Download Learning Objectives: At the end of the lecture the student should be

Learning Objectives:
At the end of the lecture the student should be able to :
1. The process of heme synthesis and its incorporation into globin chains to form
hemoglobin
2. The structure of hemoglobin from a biochemistry perspective
3. Porphyrias ,their different types ,clinical presentations and their implications
4. Various types of hemoglobin normally found in human body
5. Various types of hemoglobin associated with different diseases
6. Hemoglobin F ,its synthesis and importance of its affinity with 2,3- BPG
7. Hemoglobin A1C and its clinical implications
Lecture outline:
Hemoglobin synthesis
• Requires co-ordinated production of heme and globin
The Heme synthesis
• 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 by ALA synthase to form 5aminolevulic acid (ALA).
• This molecule is transported to the cytosol where a series of reactions produce a
ring structure called coproporphyrinogen III.
• This molecule returns to the mitochondrion where an addition reaction produces
protoporhyrin IX.
• The enzyme ferrochelatase inserts iron into the ring structure of protoporphyrin
IX to produce heme.
Clinical correlation of heme synthesis
• Deranged production of heme produces a variety of anemias
• Iron deficiency, the world's most common cause of anemia, impairs heme
synthesis thereby producing anemia.
• A number of drugs and toxins directly inhibit heme production by interfering with
enzymes involved in heme biosynthesis.
• Lead commonly produces substantial anemia by inhibiting heme synthesis,
particularly in children.
The Globin synthesis
• Synthesis occurs in the ribosomes present in the cytosol
• Two distinct globin chains (each with its individual heme molecule) combine to
form hemoglobin.
• One of the chains is designated alpha.
• The second chain is called "non-alpha".
• With the exception of the very first weeks of embryogenesis, one of the globin
chains is always alpha.
• A number of variables influence the nature of the non-alpha chain in the
hemoglobin molecule.
• The combination of two alpha chains and two non-alpha chains produces a
complete hemoglobin molecule (a total of four chains per molecule).
Structure of Hemoglobin
• Hemoglobin exhibits characteristics of both the tertiary and quaternary structures
of proteins.
• Most of the amino acids in hemoglobin form alpha helices, connected by short
non-helical segments
• Hydrogen bonds stabilize the helical sections inside this protein, causing
attractions within the molecule, folding each polypeptide chain into a specific
shape.
• Hemoglobin's quaternary structure comes from its four subunits in roughly a
tetrahedral arrangement.
• In most humans, the hemoglobin molecule is an assembly of four globular protein
subunits
• Each subunit is composed of a protein chain tightly associated with a non-protein
heme group
• Each protein chain arranges into a set of alpha-helix structural segments
connected together in a globin fold arrangement.
•
This is so called because this arrangement is the same folding motif used in other
heme/globin proteins such as myoglobin.
Adult human hemoglobin
• In adult humans, the most common hemoglobin type is a tetramer (which contains
4 subunit proteins) called hemoglobin A
• It consists of two α and two β subunits non-covalently bound, each made of 141
and 146 amino acid residues, respectively
• This is denoted as α2β2
• Each subunit has a molecular weight of about 17,000 daltons, for a total
molecular weight of the tetramer of about 68,000 daltons (64,458 g/mol)
Porphyrias
Group of disorders due to abnormalities in genes directing enzymes in bio-synthesis
of heme.
Autosomal dominant in inheritance, except: congenital erythropoietic porphyria which is
autosomal recessive.
Classification of Porphyrias
• Acute Porphyrias
• Alanine D deficient porphyria.
• Acute intermittent porphyria.
• Variegate porphyria.
• Hereditary coproporphyria.
• Non-acute porphyria:
• Porphyria cutanea tarda.
• Erythropoietic protoporphyria.
• Congenital erythropoietic porphyria.
Metabolic pathway
• If enzymatic lesion occurs early then porphyrogins accumulate in tissues and
fluids results in abdominal pain & neuropsychiatric symptoms.
