HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Dungan, S. M. Articles by Datta, P. Search for Related Content PubMed PubMed Citation Articles by Dungan, S. M. Articles by Datta, P. Social Bookmarking                      What's this?

Concerted Feedback Inhibition

PURIFICATION AND SOME PROPERTIES OF ASPARTOKINASE FROM PSEUDOMONAS FLUORESCENS

From the ¹ From the Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48104

Aspartokinase of Pseudomonas fluorescens has been purified by heat treatment, ammonium sulfate fractionation, DEAE-cellulose chromatography, and by successive gel filtration steps on Sephadex G-200 in the presence and absence of feedback modifiers. The 700-fold purified enzyme was judged to be greater than 90% pure. Sedimentation velocity centrifugation yielded an s⁰_20,w value of 6.7 S, a D_20,w value of 5.3 x 10^-7 cm² s^-1, and a molecular weight of 133,000. The Stokes radius of the native enzyme in 50 mm potassium phosphate buffer, pH 7.5, containing 200 mm KCl was found to be about 43 A. A diffusion coefficient of 5.2 x 10^-7 cm² s^-1 was calculated from the Stokes-Einstein equation. A molecular weight of 137,000 was also estimated from the gel filtration data. The molecular weight of the subunit was 43,000.

The enzyme had an absolute requirement for ATP and a divalent cation. No other nucleoside phosphates served as the phosphate donor. Addition of 200 mm KCl increased the reaction rate by about 40%; sodium and lithium ions had no effect.

The activity of this enzyme was inhibited by concerted feedback by two pairs of amino acid end products: lysine plus threonine and methionine plus threonine. Individually, lysine and methionine were slightly stimulatory; threonine showed a slight activation at low concentrations, but was weakly inhibitory at concentrations over 10 mm. Concerted inhibition by low threonine and high lysine combination was compensated by methionine; whereas, methionine did not counteract when enzyme activity was severely decreased by high concentrations of threonine plus lysine. A possible physiological significance of the dual role of methionine is discussed.

CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?