Download Methylamine Dehydrogenase: Structure and Function of Electron

A30
Biochemical Society Transactions (1999) 27
K5
Electron transfer in trimethylamine dehydrogenase
and electron transfemng flavoprotein.
K7 Ribonucleotide Reductase - Coupled ElectrodProton
Transfer Mechanisms
N. S. Scrutton, M. J. Sutcliffe. J. Basran and R.Hille
Siobere. B.M Department of Molecular Biology, University of
Stockholm, S-106 91
Departments of Biochemistry and Chemistry, University
of Leicester, University Road, UK and Department of
Medical Biochemistry, Ohio State University, Ohio, USA
Bacterial trimethylamine dehydrogenase (Th4ADH)
catalyses the oxidative demethylation of trimethylamine
to produce dimethylamine and formaldehyde. During
cleavage of the substrate C-H bond two electrons are
transferred to a 6-S-cysteinyl FMN and then individually
in two sequential one electron transfers to a ferredoxinlike 4Fe-4s centre. Reoxidation of the enzyme is
achieved by the transient formation of an electron
transfer complex with electron transfemng flavoprotein
0,
and transfer of electrons from the reduced 4Fe-4s
centre to the FAD of ETF. Complex formation is thought
to involve a large structural reorganisation in ETF and
electron transfer is facilitated by residue Tyr-442 on the
enzyme surface. In the reductive half-reaction, electron
transfer to the flavin is partially disrupted by mutation of
a novel Tyr-His-Asp triad in the active site. In two mutant
enzymes altered in the triad region the transfer of
reducing equivalents is resolved into discrete bondbreaking and electron transfer steps. Our mutagenesis
data reveal that a carbanion mechanism is unlikely to
operate in TMADH and that following cleavage of the
substrate C-H bond, H-transfer is via a H-tunnelling
process mediated by low frequency vibrational motions
of the enzyme.
K6 Methylamine Dehydrogenase: Structure and Function of
Electron Transfer Complexes. Victor L. Davidson, Dept .
of Biochemistry, University of Mississippi Medical Center,
Jackson MS 39214-4505.
Methylamine dehydrogenase [MADH] from P. denitr!ficans,
which possesses the tryptophan tryptophylquinone [TTQ]
cofactor, catalyzes the oxidative deamination of amines.
Electrons derived from these oxidations are transferred to the
respiratory chain via the type I copper protein, amicyanin, and
cytochrome c-551 i. Complex formation between these three
soluble proteins is required for this physiologic electron transfer
[ET] process. The crystal structure of this ternary protein
complex has been solved and the complex is hnctional in the
crystalline state. Site-directed mutagenesis has confirmed that
MADH and amicyanin interact in solution as seen in the crystal
structure. ET to amicyanin was studied from four different
redox forms of MADH: dithionite-reduced 0-quinol and 0semiquinone, and substrate-reduced N-quinol and Nsemiquinone. In the N forms, the substrate-derived amino group
has displaced a TTQ quinone oxygen. ET rates of the 0 forms
and the N-semiquinone vary with AGO as predicted by Marcus
theory and exhibit an electronic coupling [H-] of 12 cm-l and a
reorganizational energy [h] of 2.3 eV. Analysis of the AGO and
temperature dependencies of these reactions predicts the ET
distance that is seen in the crystal structure. In contrast, the ET
reaction of the N-quinol is gated. The rate-limiting step for that
redox reaction is deprotonation of the substrate-derived amino
group on TTQ. Analysis of the temperature dependence of the
~ 0.3 cm’l and h of 1.1
ET from copper to heme yielded an H a of
eV, and predicted an ET distance consistent with that seen in the
crystal structure. These results are providing insight into the
factors that control the rates of long range protein ET reactions.
K8
QUINOPROTEIN DEHYDROGENASES
C. Anthony. Division of Biochemistry and Molecular Biology,
School of Biological Sciences, University of Southampton,
Southampton SO16 7PX, UK
These dehydrogenases catalyse oxidation reactions in the
periplasm of bacteria and are characterised by the possession of
orthoquinone prosthetic groups: TTQ (tryptophan tryptophyl
quinone) occurs in amine dehydrogenases, and PQQ (pyrroloquinoline quinone) occurs in the soluble dehydrogenases for
alcohols, and in the membrane alcohol and glucose
dehydrogenases.All these enzymes have P-propeller structures but
this probably has no special functional significance. The electron
acceptors include periplasmic blue copper proteins or c-type
cytochromes, or membrane ubiquinone. The main questions of
interest include: what are the mechanisms of electron transfer from
substrate to prosthetic groups, and from the quinol forms of the
prosthetic groups to the electron acceptors? And what structural
features are relevant in determining ‘docking’ of interacting
proteins.