Protein Structure

Question 1

Describe primary protein structure.

Answer 1

• Linear sequence of amino acid residues
• Joined by peptide bonds
• Synthesis begins at N-terminus and ends at C-terminus
• Forms hydrogen bonds - water soluble

Question 2

Describe secondary protein structure.

Answer 2

• Forms alpha-helices and beta sheets
• Alpha-helices held by hydrogen bonds between C=O and N-H
• Beta sheets - repeated hydrogen bonds between adjacent polypeptide strands. Can be parallel/antiparallel.

Question 3

Describe tertiary protein structure.

Answer 3

• Folding into domains
• Determined by side chain interactions - ionic, hydrogen, disuplhide bonds etc.
• Globular protein folding - hydroophobic core - polar amino acids present as hydrophilic shell

Question 4

Define quaternary protein structure.

Answer 4

• Interactions between multiple polypeptide subunits
• Held by noncovalent interactiolns e.g ionic/hydrogen bonding
• Increases protein stability/aids in protein function/ difficult to unfold

Question 5

Describe sickle cell anaemia

Answer 5

• Substitution of glutamic acid in position 6 of beta chain to valine
• Long fibres form in Hb - distort biconcave RBCs and form sickle shape to clog arteries
• Leads to hypoxia/tissue damage

Question 6

Describe globular proteins

Answer 6

• Folding of polypeptide chains into spherical shape
• Water soluble
• Have catalytic/regulatory functions

Question 7

Describe fibrous proteins

Answer 7

• Arrangement of polypeptides in long strands
• Water insoluble
• Structural proteins

Question 8

Describe secondary and tertiary structures of haemoglobin

Answer 8

• SECONDARY - forms alpha helices
• TERTIARY - alpha-helical chains connected by segments and stabilised by H-bonds
• TERTIARY - folds into globin chain containing haem group. Hydrophobic core protects haem group.

Question 9

Describe quaternary structure of Haemoglobin.

Answer 9

• Tetramer of globin subunits
• Two alpha2 and two beta2 subunits
• Binds 4 O2 molecules

Question 10

What is collagen?

Answer 10

• Found in connective tissues
• Packed together in long fibrils

Question 11

How are collagen fibres formed?

Answer 11

• Repeating glycine and proline amino acids form helix
• Three helical strands combine to form triple helix
• Triple helices bundled into collagen fibrils

Question 12

What are chaperonins?

Answer 12

• Bind to and stabilise exposed hydrophobic regions in nascent polypeptides
• Prevents premature folding

Question 13

What is protein denaturation and what causes it?

Answer 13

• Irreversible unfolding of secondary and tertiary structures
• No hydrolysis of peptide bonds/disrupts hydrogen and ionic bonds
• Caused by changes in pH and heat

Question 13

What is the effect of increasing salt concentration?

Answer 13

• Increases solubility
• Reduce electrostatic interactions with neighbouring proteins

Question 14

What happens when the following occur:
- pH=pI
- pH<pI
- pH>pI

Answer 14

• Protein has both +/- charges - form salt bonds - bond protein molecules into aggregates
• POSITIVE charge
• NEGATIVE charge

Question 15

How can proteins be separated?

Answer 15

• Charge (electrophoresis)
• Molecular weight (dialysis)

Question 16

How can protein misfolding cause disease?

Answer 16

• Misfolded proteins aggregate to form amyloids
• Amyloids insoluble in water and harms tissues/damage organs where it deposits and accumulates in extracellular space
• If they form in brain, neurodegenerative disease

Question 17

Describe Alzheimer’s Disease. PART 1

Answer 17

• Amyloid beta formed by proteolytic cleavage of APP
• Beta- and gamma-secretase cleaves polypeptide, causing intra/extracellular amyloid beta

Question 18

Describe Alzheimer’s Disease. PART 2

Answer 18

• Extracellular peptides form into parallel beta sheets, aggregate into amyloid fibres, form plaques
• Amyloid plaques deposit in brains destroying nerve cells