Blueprint MCAT Full-Length 1: Passage 8 — Metabolism

Session 192

Blueprint MCAT full-length 1, passage 8 poses some tough questions about the metabolism, electrochemistry, enzymes, and free energy.

We’re joined by Phil from Blueprint MCAT, formerly Next Step Test Prep. If you would like to follow along on YouTube, go to premed.tv.

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

[03:33] Passage 8 (Questions 31-34)

Terpenes have been found to be essential building blocks of complex hormones and molecules, pigments, sterols, and even vitamins. Terpenes also play an incredibly important role by providing protection from bacteria and fungus. Terpenes have a basic structure of repeating isoprene, or (C5H8)n, units, and they are grouped according to the number of these repeating units. Monoterpenes contain 2 isoprene units; examples include menthol and pinene. Vitamin A1, a diterpene, contains 4 isoprene units. Research has revealed the Isoprene Rule, which states that adjacent isoprene units in terpenes are linked preferentially between carbon atoms located at opposite ends of the isoprene structural subunit. Head-to-head and tail-to-tail connections are rare exceptions to this rule.

Note: Anything ending in Al is going to be an aldehyde, which is going to be a double bond oxygen, but on the end of a carbon chain. So retinal is a key intermediate of vitamin A biosynthesis, the biosynthetic pathway by which retinal is synthesized in vivo from beta carotene has been identified by using specifically O-18 labeled Oxygen and O-18 labeled water.

We’re given these steps. They’ve added E1 (enzyme one), the second last is aldehyde. We also have retinol which is an alcohol since it’s ending in -ol. Then they give us a little information about what these enzymes are. So we have the betacarotene 1515 monooxygenase, and enzyme two, which is the last step is to retinal dehydrogenase.

[08:19] Question 31

Which of the following reagents could be used to complete the final step of retinol synthesis shown in Figure 2?

(A) LiAlH4

(B) O3

(D) KMnO4

Thought process:

This is a reduction reaction and you need a reducing agent to go from an aldehyde to an alcohol. And knowing that, we need to add hydrogen. Then this gives us a 50/50 chance since we now get to eliminate B and D because they don’t have hydrogens. In fact, they’re oxidizing agents, which is going to take it in the other direction going from the alcohol to the aldehyde.

So we’re looking for a reducing agent, which leaves us with A or C. Beyond that, it’s a little bit tricky and it does require a little bit more specific knowledge. H2 with palladium is a very powerful reducing agent, and it’s going to turn a carbonyl or an aldehyde into an alcohol, but it’s also going to add hydrogens to every single double bond and turn them into single bonds. And so that’s probably not going to work here because our product has 5 double bonds. And so if we used hydrogen and palladium, it’s going to get rid of this, we’re not going to have retinol. At the end of this, we’re going to have some other compound.

The H2 and palladium show up a lot in Organic Chemistry. It’s just the way to get rid of double bonds. And so there’s a good chance a student saw that in Orgo 1 or 2. So that leaves us with A as the answer. A and C are both reducing agents. They’re both going to turn this aldehyde into an alcohol. But just palladium is going to go a little bit farther. It’s far too extreme.

Correct Answer: A

[14:40] Background Knowledge is Important

'The MCAT is a critical thinking test but you also have to have the background knowledge.'Click To Tweet

Sometimes you’ll see a question that in order to answer a detail about kidneys, you need to know about hormones. You need to know about how proteins work as well as individual amino acids. So you have to build on that.

That’s why you need to keep doing practice tests because you’re able to get enough information and start putting things together and making those connections. That explains why a lot of times. students’ scores plateau at first and then sharply increase a little bit later on in their prep. So this is just to remind anyone who is prepping right now and feeling like their scores are not showing up quite yet. As you practice, you’re getting closer so don’t give up.

In undergrad, students oftentimes encounter a question that’s only asking about a single thing. But the MCAT is a bit more difficult because sometimes, you have to apply data and know 2-3 things in order to actually put it together.

