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APS 138: Molecular and cell biology > Lecture 15: Protein structure and function > Flashcards

Lecture 15: Protein structure and function Flashcards

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1
Q

Proteins are..

A

single, un-branched chains of a.a with numerous diverse functions

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2
Q

Proteins divers functions:

A
  • Catalysis
  • Energy production
  • Host/pathogen interaction: antibodies, mucus
  • Structural: keratin, fibroin, collagen
  • Motion (cytoskeleton, muscle, flagella)
  • Organisation of DNA and regulation of transcription (histones)
  • Regulation (e.g. kinases)
  • Storage (seeds, eggs etc)
  • Toxins and venoms
  • Transport: across membranes, haemoglobin.
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3
Q

catalysis:

A

enzymes accelerate biochemical reactions

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4
Q

Energy production:

A

(light harvesting, electron transport, rotary ATPases)

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5
Q

Proteins comprise chains of amino acids joined by

A

peptide bonds

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6
Q

peptic bonds formation:

A

Condensation reaction:

Loss of water.

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7
Q

peptide bond:

A 
O
       || 
(+) -C -N-  (-)
       |
      H
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8
Q

2 types of terminus

A
  • Amino ‘N’ terminus

- Carboxyl ‘C’ terminus

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9
Q

The polypeptide protein backbone has

A

amino acid side chains (can be polar and non polar)

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10
Q

3 types of amino acid

A
  • nonpolar
  • polar
  • Electrically charged
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11
Q

__ protein amino acids have side chains with ____ properties

A

20

different

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12
Q

polypeptide backbone is..

A

identical in all proteins

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13
Q

Side chins determine…

A

how the polypeptide chains of the protein interact and how the protein is folded

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14
Q

Four levels of structural organisation of proteins

A

Primary
Secondary
Tertiary
Quaternary

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15
Q

Primary

A

Linear sequence of amino acids

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16
Q

Secondary:

A

localized organization of parts of polypeptide chain (e.g.  helix or  sheet) - using hydrogen bond

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17
Q

Tertiary:

A

three‐dimensional arrangement of polypeptide chain. Non‐polar amino acids typically inside and polar side chains on outside. Stabilised by H‐bonds and disulphide bonds.

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18
Q

Quaternary:

A

association of two or more polypeptides into multi‐subunit complex. Rubisco has 16 subunits.

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19
Q

Proteins are held together by

A

different ionic interactions;

  • ionic,
  • van der Waals
  • hydrogen bonds
  • electrostatic interactions
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20
Q

ionic

A

Atraction between +ve and -ve charged ions:

O- & N+

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21
Q

van der Waals

A

Short-range weak electrical attraction & repulsion

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22
Q

Hydrogen bonds:

A

involve a H shared between O and N atoms

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23
Q

alpha helix resembles a

A

spring with h-bonds joining loop to loop

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24
Q

Beta sheet resembles a

A

folded piece of paper (fan) H-bonds lie length ways

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25
Q

polypeptide chains can be linked by

A 
covalent bonds (tertiary structure) 
-forms inter and intrachain disulphide bonds
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26
Q

intrachain disulfide bonds -

A

between the same chain

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27
Q

interchain disulphide bonds -

A

between 2 separate chains

28
Q

Where do

Proteins undergo ________ to enhance their _____ before secretion

A

In the ER.
Proteins undergo processes such as glycosylation and disulphide bond formation to enhance their stability before secretion.

29
Q

Glycosylation:

A

he controlled enzymatic modification of an organic molecule, especially a protein, by addition of a sugar molecule

30
Q

The ER lumen has a

A

Distinct environment

31
Q

Eg. of protein passage through ER

A

1) polypeptide enters rough ER, brought by ribosome.
2) Sugar chains attached = glycoprotein
3) Transport vesicle buds off
4) Secretory (glyco) protein inside transport vesicle

32
Q

Glutathione is a

A

tripeptide of Gly, Cys and Glu.

Exists i reduced (GSH) and oxidised (GSSG) forms

33
Q

The OXIDISING redox environment of secretory pathway supports

A

disulphide bond formation (~ equal concentrations of GSH and GSSG)

34
Q

In the cytosol (GSH AND GSSG)

A

[GSH]&raquo_space; [GSSG] so glutathione reduction potential is also reducing.

