Your flight ex TXL will be arriving in Terminal 1A -due to tons of connecting pax, most likely on a terminal possition close to A13. The flight to IST will propably departing in Terminal 1B. Make sure to use the tunnel (located between Gate A15 and A17) between T1A and T1B, so you don't have to go through security (only passport control) again. IMHO, the walking time from Gate to Gate is approx 15-20 min - so its definitly tight, but you should be fine.

You can also check out the terminal map on:
http://www.frankfurt-airport.com/con...entation0.html