Proteins

Question 1

Name seven types of amino acid and what group they contain

Answer 1

Aliphatic amino acids contain either a hydrogen or a carbon group.
Aromatic amino acids contain a benzene ring or other aromatic group.
Sulphur containing amino acids contain sulphur groups.
Basic amino acids contain nitrogenous groups or alkali groups.
Acidic amino acids contain acidic groups such as a carbonyl group.
Uncharged polar amino acids contain polar groups.
Other amino acids such as proline form a ring.

Question 2

Describe the primary structure of a protein

Answer 2

The sequence of amino acids in a polypeptide chain. This is the source of versatility in protein structure and function.

Question 3

Describe the secondary structure of a protein

Answer 3

The spatial arrangement of amino acid residues that are near each other in the linear sequence.
Alpha Helix is a telephone cord shape held in place by hydrogen bonds between every N-H group and the oxygen of C=O group (4aa residues apart) (troponin, myoglobin, ferritin)
Beta pleated sheets are held together by hydrogen bonds between the amide groups (fatty acids and porin)

Question 4

Describe the tertiary structure of a protein

Answer 4

The spatial arrangement of amino acid residues that are far apart in a linear sequence. 
Van der waals (non specific weak attractions between atoms 0.3-0.4nm apart)
Ionic interactions (between two close and oppositely charged groups)
Hydrogen bonds (strong and permanent)
Disulphide bridges (strong covalent bonds between two cysteine residues and can occur between and within a polypeptide)
Hydrophobic interactions (intra polypeptide)
(Chymotrypsin, transferrin, caeruloplasmin)

Question 5

Describe the quaternary structure of a protein

Answer 5

Refers to the spatial arrangement of individual polypeptide chains in a multi subunit protein.
(CRR, Haemoglobin)

Question 6

What is protein denaturation, how can this be achieved and what are the effects?

Answer 6

The disruption and possible destruction of both the secondary and tertiary structures. Denaturation reactions aren’t strong enough to break the peptide bonds so the primary structure remains the same.
Causes include acids, heat, solvents (ethanol or methanol), cross linking reagents (formaldehyde), chaotropic agents (urea) and disulphide bond reducers (2 mercaptoethanol).
It decreases the solubility, alters the water binding capacity, causes a loss of biological activity and improves digestibility of the protein.

Question 7

Name digestive protein enzymes (peptidases) and what they do

Answer 7

Peptidases cleave peptide bonds.
Endopeptidases cleave internal bonds.
Exopeptidases cleave one amino acid at a time.
Carboxypeptidases cleave at the carboxyl terminal.
Aminopeptidases cleave at the amine terminal.

Question 8

Describe how glycoproteins are made and state what they are including examples.

Answer 8

A compound composed of protein and carbohydrate. They are made by post translational modification whereby a sugar molecule binds via an amino acid to the protein (glycosylation)
Includes immunoglobulins and blood group determinants.

Question 9

State where glycosylation takes place and what the role of it is

Answer 9

It takes place in the Golgi apparatus and ER.

It is used to stabilise the protein, it affects solubility, the protein orientation, signalling and cell recognition.

Question 10

Describe the role of glycoproteins in diabetes.

Answer 10

There is a glycated form of haemoglobin (HbA1C) and this gives a hint to how high blood glucose has been over a period of time. It is better than blood glucose as this fluctuates throughout the day. This form of haemoglobin can be used to diagnose diabetes as diabetics have high levels of it and the higher it is there are increased levels of complications.

Question 11

Describe the structure and function of lipoproteins

Answer 11

Protein and lipids are either covalently or non covalently bonded together. These are used for the transport of water insoluble fats and cholesterol in the blood.
Include High density lipoprotein and Low density density protein.

Question 12

Describe the structure and function metalloproteins

Answer 12

A protein molecule with a bound metal ion.

They are used as enzymes, storage, signalling and transport.

Question 13

What is the function of globular proteins and name some examples

Answer 13

They have a varied function.

Enzymes, hormone messengers, Transporters, stock of amino acids, structural function like actin and Tubulin.

Question 14

What is the function of fibrous proteins and name some examples

Answer 14

They have a structural function.

Bone matrices, muscle fibre, tendons, connective tissue.

Question 15

What is the function of membranous proteins and name some examples

Answer 15

They are associated with cell or organelle membrane.

They relay signals, are membrane transporters, membrane enzymes and cell adhesion molecules.

Question 16

Describe the structure of haemoglobin and name a disease when it goes wrong

Answer 16

At the centre of each subunit is a haem group and at the centre of that is an iron ion. Induces cooperative binding.
If a Thymine substitutes an Adenine, changing a glutamine for a valine, then sickle cell anaemia is present. It changes the property of the haemoglobin (S) and it’s deoxygenated and has abnormally low solubility so forms crystals which changes the shape. This causes severe haemolytic anaemia.

Question 17

Describe collagen’s structure and function

Answer 17

It is a fibrous protein that has a high tensile strength and is composed of the repeat unit (glycine-X-proline)n. The polypeptide coils to form a helix then the 3 polypeptides coil around each other. This is held together by hydrogen bonds.

Question 18

Describe two diseases that can occur when collagen has a defect

Answer 18

Scurvy involves weaker collagen being produced. Vitamin C is needed to convert proline and lysine to hydroxy versions which are needed to stabilise the cross links between the chains.
Osteogenesis imperfecta occurs when a glycine is substituted for a larger amino acid so the protein can no longer fold into a tight coil. There is a loss of secondary and tertiary structure so the collagen is weak and brittle.

Question 19

What is the structure and function of the LDL receptor?

Answer 19

It is present on the surface of all cells and it is a glycoprotein. It causes the internalisation of LDL so it can be broken down in lysosomes.

Question 20

What can go wrong with LDL receptors?

Answer 20

No receptors can be produced. 
Receptors never reach the surface. 
Receptors can't bind the LDL. 
Receptors don't internalise LDL. 
Receptors don't release LDL. 

A tyrosine can be substituted into the chain instead of cysteine which affects the time of internalisation.

Question 21

Name and describe a condition arising from these LDL mutations

Answer 21

Familial hypercholesterolemia is an autosomal dominant disorder with a heterozygous prevalence of 1 in 230. The mutations of LDL receptor gene mean that LDL levels are elevated so cholesterol is deposited in skin, tendons and arteries which can lead to earlier cardiovascular disease (MI). They also get xanthomas which are fatty growths on the skin. Patients found to have the gene get put on statin treatment to reduce LDL levels by 50% and all of their family gets tested.