This volume draws together information on proteins dependent on vitamin B6, PQQ and other quinones for function. It attempts to understand these protein structures, mechanisms of action and their biological roles and functions. Emerging information regarding the participation of the proteins in enzymatic activity and their significance in biotechnology applications, such as the use of specific inhibitors, is also included. The topics treated comprise genetic regulation, evolutionary and cell biological aspects, as well as information on structures and mechanisms operating in these proteins.