The Amex Membership Rewards program has different transfer partners depending on the country in which your Amex card is issued. But even if Turkish is not a transfer partner in your country, there should certainly be one or more Star Alliance airlines that are. For example, if you are based in the U.S., you could transfer to Aeroplan, and then redeem Aeroplan miles on Turkish -- assuming that there is availability.

Note that each airline program has its own redemption rates, fees, and rules for award-travel redemptions, so you should carefully investigate before deciding which program to use, if you have more than one option.