accepted by NAD+. Glycolysis can not proceed in the absence of an electron acceptor. NADH produced by glycolysis is transported to the mitochondria where it generates a net of 2 ATPs (3 ATP are generated but one is consumed to transport NADH into the mitochondria). Erythrocytes do not have mitochondria. Methaemoglobin reductase is the only sink for NADH. Glycolysis can proceed only if NAD+ is regenerated. Conversion of pyruvate to lactate by lactate dehydrogenase regenerates NAD+ allowing glycolysis. The end product of glycolysis in erythrocytes is lactate.

The Rappaport-Luebering Glycolytic Shunt
The main functions of the erythrocyte to deliver oxygen to peripheral tissue. Anaemia decreases the oxygen carrying capacity of blood. Oxygen delivery is maintained in anaemic patients by increasing blood flow and increasing the oxygen extraction (see pathophysiology of anaemia). Patients with anaemia have a decrease in the oxygen affinity of haemoglobin allowing a greater amount to be offloaded for a given PO2. 2,3-Bisphosphoglycerate (2,3-BPG) and H+ ions decrease the oxygen affinity of haemoglobin. The Rappaport-Luebering glycolytic shunt synthesizes 2,3-BPG from 1,2-BPG. The shunt bypasses an ATP producing. The erythrocyte pays for increased oxygen affinity as ATP.

The Hexose Monophosphate Shunt
Erythrocytes are subjected to a high degree of oxidative stress from exposure to drugs and chemicals and from oxygen transport. The reactive oxygen species thus generated can affect cell visibility and function by conversion of ferrous iron of haemoglobin to ferric iron giving methaemoglobin. It can also damage lipid membranes shortening the erythrocyte lifespan. Glutathione is a tripeptide that scavenges reactive oxygen species and is oxidized in the process. Glutathione reductase regenerates glutathione by using NADPH as an electron donor. The only non-mitochondrial source of NADPH is the hexose monophosphate shunt. One molecule of glucose passing through the shunt gives two molecules of NADPH and is converted to ribulose-5-phosphate. The ribulose-5-phosphate can be converted to ribose-5-phosphate for nucleoside synthesis. It is also possible to generate 5 molecules of glucose-6-phosphate out of 6 molecules of ribulose-5-phosphate. This involves two enzymes transaldolase and transketolase both of which use thymine pyrophosphate as co-enzyme and are able to transfer carbon from one sugar to another. As the erythrocyte has only a small need of ribose it reconvert most of the ribulose-5-phosphate to glucose-6-phosphate. Normally about 10% of the glucose is metabolized through this shunt. When the erythrocyte is faced by an oxidizing stress almost 90% of the glucose may be metabolized through the shunt. The first and the rate limiting enzyme of the pathway is glucose-6-phosphate dehydrogenase (G6PD). G6PD deficiency is the commonest enzymatic deficiency causing haemolytic anaemia. It manifests as haemolysis in response to oxidative stress like exposure to drugs, chemicals of infection. The hexose monophosphate shunt is critical to the erythrocyte survival.

Red Cell Distribution Width

Features of peripheral smear like erythrocyte size, haemoglobinization and variability in size have long been useful in evaluation of patients with anaemia. The definitions of these parameters before the advent of haematological counters were subjective. The haematological counters have allowed an accurate measurement of erythrocyte size and numbers allowing reliable determination of mean cell volume (MCV), mean cell haemoglobin (MCH) and mean cellular haemoglobin concentration (MCHC). Mean cellular volume is used to classify anaemias (see Evaluating Anaemias). MCV is of little value in differentiating two of the commonest causes of anemia, iron deficiency and β-thalassaemia trait as both are microcytic hypochromic.

Anisocytosis is and abnormal variation in erythrocyte size. It is a feature of nutritional deficiencies, myelofibrosis, bone marrow infiltrations, microangiopathic haemolytic anaemia and in the presence of erythrocyte aggregates. Thalassaemias do not show anisocytosis. The erythrocyte is a disc approximately the diameter of the nucleus of a small lymphocyte and the central one-third is pale. The assessment of anisocytosis is subjective. Red cell distribution width(RDW) is quantitation of aniscytosis.

Standard deviation of a parameter is a measure of its scatter from the mean. MCV is an average of erythrocyte volumes measured by the counter. RDW is the standard deviation of these observations. It is expressed as a percentage of MCV. The normal values are 11.5-14.5%. High RDW means more anisocytosis.

Despite the apparent promise RDW has a limited role in diagnosis. A normal RDW in a microcytic anaemia can suggest the presence of on thalssaemia but can not be relied on as a sole criteria for separating iron deficiency from thalassaemia. The presence of a high RDW should alert one to the presence of one of the causes of anaemia listed above but again RDW can not be relied on for making a final diagnosis of any of these conditions. Blood transfusion in an anaemic patient increases the RDW.


Development of erythrocytes involves coordinated changes in the nucleus and the cytoplasm of erythroid processors (see Morphology of Erythroid Precursors) . The proerythroblast is a large cell with a fine chromatin, the earliest forms not being different from other blasts. As the cell matures the chromatin becomes more clumped and the nucleus reduces in size and the cytoplasm becomes acidophilic. Finally a dark pyknotic nucleus is extruded from the orthchromatophilic normoblast to give a reticulocyte.

Figure 1. Basophilic Normoblast

A group of basophilic normoblast are shown above. The cytoplasm is basophilic and the chromatin more clumped than a proerythroblast. The two cells on the right are less mature than the two on the left.

