Kinetic and magnetic resonance studies of effects of genetic substitution of a calcium-liganding amino acid in staphylococcal nuclease

Abstract

The X-ray structure of staphylococcal nuclease suggests octahedral coordination of the essential Ca2+, with Asp-21, Asp-40, and Thr-41 of the enzyme providing three of the six ligands [Cotton, F. A., Hazen, E. E., Jr., & Legg, M. J. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 2551-2555]. The Asp-40 codon was mutated to Gly-40 on the gene that had been cloned into Escherichia coli, and the mutant (D40G) and wild-type enzymes were both purified from E. coli by a simple procedure. The D40G mutant forms a (5 +/- 2)-fold weaker binary complex with Ca2+ as found by kinetic analysis and by Ca2+ binding studies in competition with Mn2+, a linear competitive inhibitor. Similarly, as found by electron paramagnetic resonance (EPR), Mn2+ binds to the D40G mutant with a 3-fold greater KD than that found with the wild-type enzyme. These differences in KD are increased by saturation of staphylococcal nuclease with the DNA substrate such that KmCa is 10-fold greater and KIMn is 15-fold greater for the mutant than for the wild-type enzyme, although KMDNA is only 1.5-fold greater in the mutant. The six dissociation constants of the ternary enzyme-Mn2+-nucleotide complexes of 3',5'-pdTp and 5'-TMP were determined by EPR and by paramagnetic effects on 1/T1 of water protons, and the dissociation constants of the corresponding Ca2+ complexes were determined by competition with Mn2+. Only small differences between the mutant and wild-type enzymes are noted in K3, the dissociation constant of the nucleotides from their respective ternary complexes. 3',5'-pdTp raises the affinities of both wild-type and mutant enzymes for Mn2+ by factors of 47 and 31, respectively, while 5'-TMP raises the affinities of the enzymes for Mn2+ by smaller factors of 6.8 and 4.4, respectively. Conversely, Mn2+ raises the affinities of both wild-type and mutant enzymes for the nucleotides by 1-2 orders of magnitude. Analogous effects are observed in the ternary Ca2+ complexes. Dissociation constants of Ca2+ and Mn2+ from binary and ternary complexes, measured by direct binding studies, show reasonable agreement with those obtained by kinetic analysis. Structural differences in the ternary metal complexes of the D40G mutant are revealed by a 31-fold decrease in Vmax with Ca2+ and by 1.4-3.1-fold decreases in the enhancement of 1/T1 of water protons with Mn2+.(ABSTRACT TRUNCATED AT 400 WORDS)

An expression system for trypsin

“…Induction may be the reason that certain expression systems yield enhanced levels of expression [56,63]. In fact, bacterial alkaline phosphatase is expressed at very high levels when induced [64,65], but at levels similar to trypsin when constitutive 166,671.…”

15N backbone dynamics of the S‐peptide from ribonuclease A in its free and S‐protein bound forms: Toward a site‐specific analysis of entropy changes upon folding

“…DNA insert sequences of plasmids expressing the desired recombinant gene were verified by Sanger dideoxy DNA sequencing. "N isotopic labeling was carried out in 1 L MOPS minimal media (Niedhardt et al, 1974;Serpersu et al, 1986) supplemented with 100 pg/mL ampicillin, and 15 mM "NH,CI. His-tagged fusion protein was purified by immobilized metal affinity chromatography (Novagen) according to the manufacturer's instructions, and dialyzed against thrombin cleavage buffer (150 mM NaCI, 2.5 mM CaC12, 20 mM Tris-HCI pH 8.4).…”

Picomole-scale characterization of protein stability and function by quantitative cysteine reactivity

“…The increase in stability of SN/L36C in the presence of 1 mM Ca 2þ or 50 μM pdTp is 0.6 AE 0.1 and 0.7 AE 0.1 kcal∕mol, which corresponds to apparent K D values of 600 AE 200 μM and 23 AE 4 μM, respectively. Binding of Ca 2þ and pdTp is synergistic, as each exhibits a greater affinity (∼2 μM) for SN in the presence of the other (31). Consequently, a 2∶1 molar solution of Ca 2þ ∶pdTp can be treated thermodynamically as a single, binary ligand Ca 2þ -pdTp.…”