Complex sentence: The pipecolate region of rapamycin structure seems necessary for rapamycin-binding to FKBP12, and this step is required for further binding of rapamycin to the mTOR kinase, which is the key enzyme in many biological actions of rapamycin.
Split into simpler sentences: The pipecolate region of rapamycin structure seems necessary for rapamycin-binding to FKBP12. This step is required for further binding of rapamycin to the mTOR kinase, which is the key enzyme in many biological actions of rapamycin.