35
Q

Secretory pathway involves

A

ER + Golgi

36
Q

Mucin is a

A

Glycosylated protein/ proteoglycan

37
Q

Man produces ____ of mucus

A

~ 1 litre/day

38
Q

Mucin has

A

Large, extracellular glycoproteins with hundreds of oligosaccharide chains linked to a protein backbone. Either anchored (transmembrane) or secreted

39
Q

High glycosylation of mucins makes them

A

resistant to acids (stomach acid), proteolysis, and adds gel-like properties found in mucosal barriers

40
Q

Examples of animals with mucin

A

Fish (hagfish), amphibians, corals

41
Q

Keratin: The formation of disulphide bridges between two cysteines on separate polypeptide chains allows for

A

the cross-linkage of the chains.

42
Q

% of amino acids in keratin are cysteine

A

25

43
Q

in keratin S-S bonds give

A

Great stability

44
Q

Beta keratins:

A

feathers, beaks & claws composed of beta-pleated sheets, twisted and cross-linked by disulphide bridges

45
Q

Alpha-helical keratins

A

mammalian hair, horns and hoof

46
Q

Beta keratins vs alpha-helical keratins

A

BETA STRONGER

47
Q

Sulphurous smell of

A

burning keratin

48
Q

Perming hair:

A

example of how secondary & tertiary structure of a protein can be modified.

1) natural hair
2) distorted in roller
3) heat/ reducing agent, ammonium thioglycolate added
4) S-S linkages broken (by reduction)
5) add H2O2
6) New S-S linkages form (by oxidation)

49
Q

Collagen is the

A

major extracellular insoluble fibrous protein and most abundant protein in animals. Help tissues withstand stretching.

50
Q

Helices cannot form in the absence of

A

scorbic acid (no hydroxyproline formed), hence blood vessels, tendons and skin become fragile, leading to scurvy

51
Q

Fibroin

A

Silk

52
Q

Glycine and alanine in silk

A

High glycine and less alanine, have small non-polar side chains

53
Q

In silk what do the small non-polar side chains allow

A

tight packing of the antiparallel β sheets, which contributes to silk’s rigid structure and tensile strength, comparable to that of high‐grade alloy steel, although density is ~6 times less

54
Q

Spinning a silk web:

Where is it stored?

A

Silk protein stored as an emulsion. C‐ terminus ensures solubility, core is hydrophobic

55
Q

Spinning a silk web: What leads to silk emulsion unfolding

A

change in the environment

56
Q

Spinning a silk web: The thread leaves the ____ through the ____ which…

A

SPINNERET
SPIGOT
…strips off residual water. The molecules are stretched out and linked together to form long strands

57
Q

Spinning a silk web:

Molecule movement

A

Water and Na+ leave lumen; K+, surfactants and lubricants enter, pH falls from 7.6 to 5.7

58
Q

Spinning a silk web: Folded conformation in aqueous environment

A
  • hydrophobic core region contains non polar side chains

- Polar side chains on the outside of the molecule can form hydrogen bonds to water

59
Q

Spinning a silk web:

unfolded polypeptide

A

long unfolded polypeptide with polar and non-polar side chains distributed along it

60
Q

Making bread:

A

Protein (gluten) is key constituent of wheat flour (~12% protein). Gluten forms a matrix trapping CO2 bubbles. Gluten proteins linked by hydrogen bonds and disulphide bridges (broken by adding ascorbate).

61
Q

Making bread: mechanical work:

A

As mechanical work stretches the dough, more hydrogen bonds can form between chains of gluten subunits

62
Q

Sickle cell anaemia - is a ____ mutation. what happens?

A

Single muta on (GAG → GTG) replaces Glu6 (polar, hydrophilic) with Val6 (nonpolar, strongly hydrophobic) in the β‐ globin chain of haemoglobin (a single‐nucleotide polymorphism).

63
Q

HbS =

A

The sickle cell genes make the body produce abnormal haemoglobin called HbS

64
Q

Sickle cell anaemia: What happens during deoxygenation (loos of O2)

A

hydrophobic residues rapidly associate. Generates rigid fibres of HbS

65
Q

Is polymerisation of HbS reversible?

A

YES
fibres ‘melt’ as O2 is taken up and the normal discoid shape returns. Sickling/unsickling cycles lead to damage to the cytoskeleton and erythrocyte membrane, resulting in sickle‐shaped cells and anaemia.

66
Q

% of birth in sub-Saharan Africa with sickle cell anaemia

A

0.74

APS 138: Molecular and cell biology (15 decks)

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