Enzymology below 200 K: the kinetics and thermodynamics of the photochemistry catalyzed by protochlorophyllide oxidoreductase.
The chlorophyll biosynthesis enzyme protochlorophyllide reductase (POR) catalyzes the light-dependent reduction of protochlorophyllide (Pchlide) into chlorophyllide in the presence of NADPH. As POR is light-dependent, catalysis can be initiated by illumination of the enzyme-substrate complex at low temperatures, making it an attractive model for studying aspects of biological proton and hydride transfers. The early stages in the photoreduction, involving Pchlide binding and an initial photochemical reaction, have been studied in vitro by using low-temperature fluorescence and absorbance measurements. Formation of the ternary POR-NADPH-Pchlide complex produces red shifts in the fluorescence and absorbance maxima of Pchlide, allowing the dissociation constant for Pchlide binding to be measured. We demonstrate that the product of an initial photochemical reaction, which can occur below 200 K, is a nonfluorescent intermediate with a broad absorbance band at 696 nm (A696) that is suggested to represent an ion radical complex. The temperature dependence of the rate of A696 formation has allowed the activation energy for the photochemical step to be calculated and has shown that POR catalysis can proceed at much lower temperatures than previously thought. Calculations of differences in free energy between various reaction intermediates have been calculated; these, together with the quantum efficiency for Pchlide conversion, suggest a quantitative model for the thermodynamics of the light-driven step of Pchlide reduction.
Robert Hill Institute for Photosynthesis and Krebs Institute for Biomolecular Research, Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, United Kingdom. d.j.hayes@shef.ac.uk
Derren J
DJ
Heyes
Alexander V
AV
Ruban
Helen M
HM
Wilks
C Neil
CN
Hunter
D001665:Binding Sites; D002384:Catalysis; D002734:Chlorophyll / Q000031:analogs & derivatives* / Q000096:biosynthesis; D000458:Cyanobacteria / Q000201:enzymology; D004798:Enzymes / Q000378:metabolism; D007700:Kinetics; D010088:Oxidoreductases / Q000378:metabolism*; D044925:Oxidoreductases Acting on CH-CH Group Donors; D010777:Photochemistry; D011994:Recombinant Proteins / Q000378:metabolism; D013053:Spectrophotometry; D013816:Thermodynamics
D004798:Enzymes; D011994:Recombinant Proteins; D002734:Chlorophyll; C016083:protochlorophyll; D010088:Oxidoreductases; D044925:Oxidoreductases Acting on CH-CH Group Donors; C023886:protochlorophyllide reductase