UBC Theses and Dissertations
Comparative studies on several catalytic properties of biosynthetic L-threonine dehydratase (Deaminating) in seven species of unicellular marine planktonic algae Kripps, Robert Stephen
Several aspects of L-threonine dehydratase from seven species of unicellular marine planktonic algae were investigated; (1) the disulfide group requirement for activity of the enzymes from two cryptomonads, (2) monovalent inorganic cation requirement for enzyme activity, (3) substrate specificity and substrate analog inhibitions, (4) allosteric activation and inhibition and diverse effects from other amino acids, (5) pH optima of the algal enzymes with particular emphasis on the elucidation of the unique pH-activity response of the enzyme from Hemiselmis virescens. The threonine dehydratases from Chroomonas salina and Hemiselmis virescens require disulfide groups for enzyme activity as exemplified by the specific inhibition exerted by all thiol reagents tested, which inhibition could be partially reversed or prevented by the appropriate treatments. Sulfhydryl group requirement: for enzyme activity was confirmed and it was demonstrated that these groups are essential for feedback inhibition from L-isoleucine. All algal enzymes appear to require monovalent alkali-metal cations for full expression of activity, more specifically K⁺₄ and NH⁺₄. Anacystis marina was exceptional in showing maximal stimulation from Li⁺. Organic cations were without effect whereas some inhibition from certain divalent cations (Zn²⁺ , Cu²⁺) and anions (N0⁻₃, I⁻ , C10⁻₃) were observed, whilst HP0²₄⁻ and SO²₄⁻ were stimulatory. Aside from L-threonine, the algal enzymes extended substrate activity to L-serine and L-aliothreonine. In addition to its known threonine dehydratase, Chroomonas salina appeared to produce a serine dehydratase which accounted for the relatively high substrate activity observed toward L-serine with this species. Inhibition from substrate analogs was limited to L-homoserine and L-serine, despite the substrate activity of the latter. The mechanism for the peculiar mode of inhibition evinced by L-homoserine remains unknown whereas that of L-serine appears to result from inactivation of the enzyme. With the exception of Cyclotella nana and to a lesser extent Hemiselmis virescens, all the algal enzymes were subject to feedback inhibition from L-isoleucine, which inhibition was pH dependent, subject to reversal by L-valine, and could be duplicated by the analog L-O-methyl threonine. Several other amino acids (L-leucine, L-norvaline, L-valine) were able to inhibit most enzymes when present at high concentration. It was proposed that the mode of inhibition by these latter amino acids may occur via interaction at the site specific for allosteric inhibition. L-Valine at low concentration effected pronounced activation of the enzymes and was thusly assigned the role of allosteric activator, acting at a site distinct from that of L-isoleucine or L-threonine. Hemiselmis virescens was distinctly unique in that, unlike the other algal enzymes, it displayed two pH-activity optima. The investigation of this phenomenon was pursued in two ways (i) examination of enzyme response to various potential effectors (nucleotides, L-methionine, L-aspartate,^ L-cystathionine) at a pH intermediate between the two optima, (ii) examination of enzyme response to known effectors (L-valine, L-isoleucine) at the two pH optima. It was concluded from these studies that Hemiselmis virescens may produce a culture-dependent mixture of two threonine dehydratases, one of which is generally similar to the other algal enzymes, the other of which is insensitive to the usual allosteric regulation yet is not a standard biodegradative isozyme.
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