Three-dimensional structure of the human transglutaminase 3 enzyme: binding of calcium ions changes structure for activation.
Transglutaminase (TGase) enzymes catalyze the formation of covalent cross-links between protein-bound glutamines and lysines in a calcium-dependent manner, but the role of Ca(2+) ions remains unclear. The TGase 3 isoform is widely expressed and is important for epithelial barrier formation. It is a zymogen, requiring proteolysis for activity. We have solved the three-dimensional structures of the zymogen and the activated forms at 2.2 and 2.1 A resolution, respectively, and examined the role of Ca(2+) ions. The zymogen binds one ion tightly that cannot be exchanged. Upon proteolysis, the enzyme exothermally acquires two more Ca(2+) ions that activate the enzyme, are exchangeable and are functionally replaceable by other lanthanide trivalent cations. Binding of a Ca(2+) ion at one of these sites opens a channel which exposes the key Trp236 and Trp327 residues that control substrate access to the active site. Together, these biochemical and structural data reveal for the first time in a TGase enzyme that Ca(2+) ions induce structural changes which at least in part dictate activity and, moreover, may confer substrate specificity.
Ahvazi
Bijan
B
Laboratory of Skin Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, MD 20892-8023, USA. Bijan@discus.niams.nih.gov
Kim
Hee Chul
HC
Kee
Sun-Ho
SH
Nemes
Zoltan
Z
Steinert
Peter M
PM
D001665:Binding Sites / Q000502:physiology; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D002135:Calcium-Binding Proteins / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D004789:Enzyme Activation / Q000502:physiology; D004792:Enzyme Precursors / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D017434:Protein Structure, Tertiary; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D011503:Transglutaminases / Q000737:chemistry* / Q000378:metabolism
D002135:Calcium-Binding Proteins; D004792:Enzyme Precursors; C498712:TGM3 protein, human; D011503:Transglutaminases; D002118:Calcium