4 pts – The two soluble globular proteins depicted below are held together by amino acid interactions between one of each of their helixes. Offer 2 possible amino acid [aa] options [provide 1 letter and full names] for each [?1, ?2, and ?3] that you would anticipate to be across from the depicted aa [A, D, Q] – chemically explain your attraction reasoning.

?1, Valine (V), Leucine (L). The amino acid Alanine (A) is hydrophobic and interacts with similar amino acids to form Van der Waal’s interaction with each other.
?2, Lysine (K) and Arginine (R). Amino acid Aspartic Acid (D) is polar, hydrophilic, and negatively charged. It forms hydrogen bonds with positively charged amino acids.
?3, Serine (S) and Threonine (T). Amino acid Glutamine (Q) is polar but uncharged or neutral and it interacts with similar neutral polar amino acids by forming hydrogen bonds.

4 pts – Next, for the circled aa [D], offer at least two different types of amino acid changes [mutation] that might alter/prevent the dimerization of the two proteins – in other words, create a loss of function allele with your change. Explain your reasoning – “why do the aa’s no longer attract each other”!

Amino acid Aspartic acid (D) is coded by two codons GAU and GAC. A mutation that alters the triplet codons can cause the incorporation of a different amino acid which can alter the dimerization of the two proteins. A transversion at the first nucleotide (G) could result in the triplet coding for histidine (CAU or CAC) which is a polar positively charged amino acid that will no longer base pair with positively charged amino acids. In case a transition mutation takes place replacing G at the first codon with A, the triplet codon formed (AAU or AAC) will code for Asparagine, a polar neutral amino acid that does not form a bond with positively charged amino acids.

2pts - The underlined amino acid [S] plays a role in regulating interaction between the two proteins – offer an explanation.

The amino acid serine (S) is a polar neutral amino acid that can adopt conformation making it easier to form hydrogen bonds with different amino acid residues. Hence it can play a vital role in regulating the interaction between these two proteins.

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