Julia Marín-Navarro

Comprehensive Transcriptional Analysis of the Oxidative Response in Yeast

The oxidative stress response in Saccharomyces cerevisiae has been analyzed by parallel determination of mRNA levels and transcription rates for the entire genome. A mathematical algorithm has been adapted for a dynamic situation such as the response to stress, to calculate theoretical mRNA decay rates from the experimental data. Yeast genes have been grouped into 25 clusters according to mRNA level and transcription rate kinetics, and average mRNA decay rates have been calculated for each cluster. In most of the genes, changes in one or both experimentally determined parameters occur during the stress response. 24% of the genes are transcriptionally induced without an increase in mRNA levels. The lack of parallelism between the evolution of the mRNA amount and transcription rate predicts changes in mRNA stability during stress. Genes for ribosomal proteins and rRNA processing enzymes are abundant among those whose mRNAs are predicted to destabilize. The number of genes whose mRNAs are predicted to stabilize is lower, although some protein folding or proteasomal genes are among the latter. We have confirmed the mathematical predictions for several genes pertaining to different clusters by experimentally determining mRNA decay rates using the regulatable tetO promoter in transcriptional expression conditions not affected by the oxidative stress. This study indicates that the oxidative stress response in yeast cells is not only conditioned by gene transcription but also by the mRNA decay dynamics and that this complex response may be particularly relevant to explain the temporary down-regulation of protein synthesis occurring during stress.

Functional Metagenomics Unveils a Multifunctional Glycosyl Hydrolase from the Family 43 Catalysing the Breakdown of Plant Polymers in the Calf Rumen

Microbial communities from cow rumen are known for their ability to degrade diverse plant polymers at high rates. In this work, we identified 15 hydrolases through an activity-centred metagenome analysis of a fibre-adherent microbial community from dairy cow rumen. Among them, 7 glycosyl hydrolases (GHs) and 1 feruloyl esterase were successfully cloned, expressed, purified and characterised. The most striking result was a protein of GH family 43 (GHF43), hereinafter designated as R_09-02, which had characteristics very distinct from the other proteins in this family with mono-functional β-xylosidase, α-xylanase, α-L-arabinase and α-L-arabinofuranosidase activities. R_09-02 is the first multifunctional enzyme to exhibit β-1,4 xylosidase, α-1,5 arabinofur(pyr)anosidase, β-1,4 lactase, α-1,6 raffinase, α-1,6 stachyase, β-galactosidase and α-1,4 glucosidase activities. The R_09-02 protein appears to originate from the chromosome of a member of Clostridia, a class of phylum Firmicutes, members of which are highly abundant in ruminal environment. The evolution of R_09-02 is suggested to be driven from the xylose- and arabinose-specific activities, typical for GHF43 members, toward a broader specificity to the glucose- and galactose-containing components of lignocellulose. The apparent capability of enzymes from the GHF43 family to utilise xylose-, arabinose-, glucose- and galactose-containing oligosaccharides has thus far been neglected by, or could not be predicted from, genome and metagenome sequencing data analyses. Taking into account the abundance of GHF43-encoding gene sequences in the rumen (up to 7% of all GH-genes) and the multifunctional phenotype herein described, our findings suggest that the ecological role of this GH family in the digestion of ligno-cellulosic matter should be significantly reconsidered.

Fructo-oligosaccharide synthesis by mutant versions of Saccharomyces cerevisiae invertase

ABSTRACT Efficient enzymatic synthesis of tailor-made prebiotic fructo-oligosaccharides (FOS) used in functional food formulation is a relevant biotechnological objective. We have engineered the Saccharomyces cerevisiae invertase (Suc2) to improve its transferase activity and to identify the enzymatic determinants for product specificity. Amino acid replacement (W19Y, N21S, N24S) within a conserved motif (β-fructosidase) specifically increased the synthesis of 6-kestose up to 10-fold. Mutants with lower substrate (sucrose) affinity produced FOS with longer half-lives. A mutation (P205V) adjacent to another conserved motif (EC) caused a 6-fold increment in 6-kestose yield. Docking studies with a Suc2 modeled structure defined a putative acceptor substrate binding subsite constituted by Trp 291 and Asn 228. Mutagenesis studies confirmed the implication of Asn 228 in directing the orientation of the sucrose molecule for the specific synthesis of β(2,6) linkages.

