Transcript Document
CLINICAL ASPECTS OF BIOCHEMISTRY
PROTEINS AND DISEASE
HAEMOGLOBIN AND HAPTOGLOBIN
Haemoglobin - revision
Haemoglobin variants - haemoglobinopathies
Haemoglobin S
Thalassaemia
Haptoglobins
HAEMOGLOBIN - REVISION
Myoglobin (Mb) Oxygen binding/storage protein in muscle;
may also play a part in local oxygen transport. O2 binds to haem.
Maintenance of haem in Fe2+ form is necessary for O2 binding.
Mb is a monomeric protein of about 150 aa.
Haemoglobin A (HbA) O2 carrier in blood (red cells).
Tetramer: 22. Quaternary structure allows allostery - cooperative binding of O2 modulated by pH (Bohr effect), CO2
binding, bisphosphoglycerate (BPG) binding. 3D structure of
each chain is similar to that of Mb.
HAEMOGLOBIN
TERTIARY STRUCTURE OF MYOGLOBIN AND
HAEMOGLOBIN SUBUNIT
Different types of Hb in man:
HbA 22
HbA2 22 ~2% of adult Hb; chain differs from at ~ 10% of residues;
function (if any) unclear
HbF 22 late foetus and neonate; replaced by HbA 3-6 months after birth;
chain differs from at ~of residues. In presence of BPG HbF has higher
affinity for O2 than HbA, allowing transfer of O2 to foetus (2 HbFs in man, chains
differing at 1 aa)
HbGower Gower I 22 Embryonic. similar to (~20% differences)
Gower II 22 Embryonic. similar to (~40% differences)
So, at least 5 different Hbs (6 chains) in normal human. , , d, chains can all
form tetramers, can't.
DEVELOPMENTAL PATTERN OF HAEMOGLOBIN IN MAN
Based on Voet & Voet (1995)
EVOLUTIONARY TREE RELATING
HUMAN GLOBIN CHAINS
Mb
Hbs in lower organisms:
Mammals. Adult Hbs all similar to human HbA, but may be
variants unlike those seen in human. Developmental patterns
of Hbs differ considerably
Other vertebrates Most vertebrates have 22 type structure.
Variant types differ considerably. Lamprey (most primitive
fish) has only a single chain - more similar to Mb than
mammalian Hbs (no allostery)
Invertebrates, plants, bacteria. Hb-like proteins frequently
found, but not 22
HAEMOGLOBIN VARIANTS - HAEMOGLOBINOPATHIES
1.
Exterior of molecule
e.g. Glu6Val [haemoglobin S (HbS)]
Glu
2.
3.
B8Lys
(harmless?)
Altered tertiary structure
Phe
CD1Ser
[Hb Hammersmith]
Gly
B6Arg
[Hb Riverdale-Bronx]
Altered 'active site’
His
F8Tyr
[Hb Iwate] (proximal His) )
His E7Tyr [Hb Boston] (distal His)
4.
)(cyanosis; methaemoglobinaemia)
Alterations at subunit interfaces
Asp G1His [Hb Yakima] )
Asn G4Thr [Hb Kansas] )
(polycythaemia or cyanosis)
SICKLE CELL ANAEMIA - HbS - FIBRES
Based on Voet & Voet (1995)
HbS - AGGREGATION
Based on Voet & Voet (1995)
HAEMOGLOBIN S (HbS)
Possible therapies:
1. Disruption of intramolecular interactions (peptides?)
2. Use of agents to increase O2 binding affinity
3. Lower HbS concentration (increase erythrocyte
permeability)
4. Keep HbF switched on (hydroxyurea)
5. Vasodilators
6. Gene therapy
and THALASSAEMIAS
0 and 0 thalassaemias- corresponding globin chain missing completely
and + thalassaemias - corresponding globin chain produced in reduced amount
thalassaemia
1.
.
3.
4.
Silent carrier state:
thalassaemia trait:
Hb H disease:
Hydrops fetalis:
[ Hb Barts: excess 4;
1 (of 4) genes missing
2 genes missing
3 genes missing
4 genes missing [lethal]
HbH: excess 4 ]
Also thalassaemia due to other causes. E.g. Hb Constant Spring:
Mutant stop codon and read-through of 31 aas, but mRNA degraded, so little
protein
HAEMOGLOBIN GENE CLUSTERS
Chromosome 16
y
Chromosome 11
y1 1 2
G
20
A
40
y
60
kbp
DELETIONS IN THE HAEMOGLOBIN GENE CLUSTER
kbp
G
20
A
40
y
60
0 thalassaemia
Hb Lepore
GA
thalassaemia
GA HPFH
G HPFH
Hb Kenya
HPFH = hereditary persistent fetal haemoglobin
thalassaemia
Point mutations that can cause thalassaemia:
1. Nonsense mutations
2. Frameshift
3. Point mutation in promoter
4. Point mutations that inactivate or generate splice sites
5. Point mutations of the AATAAA sequence
HAPTOGLOBINS
& chains; S-S linked; tend to form oligomers ()2 etc.
In human chain is polymorphic:
I (83 residues): IF (Lys54) and IS (Glu54)
II (143 residues):
54
Lys
Partial gene duplication
71
113
143
Glu
Gene frequencies: IF 0.16
IS
II
0.24
0.60
PROPOSED MECHANISM FOR PARTIAL GENE
DUPLICATION OF HAPTOGLOBIN
Hp1F
Hp1S
Hp2