Proteins 2

Question 1

Which class of accesory proteins is responsible for catalysing disulfide interchne (shuffling of disulfide bonds until correct pairing is achieved)?

Answer 1

Protein disulfide isomerases

Question 2

Which class of accesory proteins is responsible for catalysing the convertion of X-Pro peptide bonds between trans and cis conformations?

Answer 2

Peptidyl prolyl cis-trans isomerase

Question 3

Which class of accesory proteins prevents improper folding and aggregation of proteins in internal hydrophobic regions?

Answer 3

Molecular chaperones

Question 4

Heat shock proteins, chaperonins, calreticulin and lectins calnexin are examples of which kind of accesory protein?

Answer 4

Molecular chaperones

Question 5

Why do most peptide bonds adopt the trans conformation?

Answer 5

There is less steric repulsion - a lower energy state

Question 6

Why do X-Pro bonds adopt the cis conformation at higher rates?

Answer 6

Difference in steric repulsion between states is lower

Question 7

What is the cause of Alzheimer’s?

Answer 7

Protein misfolding, a small protein aggregates in the brain and kills neurons

Question 8

Prion diseases (like CJD, mad cow) cause what structural change in prion proteins?

Answer 8

Secondary alpha helical structures become beta sheets

Question 9

Why is myoglobin found in high concentrations in skeletal and cardiac muscle cells?

Answer 9

A lot of oyxygen is required to generate the energy for muscle contraction

Question 10

Myoglobin functions as

Answer 10

a store of oxygen that can be used readily

Question 11

How was the 3D structure of myoglobin discovered?

Answer 11

Through the use of X-ray crystallography - 1957 John Kendrew

Question 12

How many chains is hemoglobin made up of?

Answer 12

4

Question 13

How many chains is hemoglobin made up of?

Answer 13

1

Question 14

What is interesting about the fact that hemoglobin chains are similar to myoglobin?

Answer 14

Very different primary sequences can generate very similar structures

Question 15

How many molecules of oxygen can hemoglobin bind?

Answer 15

4

one for each heme group on each polypeptide chain

Question 16

Describe the structure of hemoglobin

Answer 16

4 polypeptide chains are packed tightlyin a tetrahedral array, forming a sphere held together by non covalent interaction

Question 17

What structure is iron held within in the heme prosthetic group?

Answer 17

A protoporyphyrin IX ring structure

Question 18

What are the 6 bonds that iron participates in in heme group?

Answer 18

4 nitrogen atoms of the poryphyrin ring
The proximal histidine residue
Oxygen

Question 19

What are the functions of the distal histidine residue, (which is near the binding site of oxygen on the heme group)?

Answer 19

Reduces affinity for carbon monoxide

Prevents close contact of neighbouring heme groups - which would cause further oxidization of iron

Question 20

Hemoglobin is an allosteric protein - what does this mean?

Answer 20

The binding of oxygen to one subunit causes a conformational change in the other subunits

Question 21

Why is the binding of oxygen to hemoglobin ‘cooperative’?

Answer 21

Binding to one subunit raises the affinity of the other units for oxygen

Question 22

Which protein has a higher affinity for oxygen, myoglobin or hemoglobin?

Answer 22

Myglobin

Question 23

What is the shape of the oxygen dissociation curve for myoglobin?

Answer 23

hyperbolic

Question 24

What is the shape of the oxygen dissociation curve for hemoglobin?

Answer 24

sigmoidal

Question 25

Why is oxygen released from hemoglobin and absorbed by myoglobin in blood capillaries?

Answer 25

Because myoglobin has a higher affinity for oxygen

Question 26

What is oxyhemoglobin?

Answer 26

Hemoglobin with 4 molecules of oxygen bound

Question 27

Binding of oxygen to deoxyhemoglobin causes __ to break and reform

Answer 27

The ionic interactions that hold together the subunits

Question 28

What is the Bohr effect?

Answer 28

The fact that the concentration of protons and caerbon dioxide in surrounding tissue affects the binding of oxygen

Question 29

Concentrations of protons and carbon dioxide are high in metabolically active tissues such as muscle. What happens to the dissociation curve of hemoglobin?

Answer 29

It shifts to the right. The release of oxygen is promoted

Question 30

The conformation of deoxyhemoglobin is preferred over that of oxyhemoglobin when in the presence of high concentrations of which 3 species?

Answer 30

Protons, carbon dioxide and BPG(bisphosphoglycerate)

Question 31

Preference of deoxyhemoglobin conformartion causes the _

Answer 31

Release of oxygen

Question 32

Why do high concentrations of carbon dioxide increase concentrations of protons?

Answer 32

Because of the action of carbonic anhydrase

Question 33

What does carbonic anhydrase catalyse?

Answer 33

The conversion between carbon dioxide + water and carbonic acid + protons + bicarbonate ions

Question 34

Proton binding sites on hemoglobin have _

Answer 34

A higher affinity for protons in the deoxyhemoglobin conformation

Question 35

What is the effect of the presence of BPG in erythrocytes?

Answer 35

It reduces the affinity of hemoglobin for oxygen, thereby promoting its release

Question 36

How does BPG bind with hemoglobin?

Answer 36

It enters the central cvity, forming ionic bonds with positively charged amino acid side chains

Question 37

Molecules that favor deoxyhemoglobin over oxyhemoglobin are examples of _

Answer 37

Allosteric effectors

Question 38

Which highly anionic organic phosphate molecule is present in erythrocytes?

Answer 38

2,3-bisphosphoglycerate

Question 39

Why are the effects of BPG, carbon dioxide and protons additive?

Answer 39

They act at different sites, so are not in competition

Question 40

What is the difference in tructure between HbA and HbF

Answer 40

HbA - 2 alpha chains, 2 beta chains | HbF - 2 alpha chains, 2 gamma chains

Question 41

Why does HbF have a higher affinity for oxygen the HbA?

Answer 41

Because it binds less BPG

Question 42

Higher affinity of HbF for oxygen optimises _

Answer 42

The transfer of oxygen from maternal circulation to fetl circulation across placenta

Question 43

What change in hemoglobin synthesis occurs near birth?

Answer 43

Synthesis of gamma chains is switched off | Synthesis of beta chains is switched on

Question 44

Describe HbS (sickle cell) molecules. What is the effect on cells?

Answer 44

They are long and fibrous, distorting the shape of erythrocytes

Question 45

What is conservative substitution?

Answer 45

The replacement of an amino acid residue by another with similar properties

Question 46

What is the fundamental alteration of hemoglobin molecules that characterises sickle cell anemia?

Answer 46

Non conservative substitution is made of a polar residue with a hydrophobic one

Question 47

Why do homozygotes of sickle cell anemia have lower lifespans?

Answer 47

There is a high risk of infection, cardiac failure etc | Due to the distorted shape of erythrocytes causing blockages in small blood vessels

Question 48

What percentage of erythrocytes are sickle cells for homozygotes and heterozygotes?

Answer 48

50% for homozygotes | 1% for heterozygotes

Question 49

Why are rates of sickle cell anemia high in Africa?

Answer 49

Heterozygotes give protection against the most lethal forms of malaria