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Biomolecules > Proteins 2 > Flashcards

Proteins 2 Flashcards

1
Q

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

A

Protein disulfide isomerases

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2
Q

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

A

Peptidyl prolyl cis-trans isomerase

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3
Q

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

A

Molecular chaperones

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4
Q

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

A

Molecular chaperones

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5
Q

Why do most peptide bonds adopt the trans conformation?

A

There is less steric repulsion - a lower energy state

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6
Q

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

A

Difference in steric repulsion between states is lower

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7
Q

What is the cause of Alzheimer’s?

A

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

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8
Q

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

A

Secondary alpha helical structures become beta sheets

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9
Q

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

A

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

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10
Q

Myoglobin functions as

A

a store of oxygen that can be used readily

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11
Q

How was the 3D structure of myoglobin discovered?

A

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

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12
Q

How many chains is hemoglobin made up of?

A

4

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13
Q

How many chains is hemoglobin made up of?

A

1

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14
Q

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

A

Very different primary sequences can generate very similar structures

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15
Q

How many molecules of oxygen can hemoglobin bind?

A

4

one for each heme group on each polypeptide chain

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16
Q

Describe the structure of hemoglobin

A

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

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17
Q

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

A

A protoporyphyrin IX ring structure

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18
Q

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

A

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

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19
Q

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

A

Reduces affinity for carbon monoxide

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

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20
Q

Hemoglobin is an allosteric protein - what does this mean?

A

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

21
Q

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

A

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

22
Q

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

A

Myglobin

23
Q

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

A

hyperbolic

24
Q

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

A

sigmoidal

25
Q

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

A

Because myoglobin has a higher affinity for oxygen

26
Q

What is oxyhemoglobin?

A

Hemoglobin with 4 molecules of oxygen bound

27
Q

Binding of oxygen to deoxyhemoglobin causes __ to break and reform

A

The ionic interactions that hold together the subunits

28
Q

What is the Bohr effect?

A

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

29
Q

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

A

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

30
Q

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

A

Protons, carbon dioxide and BPG(bisphosphoglycerate)

31
Q

Preference of deoxyhemoglobin conformartion causes the _

A

Release of oxygen

32
Q

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

A

Because of the action of carbonic anhydrase

33
Q

What does carbonic anhydrase catalyse?

A

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

34
Q

Proton binding sites on hemoglobin have _

A

A higher affinity for protons in the deoxyhemoglobin conformation

35
Q

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

A

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

36
Q

How does BPG bind with hemoglobin?

A

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

37
Q

Molecules that favor deoxyhemoglobin over oxyhemoglobin are examples of _

A

Allosteric effectors

38
Q

Which highly anionic organic phosphate molecule is present in erythrocytes?

A

2,3-bisphosphoglycerate

39
Q

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

A

They act at different sites, so are not in competition

40
Q

What is the difference in tructure between HbA and HbF

A

HbA - 2 alpha chains, 2 beta chains

HbF - 2 alpha chains, 2 gamma chains

41
Q

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

A

Because it binds less BPG

42
Q

Higher affinity of HbF for oxygen optimises _

A

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

43
Q

What change in hemoglobin synthesis occurs near birth?

A

Synthesis of gamma chains is switched off

Synthesis of beta chains is switched on

44
Q

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

A

They are long and fibrous, distorting the shape of erythrocytes

45
Q

What is conservative substitution?

A

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

46
Q

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

A

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

47
Q

Why do homozygotes of sickle cell anemia have lower lifespans?

A

There is a high risk of infection, cardiac failure etc

Due to the distorted shape of erythrocytes causing blockages in small blood vessels

48
Q

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

A

50% for homozygotes

1% for heterozygotes

49
Q

Why are rates of sickle cell anemia high in Africa?

A

Heterozygotes give protection against the most lethal forms of malaria

Biomolecules (12 decks)

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