Structure and mechanistic implications of a uroporphyrinogen III synthase-product complex.

Abstract:

:Uroporphyrinogen III synthase (U3S) catalyzes the asymmetrical cyclization of a linear tetrapyrrole to form the physiologically relevant uroporphyrinogen III (uro'gen III) isomer during heme biosynthesis. Here, we report four apoenzyme and one product complex crystal structures of the Thermus thermophilus (HB27) U3S protein. The overlay of eight crystallographically unique U3S molecules reveals a huge range of conformational flexibility, including a "closed" product complex. The product, uro'gen III, binds between the two domains and is held in place by a network of hydrogen bonds between the product's side chain carboxylates and the protein's main chain amides. Interactions of the product A and B ring carboxylate side chains with both structural domains of U3S appear to dictate the relative orientation of the domains in the closed enzyme conformation and likely remain intact during catalysis. The product C and D rings are less constrained in the structure, consistent with the conformational changes required for the catalytic cyclization with inversion of D ring orientation. A conserved tyrosine residue is potentially positioned to facilitate loss of a hydroxyl from the substrate to initiate the catalytic reaction.

journal_name

Biochemistry

journal_title

Biochemistry

authors

Schubert HL,Phillips JD,Heroux A,Hill CP

doi

10.1021/bi800635y

subject

Has Abstract

pub_date

2008-08-19 00:00:00

pages

8648-55

issue

33

eissn

0006-2960

issn

1520-4995

journal_volume

47

pub_type

杂志文章