lecture 35/36 - ice binding proteins Flashcards
what are the two groups of ice binding proteins?
antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs)
what do all AFPs/AFGPs bind?
ice - the same ligand
are AFPs diverse? describe their evolution.
- very diverse grp of structures
- convergent evolution (didn’t evolve from the same source but have the same function)
generally describe the structure of AFPs
mostly repetitive sequences and structures
what is the function of most AFPs?
to slow or stop ice crystal growth by directly interacting with ice
where are these proteins found?
found in fish, plants, insects, fungi, bacteria
what are ice nucleation proteins? what is there function?
- bacteria that infect plants
- nucleate (accelerate) ice growth
explain how AFPs work to slow ice growth
- AFP binds to the ice surface, hindering growth at the site where it is bound
- this results in growth as a cruved surface (instead of straight uniform growth - have indents where AFPs are)
- this is energetically less favourable for ice to continue growing
- results in depression of the freezing point
describe (generally) the first discovered AFP structure
- 37 aa residues
- alpha helix
- no tertiary structure associated with them
- have the sequential repeat: (TAA XAX AAA XX) 3.5 times where X = variable (usually hydrophilic) res
describe the helical wheel of type 1 AFP proteins
- top face was more variable, generally consisting of hydrophilic residues - not as important
- bottom face was more conserved, generally hydrophobic - important for struct/function
how did we test functional importance of type 1 AFPs?
what did we discover? (2)
mutagenesis experiments:
-mutated A17 -> L = inactive protein
-mutated A19 -> L = active protein
suggested that theres an “active face” of the alpha helix
-mutated T -> S = not active
-mutated T -> V = active
suggested the methyl groups of the A and T residues on the active face are the groups that are essential for binding
discuss the methyl residues on the active face of the type 1 AFP alpha helix
- these are conserved residues
- form a ~flat, hydrophobic surface
describe the “intrahelical h-bonding” of type 1 AFPs
what does this do?
- intrahelical h-bonds (i+4) occur btwn amine and carbonyl grps of the backbone stabilizing the protein
- as you get into the inner loop - can have aa with 2 h-bonds within each res (can donate from amine and accept w carbonyl grp)
- BUT first 4 can only h-bond via their c-term
- BUT last 4 can only h-bond via their n-term
what is the first aa residue of type 1 AFPs?
asp
what is the last aa residue of type 1 AFPs?
arg