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FL1 C/P passage 5
Posted: Thu Sep 05, 2019 2:20 pm
Could I get some more clarification on the following questions:
Q22 how do we know the interaction is disulfide bond formation?
Q23 arginine and aspartate are both charged amino acids? I thought groups need to be neutral for aggregation
Q24 based on the diagram the residue is serine?
Re: FL1 C/P passage 5
Posted: Mon Sep 09, 2019 3:05 am
Figure 1 for the structure of compound 1 shows 4 cysteine residues near the aliphatic tail. We are told that compound 1 self-assembles at pH 4 and freely dissolves at pH 8. We know that cysteine can form disulfide bridges under oxidizing conditions, and that these same dimers can be lysed under reducing conditions. That matches quite well with what we are observing for the behavior of compound 1, leading us to believe that in the pH 4 solution disulfide bonds are being formed and in the pH 8 solution disulfide bonds are being broken again.
This is a topic that sometimes gets lost in biochemistry review, but definitely read up on glutathione and GSH / GSSG dimers. While the topic may initially seem niche, disulfide bridges (and thioethers/thioesters) pop up everywhere in biochemistry.
Figure 2 gives one easy hint by indicating which side is facing the cellular environment, seeing as the aliphatic tails are shown aggregating here.
Charged residues will generally be excellent for interaction with cell-surface proteins, but even without trying to make a generalization like this, we can just look at the list of 5 residue types provided and reason by exclusion:
1) is on the far left
2) are our 4 cysteines
3) glycine glycine glycine
4) phosphorylated serine
5) arginine and aspartic acid
Without over-thinking it and going past the scope of the MCAT or engaging in dubious reasoning, one can realize that there are no other options.
Correct, you are told that the residue for attachment to HA is phosphorylated in compound 1, and serine is the only phosphorylated residue present.