THE 10-STRANDED BETA ALPHA BARREL IN RIBONUCLEOTIDE REDUCTASE PROTEINR1/

The large subunit of ribonucleotide reductase (RNR) contains ten-stran ded beta/alpha barrel of a new type consisting of two antiparallel hal ves. The two halves of the barrel are pseudo 2-fold-related, have simi lar folds but different additional intervening secondary structure ele ments and loops. The inner diameter of the RNR barrel, 15 Angstrom to 20 Angstrom, is significantly larger than for the (beta alpha)(8) barr els. The larger barrel forms a stable framework which holds an inserte d hairpin loop rigidly and exposes active site residues at its tip. Th e barrel organization allows three cysteine residues to be positioned close to each other without forming unfavorable disulfide bridges betw een Cys439 on the tip of the inserted loop and the redox-active cystei ne residues on the barrel strands. Redox-active cysteine residues sepa rated by more than 200 residues are held in close proximity to each ot her on adjacent barrel strands. (C) 1996 Academic Press Limited