shemos sacred names - Why is the hey in Elo-* replaced with a kuf?

As we know when we write Hashem's name, we replace the hey with a kuf. Why davka a kuf? For example: Elokai, Elokeinu, Elokah, or "Yud-Key-Vav-Key". And in "Yud-key-vav-key", why is it pronounced that way instead of "Yud-kuf-vav-kuf"