Biochem Practice Question

[yui_96]

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Collagen consists of three helices with carbon backbones that are tightly wrapped around one another in a "triple helix". Which of these amino acids is most likely to be found in the highest concentration in collagen?

A. Proline
B. Glycine
C. Threonine
D. Cysteine

The answer given is B and the explanation is that since it's a triple helix the carbon backbone needs to be packed more closely together. In order to reduce the steric hindrance, the smallest amino acid, aka glycine, should be in the highest concentration. Can someone explain further about why the carbon backbone needs to be packed more closely together in a triple helix? Also I chose cysteine because I thought cysteine can form disulfide bonds thus they link the helices together. Can someone also points out the flaw of my logic? Thank you so much!

 

[TakotsuboOkazaki]

Each amino acid has an R-group.

Glycine’s R-group is a lone hydrogen.

The single electron and proton makes for a small atomic volume and less steric hinderance.

Less steric hinderance = Closer packing of the helices.


Sent from my iPhone using Tapatalk

 

[BerkReviewTeach]

I'm going to take the BR approach to this question, so forgive me if it is a bit unorthodox at first glance. But keep in mind that you are taking a test of multiple-choice questions, and sometimes the best route to the answer is not the traditional information-heavy approach emphasized in school.

Two things to consider with this question. First, the question gives you a hint of what they want with the words "tightly wrapped." That is the distinguisher in the question stem. It means that you need amino acids that can 'scrunch' together, which implies small side chains. You are looking for the smallest side chain.

Second, you need to look at the answer choices and break them down and look for their difference and similarities. Choice A is cyclic, which causes bends and turns and disrupts the standard coiling and packing of proteins. Choice B is the smallest and does well... things that small things do. Choice C has a hydroxyl group, so it can form a hydrogen bond. Choice D can form disulfide bridges, so it can form strong tertiary and quaternary interactions. These seem different from one another, so let's look for similarities. Two of the choices result in strong interactions between side chains (hydrogen bonding and covalent bridges), so they likely take each other out in a general question like this. You are down to two remaining choices: a structurally disruptive amino acid versus a tiny amino acid. Matching back to the key word in the question makes glycine the best option for a tightly wrapped triple helix.

Improving at this exam requires looking at questions in this sort of manner. When you review a question, always consider what the key words in the question are and what the distinguishing features of answer choices are. This is what really good multiple-choice test takers doing without even realizing they are doing it.

One thing you may notice if you were to do our homework phase system is that a question like this would appear in Phase 1 and then a very similar question, but worded differently, appears in Phase 2 or Phase 3. Our goal is to emphasize the algorithm more than the facts. You need to practice how to get to an answer as opposed to memorizing detailed facts used in a solution. It takes work to overcome traditional school habits and become a better multiple-choice test taker.

Good luck!

 

[yui_96]

I'm going to take the BR approach to this question, so forgive me if it is a bit unorthodox at first glance. But keep in mind that you are taking a test of multiple-choice questions, and sometimes the best route to the answer is not the traditional information-heavy approach emphasized in school.

Two things to consider with this question. First, the question gives you a hint of what they want with the words "tightly wrapped." That is the distinguisher in the question stem. It means that you need amino acids that can 'scrunch' together, which implies small side chains. You are looking for the smallest side chain.

Second, you need to look at the answer choices and break them down and look for their difference and similarities. Choice A is cyclic, which causes bends and turns and disrupts the standard coiling and packing of proteins. Choice B is the smallest and does well... things that small things do. Choice C has a hydroxyl group, so it can form a hydrogen bond. Choice D can form disulfide bridges, so it can form strong tertiary and quaternary interactions. These seem different from one another, so let's look for similarities. Two of the choices result in strong interactions between side chains (hydrogen bonding and covalent bridges), so they likely take each other out in a general question like this. You are down to two remaining choices: a structurally disruptive amino acid versus a tiny amino acid. Matching back to the key word in the question makes glycine the best option for a tightly wrapped triple helix.

Improving at this exam requires looking at questions in this sort of manner. When you review a question, always consider what the key words int he question are and what the distinguishing features of answer choices are. This is what really good multiple-choice test takers doing without even realizing they are doing it.

One thing you may notice if you were to do our homework phase system is that a question like this would appear in Phase 1 and then a very similar question, but worded differently, appears in Phase 2 or Phase 3. Our goal is to emphasize the algorithm more than the facts. You need to practice how to get to an answer as opposed to memorizing detailed facts used in a solution. It takes work to overcome traditional school habits and become a better multiple-choice test taker.

Good luck!

WOW! I never thought there's a different approach for this question. Thank you SO MUCH I will definitely keep this kind of thinking in mind when I approach other questions!