Enzymes USP28 and USP25 (IMAGE) University of Würzburg Caption Structures of the catalytic domains of USP28 and USP25: USP28 is a constitutively active dimer and can bind and process ubiquitin (orange) at any time. In contrast, USP25 is a tetramer composed of two inter-linked USP28-like dimers (blue / gray and yellow / green) and this arrangement prevents binding of the substrate rendering the enzyme inactive. Credit Picture: Rudolf Virchow Center of experimental biomedicine / University of Wuerzburg Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.