As you already know, Turkish Airlines and Iberia has regular flights between Spain and Turkey. Turkish Airlines flies from Madrid and from Barcelona to Istanbul every day of the week, meanwhile Iberia flies from Madrid only on daily basis to Istanbul. Both airlines use Ataturk airport on the european side of Istanbul.
Now, there are two spanish low-cost airlines starting their daily flights between Barcelona and Istanbul. These new flights will operate from Sabiha Gokcen airport on the asian side of Istanbul. Spanair is starting their flights on May 6th, meanwhile Vueling will start on June 24th.
For your info.