Figure 2. A group of orthochromatophilic normoblasts

The figure above show a group of  orthochromatophilic normoblasts. The cells of the left are more mature. The nucleus is reduced to a dense body in the more mature forms. The cytoplasm still has a blue tinge. which contrasts from the megaloblasts shown below.

Figure 3. Basophilic, polychromatophilic and orthochromatophilic normoblasts

The figure above shows three stages of erythroid maturation. The cell on the top right is a basophilic normoblast, bottom right is a polychromatophilic normoblast and the bottom left is an orthochromatophlic normoblast. Note the evolution of nuclear and cytoplasmic changes.

Figure 4. Megaloblasts

The figure above shows a group of megaloblasts. Cells in the right lower corner have a cytoplasm which is fully haemoglobinized and resembles the mature erythrocyte. These cell still have a nucleus. The cell on the left upper corner has a cytoplasm resembling a orthochromatophilc normoblast (see figure 2 and 3) but nuclear features resembling a basophilic normoblast (figures 1 and 2). Megaloblastic anaemia results from conditions that hamper DNA synthesis (B12 deficiency, folate deficiency, Chemotherapy drugs). The nucleus of a megaloblast thus matures slower than the cytoplasm resulting in cells having a nuclear morphology resembling a previous stage. This, known as nucleo-cytoplasmic dissociation, is the characteristic feature of megaloblastic anaemia.

Complications of Blood and Blood Product Transfusion

Haemolytic transfusion reactions: Haemolysis following red cell transfusion occur in two forms. Acute haemolysis occurs within 24 hours, is more serious and usually intravascular. Delayed haemolysis occurs after the first 24 hours is more common, less serous and usually intravascular.
Febrile reaction: A rise of temperature usually with chills by more the 1°C after blood transfusion. Headache nausea and vomiting may be seen in serve reaction. Fever lasting more the 18 hours after transfusion are not likely to be febrile reactions. They are almost always seen with cellular components being most common with platelet transfusion.
Urticarial Reactions complicate about 1% of blood trasnfusions
Anaphylactic reactions occur in about 1:150,000 patinets. They are seen in patients with IgA deficiency. These patients have anti IgA antibodies that react to IgA in the transfused plasma. These patients must be transfused thoroughly washed  RBCs.
Transfusion Induced Acute Lung Injury manifests as sudden deterioration of lung function shortly (2-6 hours) after a blood transfusion. It is treated by respiratory support with oxygenation and positive pressure ventillation.
Graft vs. Host disease is a rare but almost uniformly fatal complication of blood transfusion, usually from a close relative, that manifests as post transfusion fever, skin, liver,  and gastrointestinal manifestations and  pancytopenia.
Immune suppression caused by blood transfusion has been used in the past for decrease renal graft rejection. Today clinical relevance this effect is not clear.
Hepatitis (B and C): The incidence of transfusion induced hepatitis has called because of testing of blood and blood products. 
HIV: HIV posed a major risk before testing became mandatory. Testing has substantially reduced but not eliminated the risk of HIV transmission.
Other Viruses: Cytomegalovirus, Epstein-Barre virus, parvovirus b19, HTLV I, HTLV II can be transmitted by blood transfusion. Though some countries mandate testing for some of these viruses the practice, unlike that for HBV, HCV and HIV is not universal.
Malaria and other parasitic diseases: Out of the parasitic diseases that can be transmitted by blood transfusion (malaria, filariasis, bebesiosis, toxiplasmosis, toxoplasmosis and trypanosomiasis (South American, African). Malaria because of it’s widespread distribution is the greatest concern. because of it’s wide spread distribution.
Transfusion induced sepsis: Transfusion induced sepsis occurs as a result on bacterial growth during storage. It is most common with platelet transfusion as platelets are the only component stored at room temperature.
Coagulopathy: Dilutional coagulopathy due to degradation of labile coagulation factors like V and VIII on storage causes coagulopathy.
Citrate toxicity: Citrate used as an anticoagulant binds calcium and causes hypocalcaemia. Clinically significant hypocalcaemia is seen only with very rapid transfusion raters (more than one unit over less than 5 mins) and in patients with liver disease (because of impaired citrate metabolism). Treated with intravenous calcium.
Hypothermia: Blood is refrigerated for storage. In massive transfusion the urgency and the rapid rate of infusion may result in hypothermia that can be prevented by the use of blood warmers.
Acid-base imbalance: Patients needing massive transfusion are likely to be acidotic due to lactic acidosis. Citrate present in the transfused blood may aggregate acidosis. On recovery citrate and lactate are converted to bicarbonate resulting in metabolic alkalosis the commonest
Hyperkalaemia: Stored blood may have unto 80mEq/L of potassium which on transfusion cause lifethreatening hyperkalaemia

Further Reading:
Vein to Vein: An online publication of the Canadian Blood Services

Normal Erythrocytes

Normal erythrocytes are round disks about 7.5μm in diameter.The central one third is paler than the periphery because of the discoid shape of the erythrocyte. The picture above is a 40X image that shows the uniformity of size and staining of normal erythrocytes. Can one say that the

se cells are 7.5μm in diameter? They could all be 10μm or be 6μm. Is it possible to known about the size of erythrocytes using an unsophisticated laboratory microscope?

The small lymphocyte comes to the rescue! The size of the small lymphocyte nucleus is approximately 8.5μm and it does not vary significantly with disease. The picture above is a 100X image comparing the normal erythrocyte with a small lymphocyte. The cells are slightly smaller than a small lymphocyte. The uniformity of size and that of the pale staining area is evident. Microcytes are smaller erythrocytes and macrocytes larger erythrocytes. Hypochromia is increase in the pale staining area and indicates decreased content haemoglobin. Anisocytosis is increased variability in erythrocyte shape.