A patient taking methotrexate for rheumatoid arthritis develops mucositis and pancytopenia. Methotrexate is a competitive inhibitor of dihydrofolate reductase (DHFR). Leucovorin rescue involves administering folinic acid (5-formyltetrahydrofolate). The Lineweaver-Burk (double reciprocal) plot for competitive inhibition shows which characteristic change compared to the uninhibited reaction?
- A Same x-intercept (Km unchanged), increased y-intercept indicating decreased Vmax
- B Both x-intercept and y-intercept change proportionally
- C Same y-intercept (1/Vmax unchanged), increased slope and x-intercept indicating apparent increase in Km ✓
- D The slope remains unchanged but the lines are parallel
Explanation
In competitive inhibition, the inhibitor competes with substrate for the active site; increasing substrate concentration can overcome inhibition, so Vmax is unchanged (same y-intercept on Lineweaver-Burk). However, the apparent Km (Km apparent = Km × (1 + [I]/Ki)) increases, causing the x-intercept to shift rightward (closer to zero) and the slope to increase. Leucovorin rescue bypasses the DHFR block by providing 5-formyl-THF directly, which can be converted to other THF derivatives without requiring DHFR. In uncompetitive inhibition, the lines would be parallel; in non-competitive inhibition, both Km and Vmax change.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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Written and medically reviewed by the StethoPrep medical team.