The structure of vasopressin is very similar to that of oxytocin (primary sequence of vasopressin: cysteine - tyrosine - phenylalanine - glutamine - asparagine - cysteine - proline - arginine - glycine), which is also a nonapeptide with a disulfide bridge. One of the most important roles of vasopressin is to regulate the body's retention of water; it is released when the body is dehydrated and causes the kidneys to conserve water, thus concentrating the urine, and reducing urine volume. In addition, it has a variety of neurological effects on the brain. What are the differences in the structures of vasopressin and oxytocin?
I. residue #3: isoleucine (oxytocin) vs. phenylalanine (vasopressin)
II. residue #7: isoleucine (oxytocin) vs. phenylalanine (vasopressin)
III. residue #8: leucine (oxytocin) vs. arginine (vasopressin)
IV. residue #4: asparagine (oxytocin) vs. cysteine (vasopressin)

Question

Quizplus · Accepted Answer

The only differences in the structures of vasopressin and oxytocin are at residues 3, 7, 8, and 4. Specifically, vasopressin has phenylalanine at positions 3 and 7, arginine at position 8, and cysteine at position 4, while oxytocin has isoleucine at positions 3 and 7, leucine at position 8, and asparagine at position 4. Therefore, choices I and III are correct.

The Structure of Vasopressin Is Very Similar to That of Oxytocin