That can depend on what your email is about.

If you’re answering a prof’s question, you can say, “Please let me know if you have further questions.”

If you’re asking for information or help, you can say something like, “I appreciate your time and guidance” or “Your advice is very much appreciated.”

If you’re asking for, say, an extension on the due date or an exemption, you can use, “Hoping for your kind consideration” or “Thank you for your kind consideration.”

You also can’t go wrong with the standard,
“Sincerely,

Your Name”

Aside from the ending, the body of your message is also important, so be sure to be concise so your prof can understand and help you better. It’s also always a good idea to thank your prof, and essentially just be polite.