Transcript PORPHYRINS AND HEMOGLOBIN

Dental Biochemistry
Lectures 39 and 40
2015
HEME AND HEMOGLOBIN
Michael Lea
Porphyrins and Hemoglobin - Lecture Outline
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1. Synthesis of Porphyrins
2. Regulation of Heme synthesis
3. Porphyrias
4. Degradation of Heme and Jaundice
5. Hemoglobin Structure and Function
6. Hemoglobin Pathology
• Suggested reading: Lippincott’s Biochemistry 6th
edition, pages 25-42, 277-285, 289
1. Porphyrin Synthesis
Porphyrins serve as prosthetic groups for
proteins that function in oxygen transport (hemoglobin
and myoglobin), breakdown of peroxide (catalase),
electron transport (cytochromes a, b and c),
hydroxylation (cytochrome P450), nitric oxide synthase
and light absorption (chlorophyll).
The porphyrins are heterocyclic ring structures
that include four pyrrole rings joined together through
carbon (methenyl) bridges. The most abundant
porphyrins in nature are found in hemoglobin and the
chlorophylls. In the center of porphyrins a metal atom is
chelated to the nitrogen atoms of the pyrrole units. In
heme and related porphyrins this atom is iron. In
chlorophyll the metal atom is magnesium.
Structure of Heme
1. Porphyrin Synthesis
The first and rate limiting reaction in porphyrin synthesis is the
reaction catalyzed by 5-aminolevulinate synthase:
glycine + succinyl CoA --> NH3+-CH2-CO-CH2-CH2-COO- +CO2+CoA
5 -aminolevulinate
Pyridoxal phosphate is a cofactor for 5-aminolevulinate synthase (daminolevulinate synthase, ALA synthase). Synthesis of 5-aminolevulinate
synthase is inhibited by heavy metal ions and there is a feedback inhibition
exerted by heme.
In the second reaction of porphyrin synthesis, two molecules of 5aminolevulinate condense to form the monopyrrole, porphobilinogen. The
combination of four porphobilinogen molecules gives a linear tetrapyrrole
and is accompanied by the loss of four molecules of ammonia . Ring
closure catalyzed by uroporphyrinogen synthase and isomerization gives
uroporphyrinogen III. Further intermediates in the synthesis of heme are
coproporphyrinogen III, protoporphyrinogen IX and protoporphyrin IX. The
conversion of protoporphyrin IX to heme is catalyzed by ferrochelatase.
The first reaction and the last three reactions of heme synthesis
occur in mitochondria with the other four reactions being in the cytosol.
ALA synthase
Porphobilinogen
Synthase
ALA dehydratase
Ferrochelatase
2. Regulation of Heme Synthesis
Regulation of hemoglobin synthesis involves control
of both porphyrin and polypeptide synthesis. The regulating
factor is an accumulation of heme, which in the free form is
spontaneously oxidized to hemin. An accumulation of hemin
diminishes the activity of 5-aminolevulinate synthase,
probably by repressing synthesis of the enzyme, as well as
directly inhibiting the existing enzyme, thereby shutting off
porphyrin synthesis. Hemin has this effect both in the
primitive red blood cell synthesizing hemoglobin and in other
cells in which the cytochromes and other heme proteins are
made.
Hemin activates the synthesis of globin peptide by
combining with an inhibitory protein. This mechanism keeps
the synthesis of heme and globin in balance.
3.Porphyrias
Disease
Enzyme
Organ Pathology
Acute Intermittent
Porphyria
Uroporphyrinogen I synthase
Neuropsychiatric
deficiency
Abdominal pain
(porphobilinogen deaminase
or hydroxymethylbilane synthase)
Congenital Erythropoietic
Porphyria
Uroporphyrinogen III
cosynthase deficiency
Porphyria Cutanea Tarda
Uroporphyrinogen decarboxylase Skin
Hereditary Coproporphyria
Coproporphyrinogen oxidase
Skin, Abdominal pain,
Neuropsychiatric
Variegate Porphyria
Protoporphyrinogen oxidase
Skin, Abdominal pain,
Neuropsychiatric
Protoporphyria
Ferrochelatase
Skin, Abdominal pain,
Neuropsychiatric
Lead poisoning
Inhibition of ALA dehydratase
and ferrochelatase
Nervous system,
blood etc.
Skin
4. Degradation of Heme
The average life span of normal human erythrocytes is
approximately 120 days. Senescent red cell are processed by the
reticuloendothelial system. Hemin, protoporphyrin and albumin bound
hemin are taken from the plasma largely by hepatic parenchymal cells
and converted almost quantitatively to bilirubin. In hemolytic anemia,
significant hemoglobinemia is unusual yet conjugated
hyperbilirubinemia frequently occurs. This indicates that the rate of
heme degradation may exceed the maximum rate of bilirubin removal
by the liver.