[16:58] Question 32

Which of the following biological substances are likely derived from terpenes?

Aldosterone
Glucose

III. Insulin

Estrogen

Thought process:

Glucose is not a hormone so we can throw that one out. Now, there are different kinds of hormones based on what they’re made out of. And so the real key here is just looking at what these molecules look like. They’re all hydrophobic carbons and hydrogens. They’re not really made of anything else, maybe an oxygen in there. And that implies that we’re going to be dealing with a steroid hormone, which is composed of these large hydrophobic molecules – basically aldosterone, testosterone, progesterone, cortisone and cortisol and then estrogen.

So if it ends in -one or it’s like estrogen, then it’s probably a steroid hormone. All the other ones generally lean towards being a peptide hormone. That includes insulin, glucagon, ADH. adrenocorticotrophic releasing hormone, TSH, luteinizing hormone and follicle-stimulating hormone and tons of them. There’s probably dozens, if not hundreds, of them. So just looking at this, it’s a pretty hydrophobic molecule so it’s going to be a steroid hormone such as aldosterone and estrogen.

Correct answer: I and IV

[21:12] Question 33

The enzyme listed in step one of the retinal synthesis listed is most likely classified as a/an:

(A) transferase

(B) lyase

(D) oxidoreductase

Thought Process:

Please check out premed.tv for some visuals on this episode.

Oxidoreductase means it does some form of redox reaction. It could be oxidation or it could be a reduction. Either way, we already added hydrogen in the last step.

Another way to think about that is when removing a bond to oxygen. If you remove a bond to oxygen, that’s a reduction. If you add a bond to oxygen, that’s oxidation.

The reverse of that last step would be an oxidation. And so would the very first step of this reaction because we have this molecule that has no oxygens on it. And then all of a sudden, it’s got to bond to oxygen in that first step. That’s going to be an oxidation reaction, which means this is an oxidoreductase.

Lyase is what’s in the third reaction. It’s like a lace where we cut this molecule basically in half and now we have two retinaldehyde.

Isomerase is not adding stuff. It’s not moving or removing things. It’s just moving stuff around. So if we move carbon from one spot to another, that would be an isomerase. Just switching it to a different isomer of that reaction.

Transferase is just going to add some functional groups like you’re going to add a carboxylic acid or phosphate or sulfate or something weird, which is not the case here.

Correct Answer: D

[26:10] Question 34

A researcher seeks to monitor the conversion of retinal (C=O) to retinol (C-OH) using infrared spectroscopy. Which of the following will indicate the reaction is complete?

(A) The disappearance of a peak in the 3200 to 3500 region

(B) The disappearance of a peak in the 1700 to 1750 region

(D) The appearance of a peak in the 1700 to 1750 region

Thought Process:

You’re definitely going to require some outside knowledge because there’s nothing in the passage about any of these IR peaks.

It’s either losing the carbonyl, losing that aldehyde, or gaining an alcohol. And so it’s got to be one of those two. The most obvious peak that people can identify in an IR spec is the alcohol. That’s this giant, big wide valley somewhere around like 3200 to 3500, which is what they’re going on for A. And the thing is, as this reaction is complete, we’re not losing an alcohol or gaining it.

A carbonyl is C=O and you have to know that that’s around 1700. And so, after knowing that, then the answer must be B in that case. In terms of IR specs, B means alcohols and carbonyls are at the top of the list. Note that those three molecules amines, alcohols and carbonyls are all found in proteins. They’re all found in DNA. Like fatty acids, they have carbonyls and alcohols, they don’t have amines. So those three cover a lot of biological molecules.

And you want to know that an alcohol is a big, wide peak around 3200 to 3500. A carbonyl (C=O) is around 1700. And then amine (CNH), that’s going to be somewhere around 3500, but it’s going to be a really sharp peak, instead of this big wide one.

So if you can just focus on those three and make sure you know them pretty well, then that’s going to cover your bases when it comes to IR spec. It’s actually a lot easier than what students are used to in undergrad.

Correct Answer: B