Photoswitchable unnatural amino acids are valuable engineering tools in biotechnology, particularly for the reversible control of enzymes with light. Here, we explore some basic principles of this protein engineering technique to simplify its approach and increase its success rate. To this end, we have selected Escherichia coli type II asparaginase (EcAII), which is a prominent chemotherapeutic enzyme that is limited by detrimental side effects associated with its promiscuous glutaminase activity. Incorporation of phenylalanine-4′-azobenzene (AzoF) combined with extensive biophysical characterizations identified two light-sensitive variants, in which glutamine hydrolysis could be reversibly (de)activated up to 9-fold, whereas asparaginase hydrolysis was only marginally light-responsive. Computationally determined conformational landscapes elucidated this substrate-specificity of photocontrol defining a clear engineering principle: An exchange between less and more productive states at the active site helps AzoF to reshape the conformational landscape and makes enzymes more susceptible toward photocontrol. Moreover, our findings mark EcAII-AzoF variants as potential chemotherapeutic precursors.