MCAT Biology : Amino Acids

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #21 : Proteins

A depletion of amino acids in a cell would slow which immediate process?

Possible Answers:

DNA replication and repair

Fatty acid use

Protein and enzyme production

All of these

Sugar metabolism

Correct answer:

Protein and enzyme production

Explanation:

A depletion of amino acids would immediately impact protein and enzyme production in the cell. All proteins, including enzymes, are made from chains of amino acids. Lack of nutrients, such as amino acids, would eventually impact other processes listed.

Example Question #21 : Amino Acids

 

Which of the following amino acid sequences would be found on the cytoplasm side of a transmembrane protein?

Possible Answers:

Proline–valine–asparagine

Glutamine–threonine–tyrosine

Cysteine–leucine–proline

Valine–leucine–proline

Threonine–valine–leucine

Correct answer:

Glutamine–threonine–tyrosine

Explanation:

Transmembrane proteins are embedded within the phospholipid bilayer of the cell membrane; therefore, the protein is exposed to the nonpolar fatty acid tails, the polar phospholipid heads, and the polar environments of the cytoplasm and extracellular space.

The questions asks which amino acids would be found facing the cytoplasm. Because the cytoplasm is polar, the amino acids interacting with the cytoplasm must also be polar. Glutamine, threonine, and tyrosine are all polar amino acids, making this the best answer. The other answers contain nonpolar amino acids (proline, leucine, cysteine, valine) causing these answers to be incorrect.

Example Question #21 : Amino Acids

A researcher stains a transmembrane protein. The membrane-spanning region of the protein is stained red, whereas the other regions are stained blue. Which of the following will you most likely find in the red region? 

I. Glycine, which has a side chain of 

II. Cysteine, which has a side chain of 

III. Alanine, which has a side chain of 

Possible Answers:

None of these amino acids will be found in the red region

I and II

II and III

I and III

Correct answer:

I and III

Explanation:

The question states that the membrane-spanning region of the transmembrane protein is stained red; therefore, you will find only hydrophobic amino acids in this region. Recall that an amino acid has a central carbon that has a hydrogen group, carboxylic acid group, amino group, and a side chain attached. The differences between amino acids arise from the different side chains.

A hydrophobic amino acid contains nonpolar side chains. Glycine and alanine contain  and  side chains, respectively. Both of these amino acids will have nonpolar properties and be found in the red region of the protein. Cysteine contains , which is a polar side chain, and will be found in the blue region.

Example Question #22 : Amino Acids

Sildenafil (commonly called Viagra) is a common drug used to treat erectile dysfunction and pulmonary arterial hypertension. Sildenafil's effect comes from its ability to cause vasodilation in smooth muscle cells. For this problem, we're only going to consider its effects on erections in males. 

Erectile dysfunction is a common medical problem in older men. Its most significant effect is the prevention of erections. Erections occur when there is an increase in blood flow via enlargement of an artery (vasodilation). Understanding the mechanism by which vasodilations occur is important in order to treat erectile dysfunction.  

Erections occur when nitric oxide  is released from an area in the penis and binds to guanylate cyclase in other cells of the penis, which creates cyclic guanosine monophosphate (cGMP) from GTP. cGMP causes a relaxation of the arterial wall in order to increase blood flow to the region, thereby causing an erection. cGMP is broken down over time by cGMP-specific phosphodiesterase type 5 (PDE5) into GTP, which reverses the effect and causes vasoconstriction on the arterial wall. Combatting this effect is the major method by which Viagra functions. 

Which of the following amino acids is most likely used to synthesize nitric oxide? 

Possible Answers:

Aspartic acid

Arginine

Leucine

Tyrosine

Proline

Correct answer:

Arginine

Explanation:

Let's consider nitric oxide, which has the chemical formula 

In most biological contexts, oxygen is relatively easy to derive since it can be derived from lots of sources, such as water or air. However, the nitrogen in nitric oxide is the more difficult atom to obtain. 

All of the listed amino acids have a backbone that has an amino group, but only one has nitrogen within its side chain, and that is arginine. Since arginine has 3 nitrogen atoms on its side chains, it is the most likely candidate to be stripped of its nitrogen to form nitric oxide. 

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