Chem 438 

Spring 2010 
Lecture Notes: 12 February 


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Chaperones  See next week's discussion.

Myoglobin is a 153 residue globular protein in the globin family. Eight alpha helices form its single domain (myoglobin fold) tertiary structure; about 80% alpha helix (high for globular proteins). (Figure 4.40) Interior almost exclusively hydrophobic residues, with water excluded from interior. Surface has mix of hydrophobic and hydrophilic residues, with ionizable groups on surface.
Myoglobin functions to store and facilitate the diffusion of oxygen in muscle. Oxygen binds to a heme {Fe (II)protoporphyrin IX} prosthetic grp. (Figure 4.39) Four of irons six ligands are to heme nitrogens, with a fifth to a histidine nitrogen. The final ligand bond goes to oxygen. (Figure 4.44, p 116) Breathing motions (see below) are necessary to allow the exchange of oxygen, since the heme is in a closed pocket. (Figure 4.45  note motion possibilities)
Hemoglobin is an alphaalphabetabeta oligomeric protein: its quaternary structure consists of a tetramer of myoglobin like subunits. (Figure 4.43, 4.42) The two types of chain are slightly shorter than myoglobin chains (alpha= 141 aa residues, beta= 146 aa residues). There are extensive contacts between an alpha and a beta subunit to give a dimer. The dimers have additional contacts to give the tetramer. Oxygen binding results in a change of conformation in Hb. (Figure 4.47, p 119) The change of conformation affects the binding of oxygen [overhead Fig 916, V&V] {oxygen binding is reduced in the "blue" form due to steric hindrance between the oxygen and the heme}.
Let's look at binding in terms of saturation, Y, where if Y = 1 every site of every Myoglobin is occupied by an oxygen molecule (thus if Y = 0.5, then 50% of the myoglobin are binding oxygen and 50% are "empty"). Mb/Hb binding curve (Figure 4.46):
Reviewing the curve in terms of saturation, Y, if Y = 1 then every site of every Myoglobin is occupied by an oxygen molecule (thus if Y = 0.5, then 50% of the myoglobin are binding oxygen and 50% are "empty").
Can describe binding as dissociation equilibrium,then:
MbO_{2} Mb + O_{2} ; &
for saturation. Substituting, , the equation of a hyperbola. If expressed as pressures, then where P_{50} = pO_{2} @ 50% saturation. Note that the binding curve for Mb is indeed hyperbolic in shape.
What about Hb? Obviously more complex. The sigmoid shape (sshape) of the curve indicates cooperativity. That is, if one site binds, another is more likely to as well (it cooperates with the first site). If the subunits of Hb are fully cooperative (if one subunit binds oxygen they all must bind oxygen, if one releases oxygen they must all release oxygen) then
;
and for saturation, substituting as above with Mb:
But this assumption of total cooperativity doesn't work, the curve is too steep.
FYIHill equation: We can rearrange our equation to find the degree of cooperativity. If we generalize:. Rearranging:
In this equation n is the cooperativity, that is the apparent number of fully cooperative sites. Hill plot: If we take logs of both sides of the Hill equation and plot the results, the exponent, n, shows up as the slope. Thus we can readily find the apparent cooperativity by plotting saturation vs. oxygen pressure (or concentration). For Hb the slope turns out to be n = 2.8. That is, Hb is partially cooperative  its 4 cooperating subunits only partially cooperate to behave like 2.8 completely cooperative subunits.
Note limiting situations at extremes with n = 1. At high concentrations this results because the effective equilibrium is:
In other words it acts like it has a single site (only one site is available at any moment) like myoglobin! At low concentrations of oxygen the opposite effect gives the same result:
Again, only one site is effectively operating (not enough O_{2} to fill more than subunit one site at any given time), so again mimics myoglobin. 
Pathway Diagrams 

Last modified 17 February 2010