10 research outputs found
Probing the Arabidopsis Flagellin Receptor: FLS2-FLS2 Association and the Contributions of Specific Domains to Signaling Function[W][OA]
Transmembrane LRR-RLKs are a major class of plant proteins. This study investigates the functional contributions of multiple FLS2 protein domains and modifications to provide insight into structure-function relationships of LRR-RLK proteins in general
Methylated <i>N</i><sup>Ï</sup>âHydroxyâlâarginine Analogues as Mechanistic Probes for the Second Step of the Nitric Oxide Synthase-Catalyzed Reaction
Nitric
oxide synthase (NOS) catalyzes the conversion of l-arginine
to l-citrulline through the intermediate <i>N</i><sup>Ï</sup>-hydroxy-l-arginine (NHA), producing
nitric oxide, an important mammalian signaling molecule. Several disease
states are associated with improper regulation of nitric oxide production,
making NOS a therapeutic target. The first step of the NOS reaction
has been well-characterized and is presumed to proceed through a compound
I heme species, analogous to the cytochrome P450 mechanism. The second
step, however, is enzymatically unprecedented and is thought to occur
via a ferric peroxo heme species. To gain insight into the details
of this unique second step, we report here the synthesis of NHA analogues
bearing guanidinium methyl or ethyl substitutions and their investigation
as either inhibitors of or alternate substrates for NOS. Radiolabeling
studies reveal that <i>N</i><sup>Ï</sup>-methoxy-l-arginine, an alternative NOS substrate, produces citrulline,
nitric oxide, and methanol. On the basis of these results, we propose
a mechanism for the second step of NOS catalysis in which a methylated
nitric oxide species is released and is further metabolized by NOS.
Crystal structures of our NHA analogues bound to nNOS have been determined,
revealing the presence of an active site water molecule only in the
presence of singly methylated analogues. Bulkier analogues displace
this active site water molecule; a different mechanism is proposed
in the absence of the water molecule. Our results provide new insights
into the steric and stereochemical tolerance of the NOS active site
and substrate capabilities of NOS
Enzymatic and Cryoreduction EPR Studies of the Hydroxylation of Methylated <i>N</i><sup>Ï</sup>âHydroxyâlâarginine Analogues by Nitric Oxide Synthase from <i>Geobacillus stearothermophilus</i>
Nitric
oxide synthase (NOS) catalyzes the conversion of l-arginine
to l-citrulline and NO in a two-step process involving the
intermediate <i>N</i><sup>Ï</sup>-hydroxy-l-arginine (NHA). It was shown that Cpd I is the oxygenating species
for l-arginine; the hydroperoxo ferric intermediate is the
reactive intermediate with NHA. Methylation of the N<sup>Ï</sup>-OH and N<sup>Ï</sup>-H of NHA significantly inhibits the conversion
of NHA into NO and l-citrulline by mammalian NOS. Kinetic
studies now show that N<sup>Ï</sup>-methylation of NHA has a
qualitatively similar effect on H<sub>2</sub>O<sub>2</sub>-dependent
catalysis by bacterial gsNOS. To elucidate the effect of methylating
N<sup>Ï</sup>-hydroxy l-arginine on the properties
and reactivity of the one-electron-reduced oxy-heme center of NOS,
we have applied cryoreduction/annealing/EPR/ENDOR techniques. Measurements
of solvent kinetic isotope effects during 160 K cryoannealing cryoreduced
oxy-gsNOS/NHA confirm the hydroperoxo ferric intermediate as the catalytically
active species of step two. Product analysis for cryoreduced samples
with methylated NHAâs, NHMA, NMOA, and NMMA, annealed to 273
K, show a correlation of yields of l-citrulline with the
intensity of the <b>g 2.26</b> EPR signal of the peroxo ferric
species trapped at 77 K, which converts to the reactive hydroperoxo
ferric state. There is also a correlation between the yield of l-citrulline in these experiments and <i>k</i><sub>obs</sub> for the H<sub>2</sub>O<sub>2</sub>-dependent conversion
of the substrates by gsNOS. Correspondingly, no detectable amount
of cyanoornithine, formed when Cpd I is the reactive species, was
found in the samples. Methylation of the NHA guanidinium N<sup>Ï</sup>-OH and N<sup>Ï</sup>-H inhibits the second NO-producing reaction
by favoring protonation of the ferric-peroxo to form unreactive conformers
of the ferric-hydroperoxo state. It is suggested that this is caused
by modification of the distal-pocket hydrogen-bonding network of oxy
gsNOS and introduction of an ordered water molecule that facilitates
delivery of the proton(s) to the one-electron-reduced oxy-heme moiety.
These results illustrate how variations in the properties of the substrate
can modulate the reactivity of a monooxygenase