Identification and characterization of a novel glucose-phosphorylating enzyme inKluyveromyces lactis

An Overview on the Mechanisms and Applications of Enzyme Inhibition-Based Methods for Determination of Organophosphate and Carbamate Pesticides

Acetylcholinesterase inactivating compounds, such as organophosphate (OP) and carbamate (CM) pesticides, are widely used in agriculture to ensure sustainable production of food and feed. As a consequence of their applications, they would result in neurotoxicity, even death. In this essence, the development of enzyme inhibition methods still shows great significance as rapid detection techniques for on-site large-scale screening of OPs and CMs. Initially, mechanisms and applications of various enzyme-inhibition-based methods and devices, including optical colorimetric assay, fluorometric assays, electrochemical biosensors, rapid test card, and microfluidic device, are highlighted in the present overview. Further, to enhance the enzyme sensitivity for detection; alternative enzyme sources or high yield enrichment methods (such as abzyme, artificial enzyme, and recombinant enzyme), as well as enzyme reactivation and identification, are also addressed in this comprehensive overview.

Crystal Structure of Kluyveromyces lactis Glucokinase (KlGlk1)

Glucose phosphorylating enzymes are crucial in the regulation of basic cellular processes, including metabolism and gene expression. Glucokinases and hexokinases provide a pool of phosphorylated glucose in an adenosine diphosphate (ADP)- and ATP-dependent manner to shape the cell metabolism. The glucose processing enzymes from Kluyveromyces lactis are poorly characterized despite the emerging contribution of this yeast strain to industrial and laboratory scale biotechnology. The first reports on K. lactis glucokinase (KlGlk1) positioned the enzyme as an essential component required for glucose signaling. Nevertheless, no biochemical and structural information was available until now. Here, we present the first crystal structure of KlGlk1 together with biochemical characterization, including substrate specificity and enzyme kinetics. Additionally, comparative analysis of the presented structure and the prior structures of lactis hexokinase (KlHxk1) demonstrates the potential transitions between open and closed enzyme conformations upon ligand binding.

The gene YALI0E20207g from Yarrowia lipolytica encodes an N-acetylglucosamine kinase implicated in the regulated expression of the genes from the N-acetylglucosamine assimilatory pathway

The non-conventional yeast Yarrowia lipolytica possesses an ORF, YALI0E20207g, which encodes a protein with an amino acid sequence similar to hexokinases from different organisms. We have cloned that gene and determined several enzymatic properties of its encoded protein showing that it is an N-acetylglucosamine (NAGA) kinase. This conclusion was supported by the lack of growth in NAGA of a strain carrying a YALI0E20207g deletion. We named this gene YlNAG5. Expression of YlNAG5 as well as that of the genes encoding the enzymes of the NAGA catabolic pathway-identified by a BLAST search-was induced by this sugar. Deletion of YlNAG5 rendered that expression independent of the presence of NAGA in the medium and reintroduction of the gene restored the inducibility, indicating that YlNag5 participates in the transcriptional regulation of the NAGA assimilatory pathway genes. Expression of YlNAG5 was increased during sporulation and homozygous Ylnag5/Ylnag5 diploid strains sporulated very poorly as compared with a wild type isogenic control strain pointing to a participation of the protein in the process. Overexpression of YlNAG5 allowed growth in glucose of an Ylhxk1glk1 double mutant and produced, in a wild type background, aberrant morphologies in different media. Expression of the gene in a Saccharomyces cerevisiae hxk1 hxk2 glk1 triple mutant restored ability to grow in glucose.

Yeast HXK2 gene reverts glucose regulation mutation of penicillin biosynthesis in P. chrysogenum

The mutant Penicillium chrysogenum strain dogR5, derived from strain AS-P-78, does not respond to glucose regulation of penicillin biosynthesis and β-galactosidase, and is partially deficient in D-glucose phosphorilating activity. We have transformed strain dogR5 with the (hexokinase) hxk2 gene from Saccharomyces cerevisiae. Transformants recovered glucose control of penicillin biosynthesis in different degrees, and acquired a hexokinase (fructose phosphorylating) activity absent in strains AS- P-78 and dogR5. Interestingly, they also recovered glucose regulation of β-galactosidase. On the other hand, glucokinase activity was affected in different ways in the transformants; one of which showed a lower activity than the parental dogR5, but normal glucose regulation of penicillin biosynthesis. Our results show that Penicillium chrysogenum AS-P-78 and dogR5 strains lack hexokinase, and suggest that an enzyme with glucokinase activity is involved in glucose regulation of penicillin biosynthesis and β-galactosidase, thus signaling glucose in both primary and secondary metabolism; however, catalytic and signaling activities seem to be independent.

