Globular

• Globular proteins?are?compact, roughly?spherical?(circular) in shape and?soluble?in water
• Globular proteins form a spherical shape when folding into their tertiary structure because:
  • their?non-polar hydrophobic R groups?are orientated towards the?centre?of the protein away from the aqueous surroundings and
  • their?polar hydrophilic R groups?orientate themselves on the?outside?of the protein

   

• This orientation enables globular proteins to be (generally)?soluble?in water as the water molecules can surround the?polar hydrophilic R groups
• The?solubility?of globular proteins in water means they play important?physiological?roles as they can be easily?transported?around organisms and be involved in?metabolic reactions
• The folding of the protein due to the interactions between the R groups results in globular proteins having?specific shapes. This also enables globular proteins to play physiological roles, for example,?enzymes?can catalyse specific reactions and?immunoglobulins?(antibodies) can respond to specific antigens
• Some globular proteins are?conjugated?proteins that contain a?prosthetic?group eg. haemoglobin which contains the prosthetic group called haem

Haemoglobin

• 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 (Fe2+) 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)

The structure of haemoglobin showing the α–globin and β–globin subunits, the prosthetic haem group with oxygen molecules bonded to form oxyhaemoglobin

• Haemoglobin is responsible for binding oxygen in the lung and?transporting?the?oxygen?to tissue to be used in aerobic metabolic pathways
• As?oxygen is not very soluble?in water and haemoglobin is, oxygen can be carried more efficiently around the body when bound to the haemoglobin
• The?presence?of the?haem group?(and Fe2+) enables small molecules like oxygen to be bound more easily because as each?oxygen molecule?binds it?alters?the?quaternary structure?(due to alterations in the tertiary structure) of the protein which causes haemoglobin to have a higher affinity for the subsequent oxygen molecules and they bind more easily
• The?existence?of the iron II ion (Fe2+) in the prosthetic haem group also allows?oxygen?to?reversibly bind?as none of the amino acids that make up the polypeptide chains in haemoglobin are well suited to binding with oxygen

Enzymes

• Enzymes are?biological catalysts
  • ‘Biological’ because they function in?living systems
  • ‘Catalysts’ because they?speed up the rate of chemical reactions?without being used up or changed

   

• Enzymes are also?globular proteins
• Critical to the enzyme's function is the?active site?where the?substrate binds
• Metabolic pathways?are controlled by enzymes in a biochemical cascade of reactions
  • Virtually every metabolic reaction within living organisms is catalysed by an enzyme – enzymes are therefore essential for life to exist

   

Enzymes Table

Insulin

• The first protein to have its sequence determined by scientists was the?hormone?insulin
• Insulin is a globular protein produced in the pancreas. It plays an important role in the?control of blood glucose concentration
• It consists of?two polypeptide chains
  • Polypeptide A has 21 amino acid residues
  • Polypeptide B has 30 amino acid residues

   

• The two polypeptide chains are held together by?three disulfide bridges