Flashcards in Cell and Molecular bio Deck (51)
What is the structure of an alpha helix?
Secondary structure of a protein, in which every 3.6 AA's a hydrogen bond forms between N-H group and C-O, creating a coil structure, turning every 3.6AA's
What is the structure of a Beta sheet?
Hydrogen bonds between adjacent AA make up the backbone, with the AA side chains giving polarity. (aromatic AA normally found in the middle of the backbone)
The sheets are lined up either parallel or anti by hydrogen bonds between C-O and N-H groups on different backbones.
What are aromatic AA's?
An amino acid that contains an aromatic ring e.g. Try, Phe, Trp, Val, Ile
Sequences at the C terminus and N terminus of proteins?
C terminus- COOH end
N terminus- NH2 start
MOst AA's prefer a ... secondary structure except for .... and ...
Proline and glycine
Hydrogen bonds between what? How?
An H shared between O and N atoms, where O is negative and N positive, so where the electrode is on the right will attract the positive N (simplified)
Attraction between positive and negative atoms.
Weak Van der Walls bonding?
Short range hydrophobic reactions, e.g. by polarity across atom, where electron one end and nucleus the other.
Chemical link between adjacent cystines, very strong even under heat
When folding a protein what side chains are folded inside and which are exposed? Why?
Hydrophobic hidden inside, and hydrophilic outside- they can form H bonds with the aqueous solution around.
An example of a quaternary structure?
2Alpha, and 2Bsubunits in haemoglobin.
What is tertiary structure of a protein?
The way the alpha helicies and beta sheets interract e.g. helix coil helix, random coil etc.
What is Edman degradation?
Add PITC to the polypeptide chain, and will bind to the terminal AA and cleave it off under acidic conditions. This AA bound forms a PTH complex, which can be run on gel electrophoresis or chromotography (HIgh performance Lipid Chromotography). The molecular weight can then be calculated by taking away the PITC. Then move onto the next AA etc, gradually sequencing
Drawback to the Edman Degradation method?
Polypeptides can't be longer than around 30 residues to accuritely sequence, but proteins could be cleaved into smaller fragments.
Can't if the N terminus is altered e.g. Acetylation
Word meaning molecule having both hydrophobic and hydrophilic parts?
After found the primary sequence what next as prediction?
Can look for patterns and predict bonding to work out secondary structure, and even programmes online can submit the sequence of AA's and it will find simialr proteins/ predict structures.
4 methods to determine a protein secondary structure accuritely?
CD- circular Dichroism
Nuclear magnetic resonance
What is Circular Dichroism? How?
Put the protein in solution and shine circular polarised light through to see the absorbtion.
Alha helices, B sheets and random coils all have characteristic shapes on the CD spectrum, so can show what percentage of protein is each but not arrangment.
Another use of Circular Dichroism?
After heating a protein and causing it to unfold, CD can be repeated at different temperatures to determine the stability of the structures by how easily they denature and if cool whether refold back.
3 advantages and one disadvantage to circular dichroism?
+No limitation on size of protein
- Low resolution
Nuclear magnetic resonance use and method?
Used to understand protein dynamics.
Pure protein labelled with isotopes which are introduced into proteins by recomination in bacteria as are grown on 15N or 13C.
The spin of particles is pared so there is no spin of the nucleus, however due to uneven protons in isotope there is a slight wobble.
Depending on the local environment the protons resonate at different frequences (chemical shift)
One advantage and three disadvantages of NMR Spectroscopy?
- Very expensive
+ Good resolution 2A
- Size limited to 50kDa
-Quick but data analysis slow
What is Xray Crystallography? Method?
First crystallise the pure protein, then an xray beam is fired at the protein. Most of the xray goes straight through but some is deflected giving a refraction pattern. From this the diffraction can be traced back to the protein structure.
Two advantage and one disadvantage of Xray Crystallography?
- Very expensive, set up £300million and £200 thousand every time used
+crystalisation slow but analysis fast
+ HIgh resolution 1A of lower.
Transmission electron Microscopy use? Method?
useful for large proteins structures.
1. Put protein on screen and dry
2. Add heavy metal (negative stain) e.g. lead
3. View down EM.
what is Cryo EM?
Liquid Nitrogen is used to freeze the specimen, and the difference in electron density is used to visualise the protein.
How can SDS PAGE be used to show protein- protein interractions?
When run, the two differenent proteins will have different molecular weights so will run at different speeds therefore producing different bands.
How do Leucine zippers interract with DNA?
2 alpha helices with leucine hydrophobic side chains which bind the two helices together. They create an X shape which straddles the DNA major groove and contact DNA bases with hydrogen bonds.
What is the structure of a zinc finger?
Zinc in the centre interracts with 2 histadines of the coiled alpha helix, and 2 cystines of the B sheet.