Revision notes topic questions past papers

CIE AS Biology (9700) 2019-2021 Resources

REVISION NOTES 

 

  TOPIC QUESTIONS 

 

  PAST PAPERS

Page 57

2.3.4 HAEMOGLOBIN

The Molecular Structure of Haemoglobin

Structure

Haemoglobin is a globular protein which is an oxygen-carrying pigment found in vast

quantities in red blood cells

It has a quaternary structure as there are four polypeptide chains. These chains or

subunits are globin proteins (two α–globins and two β–globins) and each subunit has a

prosthetic haem group

The four globin subunits are held together by disulphide bonds and arranged so that their

hydrophobic R groups are facing inwards (helping preserve the three-dimensional

spherical shape) and the hydrophilic R groups are facing outwards (helping maintain its

solubility)

The arrangements of the R groups is important to the functioning of haemoglobin. If changes

occur to the sequence of amino acids in the subunits this can result in the properties of

haemoglobin changing. This is what happens to cause sickle cell anaemia (where base

substitution results in the amino acid valine (non-polar) replacing glutamic acid (polar)

making haemoglobin less soluble)

The prosthetic haem group contains an iron II ion (Fe

2+

) which is able to reversibly combine

with an oxygen molecule forming oxyhaemoglobin and results in the haemoglobin

appearing bright red

Each haemoglobin with the four haem groups can therefore carry four oxygen molecules

(eight oxygen atoms)

---