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Tutor profile: Matthew L.

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Matthew L.
Chemistry and Biology Tutor, BA in Biochemistry, UC Berkeley
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Questions

Subject: MCAT

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Question:

In the following equation, enzyme 1 converts substrate A into substrate B, and enzyme 2 converts substrate B into substrate C. A student adds enzyme 1 and enzyme 2 to a solution containing excess substrate A. After a while, the rate of production for substrate B decreases. What is the most likely reason for this? A → B → C A) Substrate A is a competitive inhibitor of enzyme 1. B) Substrate C is an allosteric inhibitor of enzyme 1. C) Substrate C is a competitive inhibitor of enzyme 2. D) Substrate B is an allosteric inhibitor of enzyme 2.

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Matthew L.
Answer:

Based on the reaction scheme, substrate A is converted into substrate B. Once substrate B has been produced, substrate C will form. Since the reaction has been running for a while, this hints that substrate C is probably present. Since the rate of production of substrate B decreases, this might mean that enzyme 1 is being inhibited. This means that answers C and D may not be the best answers, since they do not mention enzyme 1. Furthermore, inhibiting enzyme 2 would actually slow the conversion of substrate B into substrate B and favor the accumulation of substrate B. To decide between answers A and B, we must consider substrate A and substrate C. From the question, we can infer that substrate B likely forms, since the rate of its production later declines. If substrate A inhibited enzyme 1, it seems unlikely that we would produce much substrate B. It seems more likely that substrate C is our inhibitor, since the rate of production for substrate B declines only once substrate C has had time to form. Additionally, it is irrelevant to the question whether the inhibitor acts competitively or allosterically, as we mainly care that the inhibition does occur. Therefore, B is our answer.

Subject: Biochemistry

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Question:

What is a GPCR?

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Matthew L.
Answer:

G protein-coupled receptors, or GPCRs, are one of the largest families of membrane receptors involved in signal transduction, characterized by their seven transmembrane alpha helices. During signal transduction, a signal is sent in the form of a signalling molecule called a ligand. When the ligand binds to the extracellular side of the GPCR, the GPCR becomes activated and can then bind a heterotrimeric G protein. The activated GPCR then activates the G protein, causing it to change shape and to dissociate from the GPCR. Once free, the activated G protein can cause a cellular response by interacting with other cellular molecules, which can act as secondary messengers.

Subject: Biology

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Question:

What’s the difference between innate and adaptive immunity?

Inactive
Matthew L.
Answer:

Innate immunity is the body’s first line of defense. It is composed of barrier defenses (e.g. skin, mucous membranes) and internal defenses (e.g. phagocytes, antimicrobial proteins) that target features shared by many different pathogens, which is why it provides a broad base of protection. It also acts faster than adaptive immunity. However, innate immunity is a rather blunt instrument, unable to improve with repeated exposure to a specific pathogen. Adaptive immunity, in contrast, is more precise. Composed of B-cells and T-cells that can target specific pathogens, adaptive immunity learns to recognize the pathogens it’s faced before and has a stronger response with repeated exposure to those pathogens. This learning process is called “immunological memory” and is the basis for vaccines. In short: Innate immunity is quick but broad. Adaptive immunity is slow but specific.

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