Substrate-binding model of the chlorophyll biosynthetic magnesium chelatase BchH subunit

Nickolche Sirijovski, Joakim Lundqvist, Matilda Rosenbäck, Hans Elmlund, Salam Al-Karadaghi, Robert D. Willows, Mats Hansson*

*Corresponding author for this work

Research output: Contribution to journalArticle

34 Citations (Scopus)

Abstract

Photosynthetic organisms require chlorophyll and bacterio-chlorophyll to harness light energy and to transform water and carbon dioxide into carbohydrates and oxygen. The biosynthesis of these pigments is initiated by magnesium chelatase, an enzyme composed of BchI, BchD, and BchH proteins, which catalyzes the insertion of Mg2+ into protoporphyrin IX (Proto) to produce Mg-protoporphyrin IX. BchI and BchD form an ATP dependent AAA + complex that transiently interacts with the Proto-binding BchH subunit, at which point Mg2+ is chelated. In this study, controlled proteolysis, electron microscopy of negatively stained specimens, and single-particle three-dimensional reconstruction have been used to probe the structure and substrate-binding mechanism of the BchH subunit to a resolution of 25 Å. The apo structure contains three major lobe-shaped domains connected at a single point with additional densities at the tip of two lobes termed the "thumb" and "finger." With the independent reconstruction of a substrate-bound BchH complex (BchH·Proto), we observed a distinct conformational change in the thumb and finger subdomains. Prolonged proteolysis of native apo-BchH produced a stable C-terminal fragment of 45 kDa, and Proto was shown to protect the full-length polypeptide from degradation. Fitting of a truncated BchH polypeptide reconstruction identified the N-and C-terminal domains. Our results show that the N- and C-terminal domains play crucial roles in the substrate-binding mechanism.

Original languageEnglish
Pages (from-to)11652-11660
Number of pages9
JournalJournal of Biological Chemistry
Volume283
Issue number17
DOIs
Publication statusPublished - 25 Apr 2008

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