Title

Protein-Protein Interactions and Electron Transfer Associated with Cytochrome F and Plastocyanin From the Cyanobacterium Prochlorothrix Hollandica

Date of Award

2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

George Bullerjahn

Abstract

This dissertational work describes the minimal structural requirements of interaction surfaces between two proteins involved in photosynthetic electron transfer cyt f and PC from cyanobacterium Prochlorothrix hollandica analyzed by stopped-flow absorption spectroscopy and HSQC NMR. Two mutant P.hollandica cyt f, Y102G and Y102G/F100S, yielding a modified surface-exposed loop region, were expressed and characterized to analyze the structurally unique Prochlorothrix cyt f ‘pocket-like’ region involved in the PC-cyt f complex formation and electron transfer. Stopped-flow studies showed that altering these residues slows down ket more than one order of magnitude. We propose that Tyr102 and Phe100 are actively involved in complex formation between cyt f and PC and serve to minimize distance between electron donor and acceptor. Thus, by removing these residues, the Cu-Fe distance in cyt f -PC complex increases, slowing electron transfer rates. In previous NMR studies(21) it was shown that PC from cyanobacteria interacts with cyt f differently than the comparable proteins in higher plants and algae. The PC-cyt f complex in the cyanobacterium Phormidium laminosum(21) involves a ‘head on’ contact between the hydrophobic (‘northern’) patch of PC with a hydrophobic surface surrounding the cyt f heme, with an average Cu-Fe separation of 15Å. Moreover, Prochlorothrix hollandica PC has a structurally distinct docking surface among other cyanobacteria(23) that likely makes these interactions somewhat different with additional interaction of PC with a flexible loop of cyt f forming a ‘pocket-like’ region in the vicinity of cyt f residues 99-104. There are two aromatic amino acids Tyr and Phe among them that face toward the PC and are possibly involved in protein-protein contacts. A parallel study was initiated to study the interactions of mutated cyanobacterial PC with Photosystem I (PSI) and some non-physiological electron-transfer partners (Lysine peptide and tris (2,2’-bipyridine)ruthenium (II) Ru(bpy)32+). Additionally, negatively charged P.hollandica PC mutants, mimicking the higher plant protein, should explain better the necessity of electrostatic interactions in the PC/cyt f complex in chloroplast systems.