CGIER 35 - Haemoproteins I: Structures of haemoglobin and myoglobin Flashcards
haemoglobin
in red blood cells - responsible for O2 transport in higher animals.
Contains ~ 65% of the iron in the human body
myoglobin
in muscle cells - stores O2 and transports it across muscle cells.
- Contains ~ 6% of the iron in the human body.
relative molecular mass of haemoglobin and myoglobin
Haemoglobin RMM ~ 66000 (~66 kDa)
Myoglobin RMM ~ 17800 (~17.8 kDa)
2 groups that make myoglobin
haem group
globin chain
constituents of haem
iron (II) and protoporphyrin
constituents of globin
153 – 160 amino acid residues
primary structure of protein is held by what bonds
by covalent bonds such as peptide bonds
secondary structure
refers to highly regular local sub-structures. Two main types of secondary structure, the alpha helix and the beta strand or beta sheets
what defines secondary structure
These secondary structures are defined by patterns of hydrogen bonds between the main-chain peptide groups
tertiary structure
refers to the three-dimensional structure of the protein. The alpha-helixes and beta pleated-sheets are folded into a compact globular structure.
examples of tertiary interactions
salt bridges, hydrogen bonds, and the tight packing of side chains and disulfide bonds
quarternary structure
the three-dimensional structure of a multi-subunit protein and how the subunits fit together.
structural difference of haem and myoglobin
haem - 4 subunits quarterniary structure
myoglobin - tertiary structre
protoporphyrin
Porphyrins – family of tetradentate, planar ligands.
4N donor atoms.
Heteroaromatic.
4 pyrrole-like rings joined by CH groups.
Peripheral substituents.
in haemoglobin molecule how does the haem group link with the globin group
has compact shape with protein chain
histidine residue links haem group with globin chain called proximal histidine - one with closest proximmity to haem unit
describe amphipathic qualty of myoglobin
interior - hydrophobic
exterior - hydrophilicOnly two hydrophilic residues inside, both histidines essential for biological activity.
why is Fe II unsaturated
co-ordination no = 5
has less than max co-ordination number
quality of haem group that allows complex with oxygen to form
It has a vacant coordination site which can be used to complex with O2.
The haem group is a 1.___ crevice 2.(___). This is essential otherwise 3.___
- non polar
- hydrophobic
- the oxidation of Fe(II) would occur in the presence O2 and H2O
The haem group is placed in a 1.\_\_\_\_ to prevent 2.\_\_\_\_\_ Unlike iron (II) iron (III) can not 3.\_\_\_
- non-polar crevice
- oxidation of the metal ion.
- bind O2.
forms, ion and colour of myoglobin
Myoglobin Fe(II) His purple red. Oxymyoglobin Fe(II) His & O2 bright red Metmyoglobin Fe(III) His & H2O brownish- red Metmyoglobin does not bind O2.
example of high spin IN HAEMOATOPROTEINS
Fe(II) In myoglobin,
haemoglobin
example of low spin haematoproteins
Fe(II) in oxymyoglobin
& oxyhaemoglobin
high spin - paramagnetic
fill all d orbitals first singly before in pairs - low splittings
low spin - diamagnetic
fill low energy d orbitals in pairs before filling high energy d orbitals singly because it requires more energy - high splitting
myoglobin in high spin
Iron(II), d6, is high spin, lies out of the porphyrin plane and has one empty coordination site.
oxymyoglobin
Iron(II) is low spin and smaller in size (electrons are concentrated into three orbitals), can now fit into the porphyrin plane and is coordinatively saturated (CN = 6).
where are haems in haemoglobin
Haems in non-polar crevices
3D structure of each globin in haemoglobin
3D structure of each globin chain in Hb very similar to that in Mb although amino acids are identical at only 24 positions.
how many subunits in haemoglobin - describe subunit
4 subunits – each Mb-like
2α and 2β globin chains
Salt linkages between chains
describe shape of haemoglobin with regards to subunit
A tetrameric protein (α2β2).
α chain – 141 a.a.
β chain – 146 a.a.
desecribe alpha and beta chains
Each α is in contact with both β chains.
Very few interactions between the two α chains or between the two β chains.
most important interaction with subunit in haemoglobin
salt bridge linkages. These can take various forms in
proteins.
how does co-ordination number and spin change in haemoglobin when it binds to oxygen
CN = 6 (coordinatively saturated)
low spin, diamagneticiron in porphyrin plane
iron (II) co-ordination number in myoglobin is 5 what does the iron bond to in order to get this co-ordination number
4 Nitrogens in porphyrin functional group = 4 bonds
Fe attaches to N from proximal histidine
through what process does oxymyoglobin go through to become metmyoglobin
Oxidation
how come the iron is able to fit in the porphyrin when it binds to oxygen
iron (II) changes from high spin to low spin from paramagnetic to diamagnetic and the electron density is now in the bottom rather than being distributed to all d orbitals due to high splitting, thus making it smaller. Moves from out of porphyrin plane to inside plane of ring
Now has a CN of 6 and is fully saturated
what is salt bridge interaction
A salt bridge in proteins is an interaction
between oppositely charged amino acid side chains
