Haemoglobin

Lots of different Haemoglobins - ll chemically similar (HbA is the most common in adults)

Role - Transport oxygen

What is the structure of Haemoglobin?

  • quaternary structure

  • 4 polypeptides

  • Each polypeptide contains ferrous ion (Fe2+)

  • Each Fe2+ can combine with 1 single oxygen mocelcule (O2)

Picking up and dropping off oxygen:

Also known as loading and unloading

Associating is another word used to describe oxygen binding with haemoglobin. Disassociating is the term used to describe oxygen leaving the haemoglobin at the tissues.

Affinity is another word used here. It relates to how strongly a haemoglobin molecule wants to bind with oxygen. This can vary depending on the conditions the haemoglobin is in

When oxygen is not bound:

When oxygen is not bound, a very weakly bonded water molecule fills the site

Carbon dioxide may also bind with haemoglobin but it does not compete with oxygen. Instead of binding to the iron in the heme group it binds to the amine group on the polypeptide chains attached to the heme group

Carbon monoxide on the other hand does competitively inhibit oxygen binding

What does Haemoglobin need to be good at transporting oxygen:

Haemoglobin:

  • Has a high affinity for oxygen where gas exchange takes place (i.e. to pick up oxygen)

  • This affinity also increases as oxygen binds - this is called cooperative binding

  • Have a low affinity (readily dissociate) from oxygen in areas with low oxygen (e.g. tissues)

How does haemoglobin change its affinity for oxygen?

Haemoglobin:

  • Can change its structure in the presence of substances - e.g. carbon dioxide (present in high concentrations in respiring tissues). 

  • The new shape of the haemoglobin results in oxygen binding more loosely with the haem group (iron core)

  • This causes the haemoglobin to release its oxygen

Why are there different haemoglobins?

Different organisms live in different environments, which vary in the availability of oxygen.

If you live in an area with little oxygen, you need to hold onto it, so require haemoglobin with a higher affinity than humans.

Myoglobin:

Like haemoglobin, myoglobin is a cytoplasmic protein that binds oxygen on a heme group. It harbors only one globulin group, whereas hemoglobin has four. Although its heme group is identical to those in Hb, Mb has a higher affinity for oxygen than haemoglobin. This difference is related to its different role: whereas haemoglobin transports oxygen, myoglobin's function is to store oxygen.

Partial pressure:

Gases are usually found as mixtures (e.g. air).

The partial pressure imagines a particular element is on its own in a given space and says what it’s gas pressure would be