Go to  Advanced Search

Discovery and characterization of a new zinc-bacteriochlorophyll biosynthetic pathway and photosystem in a magnesium-chelatase mutant

Show full item record

Files in this item

Files Size Format Description   View
ubc_2010_fall_jaschke_paul.pdf 25.73Mb Adobe Portable Document Format   View/Open
 
Title: Discovery and characterization of a new zinc-bacteriochlorophyll biosynthetic pathway and photosystem in a magnesium-chelatase mutant
Author: Jaschke, Paul Richard Ayre
Degree Doctor of Philosophy - PhD
Program Microbiology and Immunology
Copyright Date: 2010
Publicly Available in cIRcle 2010-08-10
Abstract: Porphyrins, for example heme and chlorophyll, are vital to biological processes such as respiration and photosynthesis. Both cofactors are synthesized through a common pathway to protoporphyrin IX (PPIX) which then branches: Fe²⁺ chelation into the macrocycle by ferrochelatase results in heme formation; by contrast, Mg²⁺ addition by Mg-chelatase commits the porphyrin to (bacterio)chlorophyll synthesis. The purple bacterium Rhodobacter sphaeroides is a model for bacteriochlorophyll a (BChl) biosynthesis and type-2 reaction center (RC) structure and function. While studying RC protein assembly it was discovered that a bchD (Mg-chelatase) mutant did not produce BChl as wild-type (wt) cells do, but instead produced small quantities of an alternative BChl in which Mg²⁺ was substituted by Zn²⁺. Zn-BChl α has been found in only one other organism before, the related acidophilic purple phototrophic bacterium Acidiphilium rubrum. The overall objectives of this thesis were two-fold: (1) to elucidate the Zn-BChl biosynthetic pathway; and (2) to utilize this biosynthesis as a tool to probe aspects of photosynthetic apparatus function. The biosynthetic pathway of Zn-BChl in the bchD mutant was found to begin at ferrochelatase, which efficiently chelated Zn²⁺ into PPIX. The resultant Zn-PPIX was utilized by the BChl-biosynthetic pathway, with metabolites early in the pathway accumulating, but with low Zn-BChl levels. Two novel intermediates I described, protoporphyrin IX monomethyl ester and divinyl-protochlorophyllide, contained Zn²⁺ instead of Mg²⁺. The demonstrated Zn-BChl biosynthetic pathway is a new way to make BChl and facilitates the further engineering of alternate forms of (Zn-)BChl. The RC in the bchD mutant (Zn-RC) was found to bind six Zn-BChls, instead of the four Mg-BChls and two bacteriopheophytins of the WT-RC. Spectroscopic examination showed that electron transfer (ET) in the Zn-RC occurs at approximately the same rate as in the WT-RC, despite substitution of Zn-BChl for bacteriopheophytin α. We showed preservation of ET was due to the unusual tetracoordination state of the Zn-BChls in the bacteriopheophytin site. This discovery allows refinement of ET rules within pigment-protein complexes by showing that the coordination state and conformation of cofactors can have an equally important role as the protein.
URI: http://hdl.handle.net/2429/27234
Scholarly Level: Graduate

This item appears in the following Collection(s)

Show full item record

All items in cIRcle are protected by copyright, with all rights reserved.

UBC Library
1961 East Mall
Vancouver, B.C.
Canada V6T 1Z1
Tel: 604-822-6375
Fax: 604-822-3893