Amino Acids & Proteins MCAT Practice Question
A 34-year-old man presents with progressive muscle weakness, tremor, and cognitive decline. Laboratory studies reveal low plasma selenium levels and elevated thyroid peroxidase antibodies. Genetic testing identifies a heterozygous mutation in the SECIS element that impairs selenocysteine incorporation during translation. Which of the following best explains the pathophysiology of this patient's clinical presentation? Which of the following structural features of selenoprotein enzymes is most critical for their catalytic function?
Answer choices
- AIncorporation of selenocysteine residues within transmembrane alpha-helical domains to facilitate electron transport across the endoplasmic reticulum
- BPositioning of selenocysteine in random coil regions to increase conformational flexibility and enhance protein-protein interactions
- CIntegration of selenocysteine into the active site of redox enzymes where its enhanced nucleophilicity enables catalysis of peroxide reduction and detoxificationCorrect answer
- DPlacement of selenocysteine at protein termini to serve as a signal sequence for post-translational proteolytic processing and enzyme maturation
- EDistribution of selenocysteine throughout beta-sheet structures to stabilize quaternary protein interactions and prevent aggregation
- FInsertion of selenocysteine into fibrous protein regions to provide mechanical rigidity and structural scaffolding for multi-protein complexes
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