2.2.12 Globular Proteins

Question 1

Globular

Answer 1

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 hydrophobic R groups orientate themselves to the outside of the protein
The orientation enables globular proteins to be (generally) soluble in water as the 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 catalyst specific reactions and immunoglobulins (antibodies) can respond to specific antigens
Some globular proteins are conjugated proteins that contain a prosthetic group
- e.g. haemoglobin which contains the prosthetic group called haem

Question 2

Haemoglobin Structure

Answer 2

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 4 polypeptide chains
These chains or subunits are globulin proteins (2 alpha-globins and 2 beta-globins) and each subunit has a prosthetic haem group
The 4 goblin subunits are held together by disulphide bonds and arrange so that their hydrophobic R groups are facing inwards (helping preserver the 3-D special shape) and the hydrophilic R groups are facing outwards (helping maintain its solubility)
The arrangements of the R groups is important to the function of the 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 animo acid valine (non-polar) replacing glutamic acid (polar) making haemoglobin less similar)
The prosthetic haem group contains an iron II ion (FE 2+) which is able to reversible combine with an oxygen molecule forming oxyhaemoglobin and results in the haemoglobin appearing bright red
Each haemoglobin within the 4 haem groups can therefore carry 4 oxygen molecules (8 oxygen atoms)

Question 3

Haemoglobin Function and How It’s Structure Aids It

Answer 3

Haemoglobin is responsible for binding oxygen in the lung and transporting the oxygen to tissue to be used in aerobic respiration
As oxygen is not very soluble in water and haemoglobin is, oxygen can be carries more efficiently when bound to the haemoglobin
The presence of the haem group (and Fe 2+) enables small molecules like oxygen to be bound more easily because each oxygen molecule binds it the quaternary structure (due to alterations in the tertiary structure) of the protein, which causes haemoglobin to have higher affinity for the subsequent oxygen molecules and they bind more easily
The existence of the iron II ions (Fe 2+0 in the prosthetic haem group allows oxygen to reversible bind as non of the amino acids that make up the peptide chains in haemoglobin are well suited to binding with oxygen

Question 4

Enzymes

Answer 4

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
Enzymes are globular proteins
Critical to the enzyme’s function is the active site where the substrate binds
Metabolic pathways are controlled by enzymes in a biological cascade of reactions
- virtually every metabolic reaction within living organisms is catalysed by an enzyme
- enzymes are therefore essential for life to exist

Question 5

Intracellular and Extracellular Enzymes Example

Answer 5

Intracellular - Catalase
- hydrogen peroxide is produced as a bi-product of many metabolic reactions
- it is harmful to cells
- catalase converts hydrogen peroxide into water and oxygen, preventing any damage to the cells
Extracellular - Amylase
- digestion is usually carried out by extracellular enzymes
- this is because the macromolecules being digested are too large to enter the cell
- amylase is involved in carbohydrate digestion
- it hydrolyses starch into simple sugars
- it is secreted by the salivary glands and the pancreas, for digestion of starch in the mouth and small intestine respectively

Question 6

Insulin

Answer 6

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 2 polypeptide chains
- polypeptide A has 21 amino acid residues
- polypeptide B has 30 amino acid residues
The 2 polypeptide chains are held together by 3 disulphide bridges