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Comparison of the activity and conformation changes of lactate dehydrogenase H4 during denaturation by guanidinium chloride.

ABSTRACT: The inactivation and unfolding of lactate dehydrogenase (LDH) during denaturation by guanidinium chloride (GuHCl) under diverse conditions have been compared. Unfolding of the native conformation, as monitored by fluorescence and c.d. measurements, occurs in two stages with increasing GuHCl concentrations, and the inactivation approximately coincides with, but slightly precedes, the first stage of unfolding. The enzyme is inhibited to about 60-70% of its original activity by cross-linking with glutaraldehyde or in the presence of 1 M-(NH4)2SO4, with its conformation stabilized as shown by the requirement for higher GuHCl concentrations to bring about both inactivation and unfolding. Low concentrations of GuHCl (0.2-0.4 M) activate the cross-linked and the (NH4)2SO4-inhibited enzyme back to the level of the native enzyme. For the enzyme stabilized by (NH4)2SO4 or by cross-linking with glutaraldehyde, inactivation occurs at a markedly lower GuHCl concentration than that required to bring about its first stage of unfolding. It is concluded that the active site of LDH is situated in a limited region relatively fragile in conformation as compared with the molecule as a whole. The GuHCl activation of LDH stabilized in (NH4)2SO4 or by cross-linking with glutaraldehyde suggests that this fragility and consequently flexibility of the active site is required for its catalytic activity.

SUBMITTER: Ma YZ

PROVIDER: S-EPMC1151211 | biostudies-other | 1991 Jul

REPOSITORIES: biostudies-other

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Project description:The conformational stability of the ?-hairpin miniprotein, CLN025, a variant of chignolin in which the N- and C-terminal glycines are replaced by tyrosines, in various concentrations of guanidinium chloride (GdmCl) and urea was examined by molecular dynamics simulations and electronic circular dichroism (ECD) spectropolarimetry. The peptide maintains its ?-hairpin conformation in GdmCl and urea solutions. In GdmCl, Gly7 influences the turn to reduce the number of Asp3-Gly7 H-bonds and the Tyr1-Trp9 H-bond is lost. The structure of the peptide is less stable in 3 M GdmCl than in water or 6 M GdmCl, because the number of Asp3-Thr8 and Tyr1-Tyr10 H-bonds are reduced and the Tyr2 side chain moves away from the Pro4 and Trp9 side chains and toward the Tyr10 side chain. This reduces the number of Tyr2-Pro4 CH-? interactions and Tyr2-Trp9 and Tyr1-Tyr10 aromatic-aromatic (Ar-Ar) interactions and increases the number of Tyr2-Tyr10 Ar-Ar interactions. In 6 M GdmCl at 300 and 333 K, the number of Tyr1-Tyr10 and Asp3-Thr8 H-bonds increases, but fewer structures have Tyr2-Pro4 CH-? and Tyr1-Tyr10 and Tyr2-Trp9 Ar-Ar interactions. In urea, Gly7 is in a mixture of ?-turn and random meander structures and the number of Asp3-Thr6 and Tyr1-Tyr10 H-bonds are reduced as are the number of Tyr2-Pro4 CH-? interactions and Tyr1-Tyr10 and Tyr2-Trp9 Ar-Ar interactions. In 4 M urea, a shorter turn places Gly7 into the ?-sheet region and Tyr10 is pushed out into the solvent. In 8 M urea, the number of Asp3-Glu5 H-bonds is increased and the ?-sheet is lost, but the electrostatic interaction between the charged termini is restored and a cation-? interaction between the indolyl ring of Trp9 and the positively charged N-terminus is formed. In 8 M urea at 333 K, the ?-hairpin conformation is almost lost. The structure of CLN025 is stable, because the weakly polar interactions and H-bonds maintain the ?-hairpin conformation in the various environments. CLN025 should not be considered a miniprotein, because it lacks a well-defined tertiary structure, it is resistant to denaturation, it does not have an increased heat capacity near its melting temperature, and the structures near and above the melting temperature retain a ?-hairpin conformation.

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Comparison of the activity and conformation changes of lactate dehydrogenase H4 during denaturation by guanidinium chloride.

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