Lecture 11 - O2 transport and Storage by Haemoglobin and Myoglobin Pt. I Flashcards
What is the primary structure of Myoglobin?
150 amino acids
What is the secondary structure of Myoglobin?
8x alpha helices (labelled from A - H) and connecting loops
What is the tertiary structure of myoglobin?
globin fold with a hydrophobic pocket
What is the quaternary structure of myoglobin?
Monomeric (single polypeptide)
Where does the Haem bind to in myoglobin?
HisF8 (8th amino acid in helix F, which is a histidine residue)
What is Haem?
a prosthetic group, or cofactor
What is the basic structure of Haem?
four pyrrole rings linked together in a plane
How many coordinate bonds does iron (Fe) have?
6
What does the iron bind to in Haem?
4 binds to Nitrogen atoms in haem, 1 binds to a nitrogen atom of HisF8 in the globin, 1 binds to O2
Is the binding of oxygen to Fe2+ in haem reversible?
yes
What does a higher concentration of a molecule mean in spectroscopy?
less transmitted light and therefore higher absorbance
What does the beer lambert law convert?
absorbance of solution converted to concentration of solution
What is the purpose of spectroscopy?
Quantifying dissolved molecules based on the principle that different wavelengths are absorbed more or less efficiently
How can you determine if Haem is bound to an oxygen?
Haem has visible absorbance and therefore colour that differs between bright red oxyhaemoglobin (HbO2) and darker red deoxyhaemoglobin (Hb)
What else can change the shape of a spectroscopy spectrum?
colour, and nature of solute
How many globin subunits make up haemoglobin?
4 (usually 2 alpha, 2 beta)
how to the globin proteins in haemoglobin associate together?
non-covalently
What is the max number of O2 molecules that haemoglobin can hold at once?
4 (one per haem)
What is the common mechanism of Myo/Haemoglobin O2 binding?
- Haem Fe2+ is attached to globin protein by coordinate linkage to His F8
- Another His (from helix E) on opposite side of haem distorts binding of gas molecules to 6th coordination position on haem Fe2+
- This reduces binding affinity of oxygen making it easier to release oxygen to the muscle cell when needed
What is the function of myoglobin?
oxygen storage in tissues, needs to hold oxygen until it is required
What is the function of haemoglobin?
must acquire O2 from lungs and deliver to tissues, must bind O2 more weakly to release at correct time.
What type of curve is myoglobins binding curve?
hyperbolic
When is myoglobin saturated with oxygen?
at low partial pressure of oxygen
When does myoglobin release O2 to cells?
when cellular partial pressure of oxygen is very low
What is the partial pressure (pO2) of oxygen in lungs?
about 100 Torr
What is the partial pressure (pO2) of oxygen in resting muscle?
about 20 Torr
What type of curve is the binding curve of Haemoglobin?
a sigmoidal curve
When will Haemoglobin give up oxygen?
In the tissues at low pO2 where as myoglobin would typically remain saturated at that pO2
What allows the local environment to control the affinity of oxygen for haemoglobin?
Allostery, and CO-operativity
What does cooperativity require?
- an oligomer (which is a tetramer in the case of haemoglobin)
- multiple interacting subunits
What is allostery?
binding of molecules, regulators or post-translational modifications away from the active site
What is the function of allostery?
can help regulate binding
Are all allosteric proteins oligomers?
No can be monomers OR oligomers
What does the ‘Allo’ mean in allosteric?
something changing away from the active site
What represents the co-operative behaviour of Haemoglobin?
Sigmoidal binding curve shows two different states that have different affinities for oxygen
What are the two states of haemoglobin?
Tense (T) state, and Relaxed (R) state
Does the T-state have high or low O2 affinity?
low
Does the R-state have high or low O2 affinity?
high
Which proteins are often co-operative?
highly regulated proteins/enzymes
When do enzymes diverge from michaelis-menten kinetics?
when they are co-operative
What do haemoglobin and myoglobin have in common? (3)
- O2 binds to iron of haem
- Shift from dull to bright red allows monitoring O2 binding
- Oxygen-bound form has small change in haem (O2 in plane)
How do myoglobin and haemoglobin differ? (4)
- Store in tissue vs transport molecule.
- Monomer vs tetramer
- tighter, hyperbolic binding (myo) versus weaker sigmoidal binding (haemo)
- Haemoglobin has co-operativity
When does Haemoglobin become saturated?
When there is a very high pO2 (ie in the lungs)
