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.