Proteins

Question 1

What are collagen fibres

Answer 1

Main component of connective tissues and found in skin, tendons, organs and bone

Question 2

Function of proteins

Answer 2

Provide structure, transport molecules, defence, biological catalysts and regulation of genes

Question 3

What is haemoglobin

Answer 3

Selective delivery of O2 to metabolic tissues. It has 4 protein subunits per molecule. Each subunit contains a haem group that can bind 1 oxygen molecule

Question 4

What is low density lipoprotein

Answer 4

It is composed of a phospholipid shell and a single molecule of apolipoprotein B

Question 5

What is the function of LDL

Answer 5

It is used to transport cholesterol between cells via the circulatory system

Question 6

How is the uptake of LDL particles mediated

Answer 6

By the LDL receptor that binds LDL and facilitates internalisation

Question 7

What is the structure of an antibody

Answer 7

2 identical heavy chains and 2 identical light chains covalently linked by disulphide bonds

Question 8

What is lysozyme

Answer 8

It catalyses the cutting of polysaccharide chains. It binds to the polysaccharide chain, catalyses the cleavage of a specific covalent bond and releases the cleaved products. It remains unchanged at the end of the reaction

Question 9

What is the function of the lac repressor

Answer 9

It controls production of proteins metabolising lactose in bacteria

Question 10

How does the lac repressor work

Answer 10

It binds to DNA and prevents expression of the gene in the absence of lactose

Question 11

What is pKa

Answer 11

It is the pH at which dissociation is 50% complete

Question 12

What influences the charge of an amino acid

Answer 12

pH

Question 13

What does dissociation largely occur over

Answer 13

2 pH units centred on the pKa

Question 14

What is receptor mediated endocytosis

Answer 14

• A reduction in pH in the endosome causes a change in conformation of the LDL receptor due to the presence of histidine residues within the protein
• LDL can no longer bind and is released to the lysosome
• Patients with familial hypercholesterolemia frequently have mutations in the histidine residues of the LDL receptor

Question 15

What is a peptide bond

Answer 15

It is a covalent bond formed when the carbon from the carboxylate group shares electrons with the nitrogen atom from the amino group of a second amino acid

Question 16

What are the constraints on a peptide bond

Answer 16

• It doesn’t permit rotation
• Bulky R-groups are positioned on either side of the backbone which limits the no. of 3D conformations possible for a polypeptide

Question 17

What is secondary structure

Answer 17

It is the initial folding patter of the linear polypeptide. It is stabilised by hydrogen bonds

Question 18

Describe the alpha helix

Answer 18

It is right handed and each turn has 3.6 amino acid residues. It is stabilised by H bonds between amino and carboxyl groups of every 4th amino acid

Question 19

Describe beta sheets

Answer 19

Extended stretches of 5 or more amino acids are called beta strands. They are organised next to each other to make beta sheets. H bonding pattern varies depending on type of sheet

Question 20

Describe the bend/loop structure

Answer 20

Polypeptide chains can fold upon themselves forming a bend or a loop. Usually 4 amino acids are required to form the turn

Question 21

Describe tertiary structure

Answer 21

It is the 3D folding of secondary structure

Question 22

What is a disulfide bond

Answer 22

The SH groups of 2 neighbouring cysteine residues form a covalent -S-S- bond

Question 23

Describe quaternary structure

Answer 23

It is the association of more than one polypeptide. The protein can be called an oligomeric protein (made up of multiple subunits). Disulfide bonds stabilise the oligomeric structure

Question 24

What is the structure of haemoglobin

Answer 24

It is a symmetrical assembly of 2 different subunits (2 alpha globin and 2 beta chains). Each of the polypeptide chains contains a haem molecule, which binds O2

Question 25

What causes sickle cell anaemia

Answer 25

• A single amino acid change at position 6 in the beta chain of haemoglobin
• Hydrophilic glutamic acid to hydrophobic valine
• This causes sickling of erythrocytes due to aggregation of mutated haemoglobin that forms stiff fibres (change in surface chemistry of the protein)

Question 26

What are the effects of pH and CO2 conc on haemoglobin

Answer 26

• O2 binding occurs with higher affinity at high pH (lungs) and lower affinity at low pH (peripheral tissues)
• O2 loading is therefore easier in the lung and unloading/delivery occurs much faster where O2 needed
• CO2 (acidic) builds up during exercise, which lowers blood pH facilitating faster oxygen delivery

Question 27

What is tropocollagen

Answer 27

It is the building block of the collagen fibre and consists of 3 polypeptide chains with a left handed twist wound together in a right handed supercoil

Question 28

What is the significance of glycine

Answer 28

It is vital for the formation of the tropocollagen triple helix as it has a small side chain that allows tight turns

Question 29

What is the significance of proline

Answer 29

It imposes left hand twists in the helix that provides main stabilising force of the structure

Question 30

What is hydroxyproline

Answer 30

It is formed when proline becomes hydroxylated. It forms strong H bonds that help stabilise the triple helix