Which letter will replace the question mark (?) in the following series?

D17E, E19F, F23G, ?

G29H

According to the above alphabet position:

D17E    ⇒ (4), 17, (5)

E19F    ⇒ (5), 19, (6)

F23G   ⇒ (6), 23, (7)

?         ⇒ (7), 29, (8) = G29H

(consecutive alphabet series at 1st and last alphabet position and there is prime number series in the middle)