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Biochemistry Flashcards

7 cards from real PCAT practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. In competitive inhibition, increasing the substrate concentration will:

    Answer: Leave Vmax unchanged and increase apparent Km

    Competitive inhibitors bind the active site reversibly; adding excess substrate outcompetes the inhibitor, so Vmax is unchanged but the apparent Km is increased.

  2. Which lipoprotein is primarily responsible for transporting dietary (exogenous) triglycerides from the intestine to peripheral tissues?

    Answer: Chylomicrons

    Chylomicrons are assembled in intestinal epithelial cells and carry absorbed dietary triglycerides through the lymphatic system to peripheral tissues.

  3. Sickle cell disease results from a single amino acid substitution in hemoglobin. What change occurs?

    Answer: Glutamate is replaced by valine at position 6 of the beta chain

    A point mutation converts the GAG codon to GTG, replacing the hydrophilic glutamate with hydrophobic valine at position 6 of the beta-globin chain, causing HbS polymerization under low oxygen conditions.

  4. Which fatty acid is classified as an omega-3 fatty acid?

    Answer: Alpha-linolenic acid (18:3)

    Alpha-linolenic acid (ALA, 18:3Δ9,12,15) is an omega-3 fatty acid because its first double bond is at the 3rd carbon from the methyl end; it is an essential fatty acid.

  5. Which enzyme is responsible for unwinding the DNA double helix during replication?

    Answer: Helicase

    Helicase uses ATP hydrolysis to break hydrogen bonds between base pairs and unwind the double helix at the replication fork.

  6. The rate-limiting enzyme of the HMP shunt (pentose phosphate pathway) is:

    Answer: Glucose-6-phosphate dehydrogenase

    Glucose-6-phosphate dehydrogenase (G6PD) catalyzes the first and rate-limiting step of the pentose phosphate pathway, producing NADPH and 6-phosphoglucono-delta-lactone.

  7. Which amino acids are considered ketogenic only (not glucogenic)?

    Answer: Leucine and lysine

    Leucine and lysine are the only purely ketogenic amino acids; their catabolism yields only acetyl-CoA and/or acetoacetate, which cannot be used for gluconeogenesis.