Passage 9 covers isolating a specific drug, amino acids and Poiseuille’s law. Once again, Bryan Schnedeker of Next Step Test Prep joins us to help us break down these questions so you can come prepared for the MCAT and be ready to rock it!
[01:45] Passage 9
Drug X is designed to assist the body with the rate limiting step of glycogenolysis. The drug molecule is 560 amino acids long and weighs 65 kDa. The drug can exist as an inactive monomer but is only pharmaceutically active as a dimer, with two matching sub-units. It’s shown to be effective with glycogen but not cellulose. X-ray crystallography has identified four biologically significant sites on the active form – catalytic site, allosteric site, glycogen site, and a phosphorylation site. Glycogen is bound to the glycogen site prior to the initiation of terminal glucose cleavage.
Further analysis shows that the phosphorylation site and the allosteric site are used in vivo for the regulation of drug X’s activity. For example, secretion of epinephrine, a potent vasoconstrictor cause phosphorylation of the inactive monomer transforming it into the active dimer.
It is hypothesized that a single enzyme phosphorylase kinase (PK) is responsible for this transformation. The proposed pathway is outlined in Figure 1 structurally, PK is polymer with four identical subunits. PK is activated by intracellular calcium levels when calcium exceeds 10–6 molar, PK binds calcium and is activated.
[04:06] Question 49
Several attempts were made to isolate the active form of Drug X from a tissue sample. All but one test failed to yield a pure extract. Based on the information presented in the passage, this successful test was due to:
- (A) Excessive centrifugation of speeds that caused the breakdown of drug PK
- (B) Elevated pH levels that caused the hydrolysis of peptide bonds in PK
- (C) Calcium contamination that led to the phosphorylation of drug X monomer
- (D) Alcohol contamination that denatured the drug X monomer
Just understand the question. In this case, the passage says that to be active, it has to get phosphorylated. Active means phosphorylated. But to get Drug X phosphorylated, you’re told in the second paragraph that calcium concentrations have to be high. So calcium activates PK and PK phosphorylates Drug X. It’s like a three step process here. So the correct answer here is C.
Students could be thrown off by the word “contamination”. And the question may seem unusual, since it says “all but one test failed.” So what happened in the successful test? We’re not used to the word “contamination” being associated with “success” but that’s exactly what happened. There was contamination that allowed us to successfully activate the drug and purify it.
[07:40] Question 50
Which of the following amino acids could a transaminase not possibly transform into a beta amino acid?
- (A) ARG
- (B) PRO
- (C) TYR
- (D) GLY
You obviously have to know your three-letter abbreviations for these various amino acids and which one could not possibly become a beta amino acid.
If you don’t know the answer to this, don’t spend a ton of time on it. Just pick and go. When in doubt with amino acids, go for Proline (PRO) because it’s weird and different since its side chain connects back to its own amino group. In that regard, proline is unique. However, it’s not the right answer here. So the rule of thumb doesn’t work here.
In this case, the key to the question is knowing what a beta amino acid is. The biological amino acids are alpha amino acids. So to have a beta amino acid, you have to have one more carbon, that beta carbon that is just one place over from the carbonyl carbon. So the alpha is the next door neighbor to carbonyl beta, two doors down. So in this case, to not be physically capable of becoming a beta amino acid, you simply pick the amino acid that doesn’t even have a beta carbon. So the answer here is D, since Glycine is the simplest amino acid, where its side chain is just a hydrogen atom. It doesn’t have a beta carbon so it can’t possibly be altered into a beta amino acid.
Rule of thumb, too, on amino acids, if you have no idea what’s going on, Glycine is the other special one. It’s the smallest, the simplest, it’s the only achiral one. With this for instance where proline and glycine are both choices, flip a coin.
[10:50] Question 51
After administration of epinephrine, the radius of the abdominal aorta is cut by one-third. According to Poiseuille’s Law, if the blood pressure is constant across the length of the vessel, blood flow through the abdominal aorta will:
- (A) Decrease to two-thirds to the fourth power of its previous value
- (B) Decrease to one-third to the fourth power of its previous value
- (C) Increase by three-halves to the fourth power of its previous value
- (D) Increase to three times its previous value
Since it says the aorta is cut to 1/3. It means it’s constricting the aorta. And when it comes to flow, you always want to flow through a fatter pipe. You want more room. Whether a current through a wire or water through a pipe, if you constrict it down, it becomes much harder to have flow.
The real world example of this is when you take a coffee stirrer and try to drink your soda through that. You can’t, obviously, because of its tiny radius.
Back to this question, since your cutting the aorta down, you’re getting less blood flow through it. So it has to be A or B here. If you’re going through a smaller pipe, you’re going to be faster, which means less pressure. But this is about blood flow and so if it’s thinner, you’re not going to get as many through.
In fact, you actually don’t have to know Poiseuille’s Law here because answer choices are both to the fourth power so they both tell us that whatever the equation is, the relationship between radius and blood flow is of fourth power.
Bryan explains this is more of an English language question where what does “cut by 1/3” mean? So to be cut by one-third means you are decreasing two-thirds your original value. So it’s A.
If you log into the Next Step Test Prep platform and after you finish the test, you get to see a little bar graphs of what everybody picks and answer choice B is a popular pick (at around 35-40%).
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