IPTG is called gratuitous inducer because it is
IPTG, isopropyl-beta-D-thiogalactopyranoside, mimics allolactose geometry required for LacI binding pocket including galactose ring recognition but contains a thio linkage between isopropyl and galactose resistant to hydrolysis by beta-galactosidase enzyme. Cells therefore cannot catabolize IPTG as carbon source, so its intracellular concentration remains stable over many generations, driving persistent derepression without consumption of inducer or metabolic side effects. This gratuitous property contrasts with lactose, which induces transiently until eventually metabolized to glucose. Non-me
Ref: Alberts Notes on IPTG gratuitous inducer non-metabolizable; NCBI Lab protocols - IPTG non-hydrolyzable lactose analog
