Zusammenfassung:
Enamidase
Die Hydrolyse von 1,4,5,6-Tetrahydro-6-oxonicotinat (THON) zu 2-Formylglutarat ist ein Schlüsselschritt im Nicotinat-Katabolismus einer Reihe von Clostridien und Proteobakterien, der von dem Enzym Enamidase (EC 3.5.2.18) katalysiert wird. Auf Grund von Sequenzver-gleichen, der Art der katalysierten Reaktion und dem Nachweis stöchiometrischer Bindung von Metallionen wurde es der Amidohydrolase-Strukturfamilie zugeordnet. Enamidase ka-talysiert eine ungewöhnliche Zweistufenreaktion: die Dezyklizierung von THON zu 2 (Enamin) glutarat und dessen weitere Hydrolyse zu (S) 2 Formylglutarat. Die hier präsen-tierte 1.9 Å Kristallstruktur der Enamidase aus Eubacterium barkeri liefert die strukturelle Grundlage für den Katalysemechanismus dieser enantioselektiven Reaktion. Das Enzym bildet 222-symmetrische Tetramere. Die Enamidase-Monomere bestehen dabei aus einer β Sandwich-Domäne, die sich aus den N- und C Termini der Polypeptidkette zusammen-setzt, und einer zentralen (α/β)8-Fass-Domäne die das aktive Zentrum enthält. Letzteres be-inhaltet ein binukleares Metallzentrum aus Zink- und Eisenionen. Damit stellt Enamidase eine spezielle Typ-II-Amidohydrolase dar.
Decarboxylase-Untereinheit der Natriumionen-Pumpe Glutaconyl-CoA-Decarboxylase
Glutaconyl-CoA-Decarboxylase (Gcd, EC 4.1.1.70) koppelt die Decarboxylierung von Gluta-conyl-CoA mit dem Aufbau eines Na+-Gradienten. Das integrale Membranenzym besteht aus vier Untereinheiten: der α-Untereinheit (GcdA), die die Carboxyl-Gruppe von Glutaconyl-CoA auf die biotinylierte γ-Untereinheit (GcdC) transferiert, der β-Untereinheit (GcdB), die die Decarboxylierung des Carboxybiotins sowie die Na+-Translokation katalysiert, und der δ-Untereinheit (GcdD), deren Funktion noch nicht bekannt ist. Die in der vorliegenden Ar-beit präsentierten Kokristallstrukturen der GcdA-Untereinheit aus Clostridium symbiosum mit dem Substrat Glutaconyl-CoA, dem Produkt Crotonyl-CoA und dem Substratanalogon Glu-taryl-CoA liefern ein hochaufgelöstes Modell für die Quartär-struktur und den Katalyseme-chanismus des Enzyms, das bemerkenswerte strukturelle Veränderungen bei der Substrat-bindung aufdeckt. Auf der Grundlage dieser Daten wird ein neuartiges, asymmetrisches Modell für den intakten Gcd-Komplex vorgeschlagen, in dem GcdA ein Tetramer bildet, das von einem Netzwerk Lösungsmittel-gefüllter Kanäle durchzogen ist.
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