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Guanidine Hydrochloride Exerts Dual Effects on the Tryptophan Synthase Alpha 2 Beta 2 Complex as a Cation Activator and as a Modulator of the Active Site Conformation

Guanidine Hydrochloride Exerts Dual Effects on the Tryptophan Synthase Alpha 2 Beta 2 Complex as a Cation Activator and as a Modulator of the Active Site Conformation

Y X Fan, P McPhie, E W Miles

Biochemistry. 1999 Jun 15;38(24):7881-90.

PMID: 10387029

Abstract:

To characterize the conformational transitions that regulate the activity and specificity of the tryptophan synthase alpha 2 beta 2 complex, we have determined some effects of low concentrations of guanidine hydrochloride (GuHCl) and of urea on functional properties. We report the novel finding that GuHCl at low concentrations (0. 02-0.08 M) is a cation activator of the tryptophan synthase alpha 2 beta 2 complex. Molecular modeling studies show that GuH+ could bind at a previously identified cation binding site in the tryptophan synthase beta subunit. Addition of increasing concentrations of GuHCl has strikingly different effects on the rates of different reactions with L-serine or beta-chloro-L-alanine in the presence or absence of indole. Spectroscopic studies demonstrate that GuHCl alters the equilibrium distribution of pyridoxal 5'-phosphate intermediates formed in reactions at the active site of the beta subunit. Data analysis shows that GuHCl binds preferentially with the conformer of the enzyme that predominates when the aldimine of L-serine is formed and shifts the equilibrium in favor of this conformer. These results provide evidence that GuHCl exerts dual effects on tryptophan synthase as a cation, stimulating activity, and as a chaotropic agent, altering the distribution of conformational states that exhibit different reaction specificities. Our finding that the nonionic urea stabilizes the aldimine of L-serine in the presence, but not in the absence, of NaCl shows that cation binding plays an important role in the conformational transitions that regulate activity and the transmission of allosteric signals between the alpha and beta sites.

Chemicals Related in the Paper:

Catalog Number	Product Name	Structure	CAS Number	Price
AP51887899	β-Chloro-L-alanine hydrochloride		51887-89-9	Price

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