•
If enzymatic lesion occurs later in pathway results in accumulation of porphyrin
products causes photosensitivity & react with molecular oxygen followed by
damage to lysosome and skin damage.
Hepatic porphyrias
• Increased plasma & urinary concentration of porphyrin precursors.
Acute Intermittent Porphyria
• Genetically autosomal dominant involving Uroporphyrogin I synthase.
• Ecologically due to gonadal steroids and low Calorie diet.
• Present with abdominal pain, neuro0psychiatricand cardiovasular symptom.
Variants and types of hemoglobin
(normally present at different ages)
In embryo:
• Gower 1 (ζ2ε2)
• Gower 2 (α2ε2)
• Hemoglobin Portland (ζ2γ2)
In fetus:
• Hemoglobin F (α2γ2)
In adults:
• Hemoglobin A (α2β2) - normal amount over 95%
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Hemoglobin A2 (α2δ2) - normal range of 1.5-3.5%
Hemoglobin F (α2γ2) – normal range < 1%
Variants and types of hemoglobin
(variants which cause diseases)
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Hemoglobin H (β4) - A variant form of hemoglobin, formed by a tetramer of β
chains, which may be present in variants of α thalassemia
Hemoglobin Barts (γ4) - A variant form of hemoglobin, formed by a tetramer of
γ chains, which may be present in variants of α thalassemia.
Hemoglobin S (α2βS2) - A variant form of hemoglobin found in people with
sickle cell disease. There is a variation in the β-chain gene, causing a change in
the properties of hemoglobin which results in sickling of red blood cells.
Hemoglobin C (α2βC2) - Another variant due to a variation in the β-chain gene.
This variant causes a mild chronic hemolytic anemia.
Hemoglobin E(α2βE2) - Another variant due to a variation in the β-chain gene.
This variant causes a mild chronic hemolytic anemia
Hemoglobin AS - A heterozygous form causing Sickle cell trait with one adult
gene and one sickle cell disease gene
Hemoglobin SC disease - Another heterozygous form with one sickle gene and
another encoding Hemoglobin C
Fetal hemoglobin (HbF)
• HbF is a tetramer consisting of teo alpha chains identical to those found in HbA
plus two gamma chains
Synthesis during development:
• In the first few weeks after conception,embryonic hemoglobin (Hb Gower 1
)composed of two zeta and two epsilon chains is synthesized by embryonic yolk
sac.
• Within few weeks,fetal liver begins to synthesize HbF in developing bone marrow
• HbF is major hemoglobin found in fetus and newborn,accounting for 60% of total
Hb in red cells during the last months of fetal life.
• HbA synthesis starts in 8th month of pregnancy and gradually replaces HbF
Binding of 2,3-BPG to HbF:
• Under physiological conditions, HbF has higher affinity for oxygen than does
HbA
• This is because of result of HbF’s weak binding to 2,3-BPG
• Because 2,3- BPG serves to reduce the affinity of HB for oxygen,the weak
interaction between2,3-BPG and HbF results in higher oxygen affinity for HbF
relative to HbA
• The higher oxygen affinity of HbF facilitates the transfer of oxygen from the
maternal circulation across placenta to the red cells of the fetus
Hemoglobin A2 (HbA2)
• A minor component of normal adult hemoglobin
• First appears 12 weeks after birth
• Constitutes 1.5 – 3.5 % of the total adult hemoglobin
• Composed of two alpha and two delta globin chains
Hemoglobin A1c
• Under physiological conditions . HbA is slowly and non-enzymatically
glycosylated and the extent depends on the plasma concentration of a particular
hexose
• The most abundant form of glycosylated hemoglobin is HbA1c
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It has glucose residues attached predominantly to the NH2 groups of the N
terminal valines of the beta globin chains
Increased amounts of HbA1c are found in the red cells of patients with diabetes
mellitus as they have got increased glucose concentrations in their plasma
Very good indicator of the Plasma glucose control over a past 3 month interval –
indicates patient’s compliance with respect to medication intake and the control of
diet