Describe the protein structure hierarchy.
Primary: amino acid sequence and disulfide bonds Secondary: local conformations of the backbone (no side chains) Tertiary: full 3D structure (includes side chains) Quaternary: non-covalent association of subunits in a multi-subunit protein
Draw the peptide bond resonance structures.
What are the characteristics of a peptide bond?
40% double bond character = no rotation
Describe and label the two angles that can rotate in a protein.
Describe the bond angle exception created by Proline.
No rotation is possible at phi due to the reduced double bond character. Both cis and trans are possible. Proline breaks the alpha helix structure and leads to sharp turns. Note that the peptide bond can be rotated next to Proline.
Regular secondary structures occur when...
...all amino acids in a chain have the same phi and same psi angles. These structures include alpha helices and beta sheets.
How many amino acids make up a turn in an alpha helix?
How are alpha helices stablized?
Via interchain hydrogen bonds (with O and with NH)
What is an amphipathic structure?
One side of the structure is hydrophilic; the other side is hydrophobic.
Describe a beta pleated sheet.
The backbone is stretched out and partially flattened. Proline can cause turns. Every other carbonyl O points in the opposite direction. Hydrogen bonding occurs to connect strands into sheets. Sheets can be parallel or antiparallel.
Where are charged/polar amino acids located? Where are non-polar amino acids located?
Charged/polar amino acids are hydrophilic and are located on the outside of the protein. Non-polar amino acids are hydrophobic and are located on the inside of the protein.
What are domains?
Domains are building blocks/modules used to construct larger proteins; small globular portion of a protein that folds independently of other domains. Domains typically have a hydrophobic core and a hydrophilic surface. They often carry a specific function, are often spatially isolated/connected by linking polypeptides, and involve covalent attachments.
What is a fold?
A fold is a specific arrangment of the secondary structural elements of a domain.
What is a superfold?
Folds that occur across proteins that are unrelated by evolution, sequence, or function.
What are several examples of folds?
Alpha helix globin folds, alpha beta domain folds (central beta barrel, twisted beta sheet, beta barrel jelly roll)
What is one method that helps to identify the fold of a protein based on its amino acid sequence?
1. Search x-ray/NMR structure databases for a similar sequence; this serves as a template.
2. Perform a sequence alignment and look for identical or similar amino acids.
3. Utilize a computer program to "thread" the sequence through the x-ray structure of the template.
What determines protein folding?
The amino acid sequence
Some DNA mutations are ____ and have no effect on the proteins. Mutations of amino acids involved in enzyme activity or basic structure are ____.
80% of the over 140,000 disease-associated mutations are found in ___, ___, and ___.
Regular secondary strecture, tight turns, and bends.
Describe the forces stabilizing protein structures.
1. Covalent (strongest)
a. H-bonds (common)
b. Ionic bonds/salt bridges (infrequent within, impossible on surface)
c. van der Waals (weak but prevalent)
d. Hydorphobic interactions (hydrophobic effect)
Define van der Waals forces.
van der Waals forces are attractive forces involving shifts in electron density such that one atom is ~ positive and the other is ~ negative. These forces are distant dependent.
Define the hydrophobic effect.
When two hydrophobic side chains interact, they displace water around them. This increases entropy, decreases free energy, and increases stability.
Describe simply the motions of proteins.
Proteins are jiggling due to the constant motion of the atoms. In addition, conformational changes can occur, such as in the reaction between hexokinase and glucose.
What is an example of a fibrous protein?
Describe the structure of collagen.
Three polyproline type II helices, each ~1,000 amino acids long.
What are the characteristics of a polyproline type type II helix?
3 amino acids/turn
Wrap in C --> N direction
Twice as extended as an alpha helix
In Type I colagen, a Gly is found every ___ residues. The second and third residues are typically ___ and ___.
Three; Proline; Hydroxyproline