CLINICAL ASPECTS OF BIOCHEMISTRY

1
CLINICAL ASPECTS OF BIOCHEMISTRY PROTEINS AND
DISEASE HAEMOGLOBIN AND HAPTOGLOBIN
Haemoglobin - revision Haemoglobin variants -
haemoglobinopathies Haemoglobin
S Thalassaemia Haptoglobins
2
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 ?2?2. Quaternary
structure allows allostery - co-operative binding
of O2 modulated by pH (Bohr effect), CO2
binding, bisphosphoglycerate (BPG) binding. 3D
structure of each chain is similar to that of Mb.
3
HAEMOGLOBIN
4
TERTIARY STRUCTURE OF MYOGLOBIN AND HAEMOGLOBIN
b? SUBUNIT
5
Different types of Hb in man HbA ?2?2 HbA2
?2?2 2 of adult Hb ? chain differs from ?
at 10 of residues function (if any)
unclear HbF ?2?2 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,
g chains differing at 1 aa) HbGower Gower I
?2?2 Embryonic. ? similar to ? (20
differences) Gower II ?2?2 Embryonic.
? similar to ? (40 differences) So, at least 5
different Hbs (6 chains) in normal human. ?, ?,
d, ? chains can all form tetramers, ? can't.
6
DEVELOPMENTAL PATTERN OF HAEMOGLOBIN IN MAN
Based on Voet Voet (1995)
7
EVOLUTIONARY TREE RELATING HUMAN GLOBIN CHAINS
e
8
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 ?2?2 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 ?2?2
9
HAEMOGLOBIN VARIANTS - HAEMOGLOBINOPATHIES

• Exterior of molecule
• e.g. ?Glu6?Val haemoglobin S (HbS)
• ?Glu B8?Lys (harmless?)
• Altered tertiary structure
• ?Phe CD1?Ser Hb Hammersmith
• ?Gly B6?Arg Hb Riverdale-Bronx
• Altered 'active site
• ?His F8?Tyr Hb Iwate (proximal His)
  )
• ???????????????His E7?Tyr Hb Boston (distal
  His) )(cyanosis methaemoglobinaemia)
• Alterations at subunit interfaces
• ????????????????Asp G1?His Hb Yakima )
• ?????????????????Asn G4?Thr Hb Kansas )
  (polycythaemia or cyanosis)

10
SICKLE CELL ANAEMIA - HbS - FIBRES
Based on Voet Voet (1995)
11
HbS - AGGREGATION
Based on Voet Voet (1995)
12
HAEMOGLOBIN S (HbS)

• Possible therapies
• Disruption of intramolecular interactions
  (peptides?)
• Use of agents to increase O2 binding affinity
• Lower HbS concentration (increase erythrocyte
  permeability)
• Keep HbF switched on (hydroxyurea)
• Vasodilators
• Gene therapy

13
a and b THALASSAEMIAS
a0 and b0 thalassaemias- corresponding globin
chain missing completely ?? and b thalassaemias
- corresponding globin chain produced in reduced
amount

• ?? thalassaemia
• Silent carrier state 1 (of 4) a genes missing
• a thalassaemia trait 2 a genes missing
• Hb H disease 3 a genes missing
• Hydrops fetalis 4 a genes missing
  lethal
• Hb Barts excess g4 HbH excess b4
• Also a 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

14
HAEMOGLOBIN GENE CLUSTERS
Chromosome 16
Chromosome 11
15
DELETIONS IN THE HAEMOGLOBIN b GENE CLUSTER
HPFH hereditary persistent fetal haemoglobin
16
b thalassaemia
Point mutations that can cause b
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
17
HAPTOGLOBINS
a b chains S-S linked tend to form
oligomers (ab)2 etc. In human a chain is
polymorphic aI (83
residues) aIF (Lys54) and aIS (Glu54)
aII (143
residues)
Partial gene duplication
Gene frequencies aIF 0.16
aIS 0.24
aII 0.60
18
PROPOSED MECHANISM FOR PARTIAL GENE DUPLICATION
OF HAPTOGLOBIN