Proteomic and functional consequences of hexokinase deficiency in glucose-repressible Kluyveromyces lactis

The analysis of glucose signaling in the Crabtree-positive eukaryotic model organism Saccharomyces cerevisiae has disclosed a dual role of its hexokinase ScHxk2, which acts as a glycolytic enzyme and key signal transducer adapting central metabolism to glucose availability. In order to identify evolutionarily conserved characteristics of hexokinase structure and function, the cellular response of the Crabtree-negative yeast Kluyveromyces lactis to rag5 null mutation and concomitant deficiency of its unique hexokinase KlHxk1 was analyzed by means of difference gel electrophoresis. In total, 2,851 fluorescent spots containing different protein species were detected in the master gel representing all of the K. lactis proteins that were solubilized from glucose-grown KlHxk1 wild-type and mutant cells. Mass spectrometric peptide analysis identified 45 individual hexokinase-dependent proteins related to carbohydrate, short-chain fatty acid and tricarboxylic acid metabolism as well as to amino acid and protein turnover, but also to general stress response and chromatin remodeling, which occurred as a consequence of KlHxk1 deficiency at a minimum 3-fold enhanced or reduced level in the mutant proteome. In addition, three proteins exhibiting homology to 2-methylcitrate cycle enzymes of S. cerevisiae were detected at increased concentrations, suggesting a stimulation of pyruvate formation from amino acids and/or fatty acids. Experimental validation of the difference gel electrophoresis approach by post-lysis dimethyl labeling largely confirmed the abundance changes detected in the mutant proteome via the former method. Taking into consideration the high proportion of identified hexokinase-dependent proteins exhibiting increased proteomic levels, KlHxk1 is likely to have a repressive function in a multitude of metabolic pathways. The proteomic alterations detected in the mutant classify KlHxk1 as a multifunctional enzyme and support the view of evolutionary conservation of dual-role hexokinases even in organisms that are less specialized than S. cerevisiae in terms of glucose utilization.

Yeast on the milky way: genetics, physiology and biotechnology of Kluyveromyces lactis

The milk yeast Kluyveromyces lactis has a life cycle similar to that of Saccharomyces cerevisiae and can be employed as a model eukaryote using classical genetics, such as the combination of desired traits, by crossing and tetrad analysis. Likewise, a growing set of vectors, marker cassettes and tags for fluorescence microscopy are available for manipulation by genetic engineering and investigating its basic cell biology. We here summarize these applications, as well as the current knowledge regarding its central metabolism, glucose and extracellular stress signalling pathways. A short overview on the biotechnological potential of K. lactis concludes this review.

In vivo phosphorylation and in vitro autophosphorylation-inactivation of Kluyveromyces lactis hexokinase KlHxk1

The bifunctional hexokinase KlHxk1 is a key component of glucose-dependent signal transduction in Kluyveromyces lactis. KlHxk1 is phosphorylated in vivo and undergoes ATP-dependent autophosphorylation-inactivation in vitro. This study identifies serine-15 as the site of in vivo phosphorylation and serine-157 as the autophosphorylation-inactivation site. X-ray crystallography of the in vivo phosphorylated enzyme indicates the existence of a ring-shaped symmetrical homodimer carrying two phosphoserine-15 residues. In contrast, small-angle X-ray scattering and equilibrium sedimentation analyses reveal the existence of monomeric phosphoserine-15 KlHxk1 in solution. While phosphorylation at serine-15 and concomitant homodimer dissociation are likely to be involved in glucose signalling, mechanism and putative physiological significance of KlHxk1 inactivation by autophosphorylation at serine-157 remain to be established.

Crystal structure of hexokinase KlHxk1 of Kluyveromyces lactis: a molecular basis for understanding the control of yeast hexokinase functions via covalent modification and oligomerization

Crystal structures of the unique hexokinase KlHxk1 of the yeast Kluyveromyces lactis were determined using eight independent crystal forms. In five crystal forms, a symmetrical ring-shaped homodimer was observed, corresponding to the physiological dimer existing in solution as shown by small-angle x-ray scattering. The dimer has a head-to-tail arrangement such that the small domain of one subunit interacts with the large domain of the other subunit. Dimer formation requires favorable interactions of the 15 N-terminal amino acids that are part of the large domain with amino acids of the small domain of the opposite subunit, respectively. The head-to-tail arrangement involving both domains of the two KlHxk1 subunits is appropriate to explain the reduced activity of the homodimer as compared with the monomeric enzyme and the influence of substrates and products on dimer formation and dissociation. In particular, the structure of the symmetrical KlHxk1 dimer serves to explain why phosphorylation of conserved residue Ser-15 may cause electrostatic repulsions with nearby negatively charged residues of the adjacent subunit, thereby inducing a dissociation of the homologous dimeric hexokinases KlHxk1 and ScHxk2. Two complex structures of KlHxk1 with bound glucose provide a molecular model of substrate binding to the open conformation and the subsequent classical domain closure motion of yeast hexokinases. The entirety of the novel data extends the current concept of glucose signaling in yeast and complements the induced-fit model by integrating the events of N-terminal phosphorylation and dissociation of homodimeric yeast hexokinases.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.