The crystal structure of the spinach plastocyanin double mutant G8D/L12E gives insight into its low reactivity towards photosystem 1 and cytochrome f.
Plastocyanin (Pc) is a copper-containing protein, which functions as an electron carrier between the cytochrome b(6)f and photosystem 1 (PS1) complexes in the photosynthetic electron transfer (ET) chain. The ET is mediated by His87 situated in the hydrophobic surface in the north region of Pc. Also situated in this region is Leu12, which mutated to other amino acids severely disturbs the ET from cytochrome f and to PS1, indicating the importance of the hydrophobic surface. The crystal structure of the Pc double mutant G8D/L12E has been determined to 2.0 A resolution, with a crystallographic R-factor of 18.3% (R(free)=23.2%). A comparison with the wild-type structure reveals that structural differences are limited to the sites of the mutations. In particular, there is a small but significant change in the hydrophobic surface close to His87. Evidently, this leads to a mismatch in the reactive complex with the redox partners. For PS1 this results in a 20 times weaker binding and an eightfold slower ET as determined by kinetic measurements. The mutations that have been introduced do not affect the optical absorption spectrum. However, there is a small change in the EPR spectrum, which can be related to changes in the copper coordination geometry.
Jansson
Hanna
H
Center for Structural Biology and Department of Biochemistry and Biophysics, Göteborg University, Box 462, SE-405 30, Göteborg, Sweden.
Okvist
Mats
M
Jacobson
Frida
F
Ejdebäck
Mikael
M
Hansson
Orjan
O
Sjölin
Lennart
L
D019943:Amino Acid Substitution; D018360:Crystallography, X-Ray; D045348:Cytochromes f / Q000378:metabolism; D007525:Isoelectric Focusing; D007700:Kinetics; D045331:Photosystem I Protein Complex / Q000378:metabolism; D010970:Plastocyanin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D018724:Spinacia oleracea / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
D045331:Photosystem I Protein Complex; D010970:Plastocyanin; D045348:Cytochromes f