Krappmann, Sven Kurt

English

Chorismate mutases (E.C. 5.4.99.5)
catalyse the Claisen rearrangement of chorismic acid to prephenate,
the first reaction of the tyrosine/phenylalanine-specific branch of
aromatic amino acid biosynthesis. Within this biosynthetic pathway
they compete with the anthranilate synthase complex for the common
substrate chorismate and both enzymes define the first branch point
of that metabolic pathway. The ARO7-encoded chorismate mutase of
the bakerís yeast Saccharomyces cerevisiae which has previously
been characterized in detail serves as model system for allosteric
regulation of catalytic activity. Other chorismate mutases of fungi
are hardly characterized. In this thesis, the mechanisms regulating
the enzymatic activities that channel chorismate into the two main
branches in S. cerevisiae were analysed. The impact of an
allosterically unregulated chorismate mutase was investigated in
combination with genetically engineered variations in
transcriptional regulation of expression of both branch point
genes. It turned out that only the regulatory pattern as it exists
at the branch point is sufficient under amino acid starvation
conditions. The results imply that in the bakerís yeast
transcriptional and allosteric regulation have evolved in
accordance to guarantee optimal flux of the intermediate compound
into both branches. For comparison, the chorismate mutase activity
of a methylotrophic yeast, the HARO7 gene product of Hansenula
polymorpha, was characterized. The allosteric enzyme is strictly
regulated by the end products tyrosine and tryptophan. In the
presence of methanol as sole carbon source, transcription of the
encoding gene is induced, whereas under amino acid starvation
conditions no additional transcriptional regulation is present. In
order to characterize the chorismate mutase of a filamentous
fungus, the aroC-encoded enzyme of Aspergillus nidulans was
investigated. Here, again no transcriptional regulation upon
starvation conditions is present, but allosteric regulation by the
heterotropic effectors tyrosine and tryptophan. Catalytic
properties of the gene product were determined as well as its
quaternary structure. Furthermore, it was shown that the allosteric
intramolecular signal transduction pathway is not conserved with
respect to the bakerís yeast enzyme

Georg-August-University Göttingen

Biosynthesis of Aromatic Amino Acids in Yeast and Aspergillus

https://ediss.uni-goettingen.de/bitstream/11858/00-1735-0000-000D-F20C-1/1/krappmann.pdf

Biosynthesis of Aromatic Amino Acids in Yeast and Aspergillus

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Georg-August-University Göttingen