Hydrophobic substrate utilisation by the yeast Yarrowia lipolytica, and its potential applications.

The alkane-assimilating yeast Yarrowia lipolytica degrades very efficiently hydrophobic substrates such as n-alkanes, fatty acids, fats and oils for which it has specific metabolic pathways. An overview of the oxidative degradation pathways for alkanes and triglycerides in Y. lipolytica is given, with new insights arising from the recent genome sequencing of this yeast. This includes the interaction of hydrophobic substrates with yeast cells, their uptake and transport, the primary alkane oxidation to the corresponding fatty alcohols and then by different enzymes to fatty acids, and the subsequent degradation in peroxisomal beta-oxidation or storage into lipid bodies. Several enzymes involved in hydrophobic substrate utilisation belong to multigene families, such as lipases/esterases (LIP genes), cytochromes P450 (ALK genes) and peroxisomal acyl-CoA oxidases (POX genes). Examples are presented demonstrating that wild-type and genetically engineered strains of Y. lipolytica can be used for alkane and fatty-acid bioconversion, such as aroma production, for production of SCP and SCO, for citric acid production, in bioremediation, in fine chemistry, for steroid biotransformation, and in food industry. These examples demonstrate distinct advantages of Y. lipolytica for their use in bioconversion reactions of biotechnologically interesting hydrophobic substrates.

Enzymatic phosphorylation of food proteins by purified and recombinant protein kinase CK2.

Protein kinase CK2 formerly called casein kinase II is a protein kinase able to phosphorylate more than 100 proteic substrates. We have purified protein kinase CK2 from the yeast Y. lipolytica to phosphorylate milk and plant reserve proteins to a significant extent. In the case of plant reserve proteins, which are polymeric substrates, not all subunits are substrate for protein kinase CK2, even if non phosphorylated subunits contain significant potent phosphorylations sites. Best substrates were soy beta-conglycinin (0.72 P/mol) and dephosphorylated caseins (0.5 P/mol). We have studied some functional properties of phosphorylated caseins. Solubility was improved for all pH values but pI. Sensitivity to calcium has also been assessed, and it is slightly improved upon phosphorylation. We have cloned the catalytic subunit of protein kinase CK2 from yeast Y. lipolytica. The recombinant catalytic subunit expressed in E. coli was active and displayed kinetic properties similar to those of the purified enzyme. The recombinant catalytic subunit was able to phosphorylate plant reserve proteins and milk proteins to a significant extent. Best substrates were soy beta-conglycinin (1.0 P/mol), and glycinin (0.59 P/mol).

Expression and characterization of the recombinant catalytic subunit of casein kinase II from the yeast Yarrowia lipolytica in Escherichia coli.

The alpha catalytic subunit of casein kinase II from the yeast Yarrowia lipolytica has been cloned and overexpressed using the pT7-7 expression vector in Escherichia coli. Casein kinase activity is found in the bacterial extracts. The catalytic subunit is partially expressed in a soluble and active form, which is purified to electrophoretic homogeneity. Most of the enzyme was found in inclusion bodies. In this form, the enzyme, which is almost pure, exhibits a low specific activity. We have focused our efforts on methods to activate the protein from the inclusion bodies. We have studied the renaturation of urea-denaturated CKII catalytic subunit. We have tried different renaturation buffers and found that renaturation by dilution was more efficient than renaturation by dialysis. Treatment of the enzyme found in the inclusion bodies with different nondetergent sulfobetaines (NDSB) led to a time-dependent activation. NDSB195 is a V-type activator of the recombinant catalytic subunit of casein kinase II. The NDSB195-activated enzyme remained active at the room temperature for weeks. Kinetic properties of the recombinant casein kinase II subunit are similar to those of the purified holoenzyme (low Km for ATP and inhibition by heparin). Kinetic study indicates that the beta subunit could interact with the alpha subunit at the level of the catalytic site to enhance activity and to modify the kinetic behavior of the enzyme.

Cloning of the gene encoding the catalytic subunit of casein kinase II from the yeast Yarrowia lipolytica.

Casein kinase II from the yeast Yarrowia lipolytica is a heterotetramer of the form alpha alpha' beta 2. We report on the cloning and sequencing of a partial cDNA and of the complete genomic DNA coding for the catalytic alpha subunit of the casein kinase II from this yeast species. The sequence of the gene coding for this enzyme has been analyzed. No intron was found in the gene, which is present in a single copy. The deduced amino acid sequence of the gene shows high similarity with those of alpha subunit described in other species, although, uniquely, Y. lipolytica CKII alpha lacks cysteines. We find that the alpha subunit sequence of Y. lipolytica CKII is shown greater homology with the corresponding protein from S. pombe than with that from S. cerevisiae. We have analyzed CKII alpha expression and CKII alpha activity. We show that expression of this enzyme is regulated. The catalytic subunit is translated from a single mRNA, and the enzyme is present at a very low level in Y. lipolytica, as in other yeasts.

L-tryptophan 2',3'-oxidase from Chromobacterium violaceum. Substrate specificity and mechanistic implications.

L-Tryptophan 2',3'-oxidase, an amino acid alpha,beta-dehydrogenase isolated from Chromobacterium violaceum, catalyzes the formation of a double bond between the C alpha and C beta carbons of various tryptophan derivatives provided that they possess: (i) a L-enantiomeric configuration, (ii) an alpha-carbonyl group, and (iii) an unsubstituted and unmodified indole nucleus. Kinetic parameters were evaluated for a series of tryptophan analogues, providing information on the contribution of each chemical group to substrate binding. The stereochemistry of the dehydro product was determined to be a Z-configuration from proton nuclear magnetic resonance assignments. No reaction can be observed in the presence of other aromatic beta-substituted alanyl residues which behave neither as substrates nor as inhibitors and therefore do not compete against this reaction. The enzymatic synthesis of alpha,beta-dehydrotryptophanyl peptides from 5 to 24 residues was successfully achieved without side product formation, irrespective of the position of the tryptophan residue in the amino acid sequence. A reactional mechanism involving a direct alpha,beta-dehydrogenation of the tryptophan side chain is proposed.