Section 9.6 describes haemoglobin as an allosteric macromolecule i.e. that the binding of a molecule (e.g. oxygen) on one part of the macromolecule can affect the binding of a spatially distant binding site. Allostery is important for haemoglobin as it must be able to bind oxygen strongly enough at the lungs and release oxygen quickly enough at the tissues. If haemoglobin displayed simple binding properties this would not be possible.
It mentions that CO2 has an effect on the binding of oxygen to haemoglobin. I am told that it is the presence of CO2 in the blood and not that lack of O2 that stimulates breathing. This is a reason often given for people accidentally drowning while holding their breath as they often expel air (including CO2) before diving to increase the dive time.
Yeah, I understood that to be the case too. I think a higher partial pressure of CO2 causes the local pH to increase (as H20+CO2<->2H+ + CO3(2-)), which in turn effects the binding between O2 and haemoglobin. However, I imagine it would be difficult to separately measure experimentally the effect of the change in pH and the effect of the change in CO2 concentration on the binding of O2 to haemoglobin.
ReplyDeleteIt seemed weird to me that Nelson feigned suprise that the binding of a molecule at one site in the protein can affect a spatially distant site.
ReplyDeleteI have two main reasons for this. Firstly the existence of long range effects such as London dispersion forces etc, electron transfer can also be across long distances in a protein. Secondly a protein is made of an amino acid chain, i.e. pulling on one end will move the other. Furthermore the chain is linked in a number of different ways as the protein folds. This would only increase the effect felt at one end of the protein when the other is dirupted.