Sample MCAT Biology & Biochemistry Questions
Below are 10 practice questions to help you study for the Biology & Biochem portion of the MCAT exam.
Part 1: Biological and Biochemical Foundations of Living Systems
Use the following passage to answer questions 1 through 4.
Hospitals and clinics in the United States and around the world face constant shortages in the supply of blood for transfusions. One alternative currently being explored is to make recombinant human hemoglobin (rHb). In addition to increasing supply for transfusions, rHb has a longer shelf life than red blood cells, is compatible with all blood types, and poses lower risk of transmitting disease.
Despite these advantages, neither rHb products nor hemoglobin purified from animals has been licensed as therapy in the United States, mainly because of reports of hypertension and associated cardiovascular and cerebrovascular problems. (A product called HBOC-201, which is purified from cows, has been approved for human use in South Africa and Russia.)
The toxicity of rHb has been attributed to the protein's presence in the plasma, where it can scavenge nitric oxide (NO), undergo oxidative reactions that damage tissues, denature, aggregate, and precipitate, among other activities. In contrast, endogenous hemoglobin localizes in erythrocytes, where the cell membrane and cellular reduction pathways reduce or prevent these activities.
It has been proposed that the hypertensive effects of rHb could be due to the protein having too low an affinity for oxygen, causing excessive release of oxygen into the arterioles and vasoconstriction. Alternatively, it has been proposed that NO scavenging by rHb from the vasculature leads to depletion of NO, an increase in intracellular calcium, and muscle constriction.
Since the 1980s when researchers started studying rHb, various conjugates and amino acid substitutions have been tested for the ability to reduce issues with stability and toxicity. Through mutagenesis strategies, it has been possible to identify rHb variants with different rates of oxygen binding and release, as well as some that are not associated with increases in blood pressure or mean arterial pressure (MAP).
However, more work is needed to identify variants with reduced loss of hemin (a protoporphyrin ring containing chelated iron) and globin denaturation. Work is also underway to lower production costs of rHb variants that are being developed as potential therapeutic products.
1. The table summarizes the difference in blood pressure associated with six variants of rHb along with several parameters in vitro.
Based on the data in the table, which parameter is most likely to be responsible for the increase in MAP observed for some rHb variants?
a. NO is being scavenged and depleted.
b. Too much oxygen is being released.
c. The affinity for oxygen is too high.
d. rHb is unstable and autoxidation is occurring.
2. In the figure below, the four curves represent rHb variants harboring point mutations and their oxygen-binding curves.
Which curve is most similar to the binding curve of endogenous hemoglobin, and why?
a. The curve for α(L29F)β(WT) because hemoglobin has maximal affinity for oxygen at low and high concentrations.
b. The curve for α(V96W)β(WT) because hemoglobin has high affinity for oxygen at low and high concentrations.
c. The curve for α(L29F/V96W)β(N108K) because hemoglobin has low affinity for oxygen at low concentrations and high affinity for oxygen at high concentrations.
d. The curve for α(V96W)β(N108K) because hemoglobin has low affinity for oxygen at low and high concentrations.
3. Engineering a cellular rHBOC involves recombinant DNA technology. What is necessary to make recombinant hemoglobin?
a. Expression of hemoglobin protein in bacteria
b. Knowing the sequence of the hemoglobin gene
c. Knowing the hemoglobin gene's splice donor and acceptor sites
d. Having the crystal structure of hemoglobin protein
4. Which condition would NOT be expected to result in increases in extracellular hemoglobin?
a. Liver cirrhosis
b. Blood transfusion
c. Inflammatory bowel disease
d. Malaria infection
5. Which of the following amino acids does NOT have an isoelectric point (pI) between 5.5 and 6.0?
a. Alanine
b. Glutamic acid
c. Glycine
d. Glutamine
6. Which amino acid would most likely be found on the surface of a protein molecule at physiological pH?
a. Isoleucine
b. Lysine
c. Alanine
d. Proline
7. Which of the following statements about terpenes is NOT true?
a. They are a type of terpenoid.
b. They all contain double bonds.
c. They are all made up of 5-carbon units.
d. They all contain oxygen.
8. How are the plasma membranes of mammalian and bacterial cells similar?
a. They typically contain cholesterol.
b. They have negatively charged lipids on their surfaces.
c. They contain lipids that are involved in signal transduction.
d. They are made up of many different types of phospholipids.
Use the following passage to answer questions 9 and 10.
Under conditions of cell stress, such as exposure to heat, the weak bonds within a protein can be broken, leading to protein misfolding and self-association. When the concentration of misfolded polypeptides becomes high enough, they can form larger aggregates that are very stable because strong bonds occur between the molecules.
Many age-related diseases, including Alzheimer's, Parkinson's, and type 2 diabetes, are considered to be the result of protein aggregates, which can eventually cause tissue death. Chaperone molecules bind with high affinity to exposed hydrophobic regions on the surface of misfolded polypeptides and reduce aggregation.
However, there is increasing evidence that chaperones do not merely act like sponges that associate with misfolded polypeptides and prevent them from aggregating. Along with a complex of molecules, chaperones can help misfolded substrates unfold and are thus also called unfoldases. Once the substrates are completely unfolded, they can then spontaneously refold into their native conformation.
Although chaperones can act on different types of proteins, they unfold them with varying degrees of efficiency. For example, one of the major families of chaperones, which are highly conserved from bacteria to eukaryotes, is called GroEL/GroES. Compared with other chaperones, GroEL/GroES can rapidly convert misfolded rhodanese (rho), a mitochondrial enzyme involved in detoxifying cyanide, into its unfolded form.
However, it is unclear in general whether a vast excess of native, properly folded proteins could compete with misfolded substrates for binding to chaperones. Indeed, native proteins are present in a cell at a much higher concentration relative to misfolded species. Furthermore, if chaperones bind inappropriately to substrates that they are not able to completely unfold, the chaperone molecules may not dissociate rapidly from the partially unfolded species and their activity could get stalled.
Another area of question is the role of ATP in the unfolding and refolding of polypeptide substrates. When GroEL/GroES is bound to substrate, ATP hydrolysis leads to conformational changes in GroEL and the release of GroES from the complex.
One model is that the ATP is not required for the binding of chaperones to misfolded polypeptides but that, at least for some chaperones, ATP hydrolysis is required for the release of the unfolded polypeptide from the chaperone complex so it can refold in solution.
9. The graph below shows the concentration of rhodanese (Rho) that is refolded into its native confirmation in the presence (solid shapes) or absence (open shapes) of an excess concentration of native protein, called MDH. Rhodanese was also incubated with the chaperone GroEL/GroES (LS, ELS) (top four curves) or without (bottom two curves) and with ATP (top two curves) or without (bottom four curves).
Based on the data presented in the graph, how does native MDH affect the refolding of rhodanese by the chaperone molecules?
a. The refolding is not affected because the chaperones bind MDH and Rho simultaneously.
b. The refolding rate is reduced but the yield of refolding is not changed.
c. The refolding does not occur because MDH stalls the chaperones.
d. The refolding yield is reduced because MDH depletes ATP.
10. Which of the following is NOT a chaperone molecule?
a. Heat-shock protein (Hsp) 70
b. SecB
c. GroEL/GroES
d. PI3K
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