1. Dr osophila Cryptochrome (CRY) is structurally related to DNA photolyase, an

Question

1. Dr osophila Cryptochrome (CRY) is structurally related to DNA photolyase, an enzyme that uses blue light to repair UV-induced DNA damage. In the dark,aflexible terminal tail (yellow) is bound in a pocket near the light-absorbing cofactor FAD (blue) to make a 'closed' state (panel a), but the tail is released from the rest of the protein after light absorption to create an 'open signaling state (panel b). hv dark (closed) light (open) lose A close-up view of the pocket in panel g above depicts 3 amino acids in the tail that are critical for making the 'closed state in the dark. a) Underline the critical tripeptide region pictured in panel c in the tail sequence below. 2 pts TPPPHCRPSNEEEVR i. If you were to mutate this tripeptide sequence to KEK', would you expect the Ko for the tail to go up or down? Explain your reasoning. [4 pts) the ka wo b) The reversion of light-excited FAD back to its dark state follows first-order kinetics. Once in the dark state, the tail can re-bind the pocket. By controlling the conformation of the protein, this process also controls how long CRY signaling occurs after light in Drosophila. If the rate constant for dark state reversion is k = 0.0004 s", what is the half-life of the light- excited signaling state in minutes? [4 pts] Page 2

Explanation / Answer

(b)

For first order kinetics,

Half life = 0.693 / k = 0.693 / 0.0004 seconds = 1732.5 seconds = 28.875 minutes.

(a)

i. FFW is the tail sequence.

ii. KD will go down as the affinity will increase due to replacement (mutation) with polar amino acids. Lower the KD higher the affinity of the bond.

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