3 research outputs found
Aspects of the hemes and modulation of hydrogen donors in catalases from bovine liver, yeast, and escherichia coli
Catalase is the enzyme which decomposes hydrogen peroxide to
water and oxygen. Escherichia coli contains two catalases.
Hydroperoxidase I (HPI) is a bifunctional catalase-peroxidase.
Hydroperoxidase II (HPII) is only catalytically active toward H202.
Expression of the genes encoding these proteins is controlled by
different regimes. HPJI is thought to be a hexamer, having one heme d
cis group per enzymatic subunit. HPII wild type protein and heme
containing mutant proteins were obtained from the laboratory of P.
Loewen (Univ. of Manitoba). Mutants constructed by oligonucleotidedirected
mutagenesis were targeted for replacement of either the
His128 residue or the Asn201 residue in the vicinity of the HPII heme
crevice. His128 is the residue thought to be analogous to the His74
distal axial ligand of the heme in the bovine liver enzyme, and Asn201
is believed to be a residue critical to the function of the enzyme
because of its role in orienting and interacting with the substrate
molecule. Investigation of the nature of the hemes via absorption
spectroscopy of the unmodified catalase proteins and their derived
pyridine hemochromes showed that while the bovine and Saccharomyces
cerevisiae catalase enzymes are protoheme-containing, the HPII wild
type protein contains heme d, and the mutant proteins contain either
solely protoheme, or heme d-protoheme mixtures. Cyanide binding
studies supported this, as ligand binding was monophasic for the
bovine, Saccharomyces cerevisiae, and wild type HPII enzymes, but
biphasic for several of the HPII mutant proteins.
Several mammalian catalases, and at least two prokaryotic
catalases, are known to be NADPH binding. The function of this cofactor
appears to be the prevention of inactivation of the enzyme, which
occurs via formation of the inactive secondary catalase peroxide
compound (compound II). No physiologically plausible scheme has yet
been proposed for the NADPH mediation of catalase activity. This study
has shown, via fluorescence and affinity chromatography techniques,
that NADPH binds to the T (Typical) and A (Atypical) catalases of
Saccharomyces cerevisiae, and that wild type HPII apparently does not
bind NADPH. This study has also shown that NADPH is unlike any other
hydrogen donor to catalase, and addresses its features as a unique
donor by proposing a mechanism whereby NADPH is oxidized and
catalase is protected from inactivation via the formation of protein
radical species. Migration of this radical to a position close to the
NADPH is also proposed as an adjunct hypothesis, based on similar
electron migrations that are known to occur within metmyoglobin and
cytochrome c peroxidase when reacted with H202. Validation of these
hypotheses may be obtained in appropriate future experiments
Four lectures on secant varieties
This paper is based on the first author's lectures at the 2012 University of
Regina Workshop "Connections Between Algebra and Geometry". Its aim is to
provide an introduction to the theory of higher secant varieties and their
applications. Several references and solved exercises are also included.Comment: Lectures notes to appear in PROMS (Springer Proceedings in
Mathematics & Statistics), Springer/Birkhause