When hemoglobin is catabolized the apoprotein is
degraded to the constituent amino acids and the heme prosthetic
group is opened and subject to oxidation to yield biliverdin, iron and
carbon monoxide. The iron is reutilized. Biliverdin is reduced to
bilirubin by biliverdin reductase. Bilirubin is transferred to the liver as a
bilirubin-albumin complex. The bilirubin is made more soluble by
conjugation with glucuronic acid from UDP-glucuronic acid in a
reaction catalyzed by glucuronate transferase.
Jaundice is defined as the clinical syndrome associated
with elevation of the serum bilirubin level above the normal level of 1
mg/dl.
Types of Jaundice (Icterus)
1. Prehepatic jaundice (hemolytic jaundice)
a. acute hemolytic anemia
b. neonatal physiologic jaundice
c. chronic hemolytic anemia
2. Hepatic jaundice
a. conjugation failure: neonatal physiologic jaundice, CriglerNajjar disease, Gilbert's syndrome,
b. bilirubin transport disturbances: Dubin-Johnson disease
c. diffuse hepatocellular damage or necrosis
d. intrahepatic obstruction
3. Post-hepatic jaundice
Obstruction of the common bile duct by stones, neoplasms or
spasms.
5. Hemoglobin Structure and Function
Suggested reading: Lippincott’s
Biochemistry 6th edition, pages 25-42
5. Hemoglobin Structure and Function
Oxygen transport is mediated by two heme proteins, myoglobin and
hemoglobin.
Myoglobin occurs in muscle and has a single polypeptide chain.
Hemoglobin occurs in red blood cells and has four polyeptide chains.
Hemoglobin A is the major type in adults and has two alpha and two beta chains. Each
subunit has a hydrophobic pocket containing the heme unit.
The oxygen binding curve for myoglobin is a rectangular hyperbola whereas
the oxygen binding curve for hemoglobin is sigmoidal in shape.
Myoglobin has a greater affinity for oxygen than hemoglobin.
The sigmoidal curve for oxygen binding to hemoglobin is an indication of the
cooperativity which exists in the binding of the four oxygen molecules.
The binding of oxygen to hemoglobin is decreased by a decrease in pH (Bohr
effect) and by an increase in the level of 2,3-bisphosphoglycerate.
5. Hemoglobin Structure and Function
Unlike the iron in cytochromes which is alternately reduced and
oxidized as part of the mechanism of action, the iron in hemoglobin
must be maintained in the reduced state for the transport of oxygen.
Oxidation of the iron of hemoglobin from the Fe2+ to the Fe3+ state
results in the formation of methemoglobin.
Oxidation of hemoglobin to methemoglobin can be caused by a number
of agents including nitrite and some drugs. The reverse of this process
can be achieved by several NADH or NADPH-linked enzymes.
Cooperativity in Oxygen binding to hemoglobin
The sigmoid curve for the association of oxygen
with hemoglobin reflects the cooperativity that
occurs and involves two conformational states.
The two conformational states are designated T
(tense) and R (relaxed). Binding of oxygen to one
of the four subunits favors conversion from the T
to the R state where the R state has a higher
affinity for oxygen.
6. Hemoglobin Pathology
1. Glucosylation of hemoglobin occurs in diabetes mellitus
and may serve to measure chronic elevation of blood glucose
levels.
2. One of the most important hemoglobinopathies of genetic
origin is sickle cell anemia. This occurs when the individual is
homozygous for hemoglobin S in which there is a point
mutation resulting in a substitution of valine for glutamate in
the beta subunits. There is hemolytic anemia, painful crises
and poor circulation.
3. The thalassemias are a group of hereditary diseases in
which there are defects in the synthesis of either the alpha or
the beta chains.
4. Carbon monoxide binds to hemoglobin more tightly than
oxygen and it prevents oxygen release.
Hemoglobin Learning Objectives (taken from Baynes
and Dominiczak)
You should be able to
1.
Describe the mechanism of oxygen binding to myoglobin and
hemoglobin.
2.
Describe the conformational differences between deoxygenated and
oxygenated hemoglobin.
3.
Define the concept of cooperativity in oxygen binding to hemoglobin.
4.
Describe the Bohr effect and its role in modulating the binding of oxygen
to hemoglobin.
5.
Explain how 2,3-bisphosphoglycerate interacts with hemoglobin and
influences oxygen binding.
6.
Summarize the processes by which carbon dioxide is transported from
peripheral tissues to the lungs.
7.
Describe the major classifications of hemoglobinopathies.
8.
Describe the molecular basis of sickle cell disease.