The influence of the amino acid residues sandwiching the flavin ring in flavodoxin (Fld) from the cyanobacteriumAnabaena sp. PCC 7119 in complex formation and electron transfer (ET) with its natural partners, photosystem I (PSI) and ferredoxin-NADP+ reductase (FNR), was examined in mutants of the key residues Trp57 and Tyr94. The mutants' ability to form complexes with either FNR or PSI is similar to that of wild-type Fld. However, some of the mutants exhibit altered kinetic properties in their ET processes that can be explained in terms of altered flavin accessibility and/or thermodynamic parameters. The most noticeable alteration is produced upon replacement of Tyr94 by alanine. In this mutant, the processes that involve the transfer of one electron from either PSI or FNR are clearly accelerated, which might be attributable to a larger accessibility of the flavin to the reductant. However, when the opposite ET flow is analyzed with FNR, the reduced Y94A mutant transfers electrons to FNR slightly more slowly than wild type. This can be explained thermodynamically from a decrease in driving force due to the significant shift of 137 mV in the reduction potential value for the semiquinone/hydroquinone couple (E 1) of Y94A, relative to wild type (Lostao, A., Gómez-Moreno, C., Mayhew, S. G., and Sancho, J. (1997) Biochemistry 36, 14334–14344). The behavior of the rest of the mutants can be explained in the same way. Overall, our data indicate that Trp57 and Tyr94 do not play any active role in flavodoxin redox reactions providing a path for the electrons but are rather involved in setting an appropriate structural and electronic environment that modulates in vivo ET from PSI to FNR while providing a tight FMN binding.

Comisión Interministerial de Ciencia y Tecnología BIO2000-1259, BQU2001-2520

Dirección General de Estudios Superiores PB97-1027 and BMC 2001-2522

European Union Networks ERB-FMRXCT98-0218 and HPRN-CT1999-00095

Dirección General de Investigación BMC2000-0444

Diputación General de Aragón P006/2000

Junta de Andalucía CVI-0198