Lecture 15: Protein structure and function Flashcards
Proteins are..
single, un-branched chains of a.a with numerous diverse functions
Proteins divers functions:
- 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.
catalysis:
enzymes accelerate biochemical reactions
Energy production:
(light harvesting, electron transport, rotary ATPases)
Proteins comprise chains of amino acids joined by
peptide bonds
peptic bonds formation:
Condensation reaction:
Loss of water.
peptide bond:
O
||
(+) -C -N- (-)
|
H2 types of terminus
- Amino ‘N’ terminus
- Carboxyl ‘C’ terminus
The polypeptide protein backbone has
amino acid side chains (can be polar and non polar)
3 types of amino acid
- nonpolar
- polar
- Electrically charged
__ protein amino acids have side chains with ____ properties
20
different
polypeptide backbone is..
identical in all proteins
Side chins determine…
how the polypeptide chains of the protein interact and how the protein is folded
Four levels of structural organisation of proteins
Primary
Secondary
Tertiary
Quaternary
Primary
Linear sequence of amino acids
Secondary:
localized organization of parts of polypeptide chain (e.g. helix or sheet) - using hydrogen bond
Tertiary:
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.
Quaternary:
association of two or more polypeptides into multi‐subunit complex. Rubisco has 16 subunits.
Proteins are held together by
different ionic interactions;
- ionic,
- van der Waals
- hydrogen bonds
- electrostatic interactions
ionic
Atraction between +ve and -ve charged ions:
O- & N+
van der Waals
Short-range weak electrical attraction & repulsion
Hydrogen bonds:
involve a H shared between O and N atoms
alpha helix resembles a
spring with h-bonds joining loop to loop
Beta sheet resembles a
folded piece of paper (fan) H-bonds lie length ways
polypeptide chains can be linked by
covalent bonds (tertiary structure) -forms inter and intrachain disulphide bonds
intrachain disulfide bonds -
between the same chain
interchain disulphide bonds -
between 2 separate chains
Where do
Proteins undergo ________ to enhance their _____ before secretion
In the ER.
Proteins undergo processes such as glycosylation and disulphide bond formation to enhance their stability before secretion.
Glycosylation:
he controlled enzymatic modification of an organic molecule, especially a protein, by addition of a sugar molecule
The ER lumen has a
Distinct environment
Eg. of protein passage through ER
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
Glutathione is a
tripeptide of Gly, Cys and Glu.
Exists i reduced (GSH) and oxidised (GSSG) forms
The OXIDISING redox environment of secretory pathway supports
disulphide bond formation (~ equal concentrations of GSH and GSSG)
In the cytosol (GSH AND GSSG)
[GSH]»_space; [GSSG] so glutathione reduction potential is also reducing.
Secretory pathway involves
ER + Golgi
Mucin is a
Glycosylated protein/ proteoglycan
Man produces ____ of mucus
~ 1 litre/day
Mucin has
Large, extracellular glycoproteins with hundreds of oligosaccharide chains linked to a protein backbone. Either anchored (transmembrane) or secreted
High glycosylation of mucins makes them
resistant to acids (stomach acid), proteolysis, and adds gel-like properties found in mucosal barriers
Examples of animals with mucin
Fish (hagfish), amphibians, corals
Keratin: The formation of disulphide bridges between two cysteines on separate polypeptide chains allows for
the cross-linkage of the chains.
% of amino acids in keratin are cysteine
25
in keratin S-S bonds give
Great stability
Beta keratins:
feathers, beaks & claws composed of beta-pleated sheets, twisted and cross-linked by disulphide bridges
Alpha-helical keratins
mammalian hair, horns and hoof
Beta keratins vs alpha-helical keratins
BETA STRONGER
Sulphurous smell of
burning keratin
Perming hair:
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)
Collagen is the
major extracellular insoluble fibrous protein and most abundant protein in animals. Help tissues withstand stretching.
Helices cannot form in the absence of
scorbic acid (no hydroxyproline formed), hence blood vessels, tendons and skin become fragile, leading to scurvy
Fibroin
Silk
Glycine and alanine in silk
High glycine and less alanine, have small non-polar side chains
In silk what do the small non-polar side chains allow
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
Spinning a silk web:
Where is it stored?
Silk protein stored as an emulsion. C‐ terminus ensures solubility, core is hydrophobic
Spinning a silk web: What leads to silk emulsion unfolding
change in the environment
Spinning a silk web: The thread leaves the ____ through the ____ which…
SPINNERET
SPIGOT
…strips off residual water. The molecules are stretched out and linked together to form long strands
Spinning a silk web:
Molecule movement
Water and Na+ leave lumen; K+, surfactants and lubricants enter, pH falls from 7.6 to 5.7
Spinning a silk web: Folded conformation in aqueous environment
- hydrophobic core region contains non polar side chains
- Polar side chains on the outside of the molecule can form hydrogen bonds to water
Spinning a silk web:
unfolded polypeptide
long unfolded polypeptide with polar and non-polar side chains distributed along it
Making bread:
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).
Making bread: mechanical work:
As mechanical work stretches the dough, more hydrogen bonds can form between chains of gluten subunits
Sickle cell anaemia - is a ____ mutation. what happens?
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).
HbS =
The sickle cell genes make the body produce abnormal haemoglobin called HbS
Sickle cell anaemia: What happens during deoxygenation (loos of O2)
hydrophobic residues rapidly associate. Generates rigid fibres of HbS
Is polymerisation of HbS reversible?
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.
% of birth in sub-Saharan Africa with sickle cell anaemia
0.74
