Space-filling model of the 24-meric cubic core and its constituent trimers. (Left panel) The trimer, where the three catalytic (inner-core) domains (depicted in red, yellow, and blue) from different E2 subunits form the basal functional and assembly unit. N and C = the amino and carboxy termini, respectively. (Right panel) The 24-meric cubic core, where eight trimers are assembled into a hollow cube (~130-Å edge), with each trimer occupying one of the eight vertices. The ovals (in white) depict trimer-trimer interfaces comprising the C-termini from the neighboring trimers. Substrate CoA enters the 30-Å-long active-site channel from inside the cube (green arrow), whereas the second substrate LBD approaches the active-site channel from the outside. Source: Maple Syrup Urine Disease (Branched-Chain Ketoaciduria), The Online Metabolic and Molecular Bases of Inherited Disease Citation: Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson K, Mitchell G. The Online Metabolic and Molecular Bases of Inherited Disease; 2014 Available at: https://ommbid.mhmedical.com/DownloadImage.aspx?image=/data/books/971/ch87fg1.png&sec=62675443&BookID=971&ChapterSecID=62675436&imagename= Accessed: October 05, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved