PubMed Health⌕ Search

Biomedical subjects

C B Libby

Publications and source records attributed to C B Libby.

4 recordsLinked to original sources
PubMed1994

Effect of amino acid deletions in the O-glycosylated region of Aspergillus awamori glucoamylase.

Aspergillus awamori glucoamylase (GA) contains globular catalytic and starch-binding domains (residues 1-471 and 509-616, respectively). A heavily O-glycosylated sequence comprises two parts. The first (residues 441-471) in the crystal structure wraps around an alpha/alpha-barrel formed by residues 1-440. The second (residues 472-508) is an extended, semi-rigid linker between the two domains. To investigate the functional role of this linker, we made internal deletions to remove residues 466-512 (GA delta 1), 485-512 (GA delta 2) and 466-483 (GA delta 3). GA delta 2 and GA delta 3 were expressed in Saccharomyces cerevisiae culture supernatants at approximately 60 and 20% the wild-type level, respectively, while GA delta 1 was almost undetectable. Western blots comparing extracellular and intracellular fractions indicated that the region deleted in GA delta 3 was critical for secretion, while the region deleted in GA delta 2 contributed to the production of a stable enzyme structure. The activities of purified GA delta 2 and GA delta 3 on soluble and insoluble starch were similar to those of wild-type GA, indicating that for soluble starch their deletions did not affect the catalytic domain and for insoluble starch the linker does not coordinate the activities of the catalytic and starch-binding domains. The deletions had a significant negative effect on GA delta 2 and GA delta 3 thermostabilities.

Amino Acid Sequence↗
PubMed1975

Kinetic and binding studies of Mn (II) and fructose 1,6-bisphosphate with rabbit liver hexosebisphosphatase.

The separate interaction of the substrate fructose 1,6-bisphosphate and a metal ion cofactor Mn2+ with neutral hexosebisphosphatase has been studied under equilibrium conditions at pH 7.5 with gel filtration and electron paramagnetic resonance measurements, respectively. Binding data for both ligands to the enzyme yielded nonlinear Scatchard plots that analyze in terms of four negatively cooperative binding sites per enzyme tetramer. Graphical estimates of the binding constants were refined by a computer searching procedure and nonlinear least squares analysis. These results are qualitatively similar to those obtained from binding studies involving teh alkaline enzyme, a modified form of hexosebisphosphatase whose pH optimum is in the alkaline pH region. Both forms of the enzyme enhance the proton relaxation rate of water protons by a factor of approximately 7 to 8 at 24 MHz, demonstrating similar metal ion environments. Teh activator Co(III)-EDTA did not affect Mn2+ binding to the neutral enzyme. In the presence of (alpha + beta)methyl-D-fructofuranoside 1,6-bisphosphate, however, two sets--each containing four Mn2+ binding sites--were observed per enzyme tetramer with loss of the negatively cooperative interaction. These results are viewed in terms of four noncatalytic and four catalytic Mn2+ binding sites. Parallel kinetic investigations were conducted on the neutral enzyme to determine specific activity as a function of Mn2+ and fructose 1,6-bisphosphate concentration. A pro-equilibrium sequential pathway model involving Mn2+-enzyme and the Mn2+-fructose 1,6-bisphosphate complex both as substrate and as an allosteric inhibitor satisfactorily fit the kinetic observations. All possible enzyme species were computed from the determined binding constants and grouped according to the number of moles of Mn2+-fructose 1,6-bisphosphate complex bound to the Mn2+-enzyme, and individual rate constants were calculated. The testing of other models and their failure to describe the kinetic observations are discussed.

Animals↗