MCAT Biology : Amino Acids
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Example Questions
Example Question #21 : Amino Acids
A depletion of amino acids in a cell would slow which immediate process?
DNA replication and repair
Fatty acid use
Sugar metabolism
Protein and enzyme production
All of these
Protein and enzyme production
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 #22 : Amino Acids
Which of the following amino acid sequences would be found on the cytoplasm side of a transmembrane protein?
Valine–leucine–proline
Cysteine–leucine–proline
Threonine–valine–leucine
Proline–valine–asparagine
Glutamine–threonine–tyrosine
Glutamine–threonine–tyrosine
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 #23 : 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
I and II
None of these amino acids will be found in the red region
I and III
II and III
I and III
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 
Example Question #24 : 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 
Which of the following amino acids is most likely used to synthesize nitric oxide?
Proline
Leucine
Arginine
Tyrosine
Aspartic acid
Arginine
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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