Characterization of a novel 4-guanidinobutyrase from Candida parapsilosis.

Enzymes of the ureohydrolase superfamily are specific in recognizing their substrates. While looking to broaden the substrate specificity of 4-guanidinobutyrase (GBase), we isolated a yeast, typed as Candida parapsilosis (NCIM 3689), that efficiently utilized both 4-guanidinobutyrate (GB) and 3-guanidinopropionate (GP) as a sole source of nitrogen. A putative GBase sequence was identified from its genome upon pBLAST query using the GBase sequence from Aspergillus niger (AnGBase). The C. parapsilosis GBase (CpGBase) ORF was PCR amplified, cloned, and sequenced. Further, the functional CpGBase protein expressed in Saccharomyces cerevisiae functioned as GBase and 3-guanidinopropionase (GPase). S. cerevisiae cannot grow on GB or GP. However, the transformants expressing CpGBase acquired the ability to utilize and grow on both GB and GP. The expressed CpGBase protein was enriched and analyzed for substrate saturation and product inhibition by γ-aminobutyric acid and ß-alanine. In contrast to the well-characterized AnGBase, CpGBase from C. parapsilosis is a novel ureohydrolase and showed hyperbolic saturation for GB and GP with comparable efficiency (Vmax/KM values of 3.4 and 2.0, respectively). With the paucity of structural information and limited active site data available on ureohydrolases, CpGBase offers an excellent paradigm to explore this class of enzymes.

Protein expression and secretion by filamentous fungi.

In the search for optimal platforms for protein expression and secretion, filamentous fungi in principle provide some of the best microbial cell factories. They are inherently endowed with the ability to secrete proteins. Fungi belonging to Aspergillus and Trichoderma species are well-studied for industrial production of proteins and enzymes. Our understanding of these organisms at the level of transcription, translation, post-translational processing and the secretory pathways has improved significantly in recent years. Despite this, the ability of these fungal secretion platforms has not yet been able to match their intrinsic secretion capacity to produce foreign proteins. Details of the molecular mechanisms of the secretory pathways in filamentous fungi are emerging. This knowledge can be gainfully employed to enhance protein production in filamentous fungi, particularly in the secretion of heterologous proteins of value.