23. Which of the sites on phosphofructokinase has higher affinity for ATP? A. Al

Question

23. Which of the sites on phosphofructokinase has higher affinity for ATP? A. Allosteric site. B. Active site. C. They are equal. 24. Where would a protein destined for the inner membrane space of the mitochondria end up if it was mutated to lack a stop transfer sequence? A. The cytosol. B. Mitochondrial matrix. C. Inner membrane space. D. It would stay in the ER. 25. How do SNAREs mediate vesicle fusion to target membranes? A. The SNARE proteins make the polar phospholipid heads nonpolar so the membranes can fuse. B. The Rabs on the vesicle bind to the Rab effectors on the target and mediate tetherin C. The SNAREs synthesize a new piece of membrane to help join the two together. D. They use a mechanism similar to forming a coat, except water activates the Sarl protein. E. S NAREs wrap around each other and force out the water molecules associated with each membrane 6. Which of the following would likely not undergo co-translational transport during protein sorting? A. Pex5 receptor B. LDL receptor. C. Na/K pump D. GLUTI E. All of the above undergo co-translational transport.

Explanation / Answer

23) ATP has two binding sites on Phosphofructokinase. One, ATP is a substrate and it binds to the active site. Two, it is a key allosteric inhibitor of the enzyme and it binds to an allosteric site. Out of these two sites, it binds to the active site with higher affinity.

This ensures that when ATP is more in the cytosol, it binds to the allosteric site and stops the enzyme, which turns off glycolytic pathway. When the ATP levels are low, the higher affinity for the active site ensures that ATP is used mainly as substrate and the enzyme carries out its function and keeps glycolysis going.

24) Proteins containing signal sequences are translocated across the inner mitochondrial membrane into the matrix. The stop-transfer sequence stops this translocation process by anchoring it to the membrane. So, we can expect mutations that render the peptide without the stop-transfer sequence to end up in the mitochondrial matrix.

25) The major energetic hurdle to get two membranes to fuse is posed by the water molecules present associated with the membranes. When v-SNAREs present on the vesicles and t-SNAREs present on the target membranes form specific coiled coils by wrapping around each other, they bring the two membranes close together. The binding of the complementary SNAREs provides the energy to displace the water molecules and bring the membranes close enough to fuse. The answer is E.

26) Proteins that need to be targeted to plasma membrane are usually sorted by the co-translational transport route. GLUT1, LDL receptor and Na+/K+ pumps are all plasma membrane proteins. Pex5 receptor is a cytosolic receptor that recognizes PTS1 sequence in soluble cytosolic proteins and carries it to the peroxisome membrane. Since it is present in the cytosol, Pex5 receptor is therefore likely to not undergo co-translational transport.

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