BR Bio I (Passage 13 Q83 and Q84)

[Synapsis]

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In this passage the questions asks, "Which of the following is true of Trp (Tryptophan)?"

I. It's a neurotransmitter
II. It's an essential amino acid.
III. It's an acidic amino acid.

The answer was B, which is "II only". I have two questions. I know it's not a neurotransmitter but...

1. The passage doesn't tell us whether or not it's an essential amino acid, so that must have been a memory thing. Should we be memorizing the essential amino acids?

2. Here's the structure of Trp as given in the passage. The answer explanation says that since Trp is neutral (no charge), it isn't acidic. I kind of agree, but when I look at that structure they gave us, I see the carboxylic acid group. And we know from organic chemistry that carboxylic acids have a pKa of roughly 5. I said it was acidic. What's wrong with my thinking?


Now for Question 84, it says that researchers inhibit an enzyme involved in the two step conversion of Trp to serotonin synthesis in the brain. What can be used to estimate the synthesis rate of serotonin?

A. Accumulation of an intermediate metabolite in the brain
B. Concentration of Trp in blood.
C. Concentration of the inhibitor in the brain.
D. Accumulation of the inhibitor in the blood.

The answer was A. I understand why answers C and D could be eliminated easily, but why B? The explanation says that the intermediate is not being fully processed, which, I understand, can give us some indication of the rate of synthesis of serotonin. But if the intermediate comes from Trp, then can't you say the same thing for choice B? Wouldn't the accumulation of intermediate be proportional to the decrease in concentration of Trp, and both of those could give estimates to the rate of synthesis of serotonin?

Thanks!

 

[dcamkl1]

In this passage the questions asks, "Which of the following is true of Trp (Tryptophan)?"

I. It's a neurotransmitter
II. It's an essential amino acid.
III. It's an acidic amino acid.

The answer was B, which is "II only". I have two questions. I know it's not a neurotransmitter but...

1. The passage doesn't tell us whether or not it's an essential amino acid, so that must have been a memory thing. Should we be memorizing the essential amino acids?

2. Here's the structure of Trp as given in the passage. The answer explanation says that since Trp is neutral (no charge), it isn't acidic. I kind of agree, but when I look at that structure they gave us, I see the carboxylic acid group. And we know from organic chemistry that carboxylic acids have a pKa of roughly 5. I said it was acidic. What's wrong with my thinking?

Amino acids all have an amino group and a carboxyl group, but what gives them their unique characteristics is their -R group. The R group in tryptophan is the double ring with the nitrogen. The nitrogen has a lone pair, which makes it basic.

I'm not sure if you need to memorize essential amino acids. Can someone chime in?

Now for Question 84, it says that researchers inhibit an enzyme involved in the two step conversion of Trp to serotonin synthesis in the brain. What can be used to estimate the synthesis rate of serotonin?

A. Accumulation of an intermediate metabolite in the brain
B. Concentration of Trp in blood.
C. Concentration of the inhibitor in the brain.
D. Accumulation of the inhibitor in the blood.

The answer was A. I understand why answers C and D could be eliminated easily, but why B? The explanation says that the intermediate is not being fully processed, which, I understand, can give us some indication of the rate of synthesis of serotonin. But if the intermediate comes from Trp, then can't you say the same thing for choice B? Wouldn't the accumulation of intermediate be proportional to the decrease in concentration of Trp, and both of those could give estimates to the rate of synthesis of serotonin?

Correct me if I'm wrong, but in most reactions, creation of the intermediate is often the rate-determining step. So determining the concentration of the intermediate may be the better way to determine the synthetic rate.

 

[Synapsis]

Amino acids all have an amino group and a carboxyl group, but what gives them their unique characteristics is their -R group. The R group in tryptophan is the double ring with the nitrogen. The nitrogen has a lone pair, which makes it basic.

I'm not sure if you need to memorize essential amino acids. Can someone chime in?




Correct me if I'm wrong, but in most reactions, creation of the intermediate is often the rate-determining step. So determining the concentration of the intermediate may be the better way to determine the synthetic rate.

Ah right. I had completely overlooked the fact that R groups are the only difference among amino acids. Thanks!

I still don't quite understand the second one. In GChem we usually write the RDS as the slow step.

 

[milski]

Amino acids all have an amino group and a carboxyl group, but what gives them their unique characteristics is their -R group. The R group in tryptophan is the double ring with the nitrogen. The nitrogen has a lone pair, which makes it basic.

I'm not sure if you need to memorize essential amino acids. Can someone chime in?

It's hard to come with ultimate answer, since no one has seen all the test. Based on the feel that I got from practice+real tests, I would not rule out a discrete which asks which one of the following is/is not essential amino acid. I am a lot more confident in saying that I would not bother memorizing their structures.

 

[dcamkl1]

I still don't quite understand the second one. In GChem we usually write the RDS as the slow step.

Since it's a two-step reaction, I think about the SN1 reaction as an analogy. Creating the carbocation intermediate is the slowest step, thus it is the rate-determining step (RDS) and the RDS is what mostly determines the overall reaction rate.

In your BR question, the two-step reaction is:

Tryptophan ------> [intermediate] ------> Serotonin

If you remember a typical reaction coordinate, intermediates are like transition states that are high in energy. Since they are high in energy, they require a big activation energy to proceed, which means making the intermediate is the slow(er) step.

Hope that helps?

 

[Synapsis]

Since it's a two-step reaction, I think about the SN1 reaction as an analogy. Creating the carbocation intermediate is the slowest step, thus it is the rate-determining step (RDS) and the RDS is what mostly determines the overall reaction rate.

In your BR question, the two-step reaction is:

Tryptophan ------> [intermediate] ------> Serotonin

If you remember a typical reaction coordinate, intermediates are like transition states that are high in energy. Since they are high in energy, they require a big activation energy to proceed, which means making the intermediate is the slow(er) step.

Hope that helps?

It does a bit, but it's still not crystal clear. I completely see A as a fair answer choice, I'm just struggling to see why B isn't just as fair. If one molecule of Trp is converted to an intermediate, which is converted to serotonin, then that means the Trp loss and intermediate formation have a 1:1 correlation. So it just seems that the rate that we lose Trp should be just as useful a tool as the accumulation of intermediate. Sorry if I'm moving us in circles, it's really this point that I'm stuck on.

 

[dcamkl1]

It does a bit, but it's still not crystal clear. I completely see A as a fair answer choice, I'm just struggling to see why B isn't just as fair. If one molecule of Trp is converted to an intermediate, which is converted to serotonin, then that means the Trp loss and intermediate formation have a 1:1 correlation. So it just seems that the rate that we lose Trp should be just as useful a tool as the accumulation of intermediate. Sorry if I'm moving us in circles, it's really this point that I'm stuck on.

Okay, so I opened my TBR book and read the passage. You can ignore my previous explanation since this may be the best explanation:

The passage states that brain tryptophan "reflects the uptake of tryptophan from the blood into the brain." And the Q84 deals with the brain converting tryptophan (in the brain) into serotonin.

Thus, the concentration of blood tryptophan is irrelevant; it's the level of brain tryptophan that is involved in the reaction. So use POE and choose A.

 

[Synapsis]

I see...so I guess I just needed to read more carefully.