Three-dimensional Structure of Saccharomyces Invertase ROLE OF A NON-CATALYTIC DOMAIN IN OLIGOMERIZATION AND SUBSTRATE SPECIFICITY

Background: Invertase is a fundamental enzyme for sugar metabolism in yeast and a classical model in early biochemical studies. Results: Invertase shows an unusual octameric quaternary structure composed of two types of dimers. Conclusion: A peculiar pattern of monomer assembly through non-catalytic domain interactions determines invertase specificity. Significance: Unraveling the structural features that rule enzyme modularity casts new light on protein-carbohydrate recognition. Invertase is an enzyme that is widely distributed among plants and microorganisms and that catalyzes the hydrolysis of the disaccharide sucrose into glucose and fructose. Despite the important physiological role of Saccharomyces invertase (SInv) and the historical relevance of this enzyme as a model in early biochemical studies, its structure had not yet been solved. We report here the crystal structure of recombinant SInv at 3.3 Å resolution showing that the enzyme folds into the catalytic β-propeller and β-sandwich domains characteristic of GH32 enzymes. However, SInv displays an unusual quaternary structure. Monomers associate in two different kinds of dimers, which are in turn assembled into an octamer, best described as a tetramer of dimers. Dimerization plays a determinant role in substrate specificity because this assembly sets steric constraints that limit the access to the active site of oligosaccharides of more than four units. Comparative analysis of GH32 enzymes showed that formation of the SInv octamer occurs through a β-sheet extension that seems unique to this enzyme. Interaction between dimers is determined by a short amino acid sequence at the beginning of the β-sandwich domain. Our results highlight the role of the non-catalytic domain in fine-tuning substrate specificity and thus supplement our knowledge of the activity of this important family of enzymes. In turn, this gives a deeper insight into the structural features that rule modularity and protein-carbohydrate recognition.

Fermentation of cellobiose to ethanol by industrial Saccharomyces strains carrying the β-glucosidase gene (BGL1) from Saccharomycopsis fibuligera.

Constructs carrying the Saccharomycopsis fibuligera β-glucosidase gene (BGL1) under the control of a constitutive actin or a galactose-inducible promoter were introduced into eleven Saccharomyces strains. In ten of these recombinant strains, BGL1 expression driven by the actin promoter was between 1.6- and 18-fold higher than that obtained with the galactose-inducible promoter. Strains carrying the actin promoter yielded ethanol concentrations from cellobiose of between 0.5% and 14%, depending on their ability to accumulate Bgl1 (between 30 and 250 mU/mL) but also on their genetic background. Comparative analysis of a S. cerevisiae strain and its corresponding petite version showed similar ethanol yields, despite a 3-fold lower β-glucosidase production of the latter, suggesting that respiratory activity could be one of the factors influencing ethanol production when using carbon sources other than glucose. This study provides a selection of strains that may be good candidates as hosts for ethanol biosynthesis from cellulosic substrates.

Thermostable microbial xylanases for pulp and paper industries: trends, applications and further perspectives.

Xylanases are enzymes with biotechnological relevance in a number of fields, including food, feed, biofuel, and textile industries. Their most significant application is in the paper and pulp industry, where they are used as a biobleaching agent, showing clear economic and environmental advantages over chemical alternatives. Since this process requires high temperatures and alkali media, the identification of thermostable and alkali stable xylanases represents a major biotechnological goal in this field. Moreover, thermostability is a desirable property for many other applications of xylanases. The review makes an overview of xylanase producing microorganisms and their current implementation in paper biobleaching. Future perspectives are analyzed focusing in the efforts carried out to generate thermostable enzymes by means of modern biotechnological tools, including metagenomic analysis, enzyme molecular engineering and nanotechnology. Furthermore, structural and mutagenesis studies have revealed critical sites for stability of xylanases from glycoside hydrolase families GH10 and GH11, which constitute the main classes of these enzymes. The overall conclusions of these works are summarized here and provide relevant information about putative weak spots within xylanase structures to be targeted in future protein engineering approaches.

Structural and functional analysis of hybrid enzymes generated by domain shuffling between Saccharomyces cerevisiae (var. diastaticus) Sta1 glucoamylase and Saccharomycopsis fibuligera Bgl1 β-glucosidase

Saccharomyces cerevisiae Sta1 glucoamylase and Saccharomycopsis fibuligera Bgl1 β-glucosidase, two relevant enzymes from a biotechnological point of view, are proteins with multidomain structure. Starting with homology-based structural models of Sta1 and Bgl1, we have constructed a series of hybrid enzymes by interchanging domains of the two proteins. The first purpose of these constructs was to check available hypotheses about the uncertain biological functions of two domains: the serine/threonine-rich domain (STRD) of Sta1 and a β-sandwich domain present in Bgl1 that we have designated fibronectin-like domain (FLD). While, according to the initial hypothesis, proteins carrying the FLD tend to adhere to the cell wall, our results argued against the idea of an involvement of the STRD in protein secretion that stemmed from the presence of similar domains in different proteins secreted by yeast. The second objective of this work was to increase the enzymatic repertoire by generating enzymes with new structural and functional properties.

Identification and Structural Analysis of Amino Acid Substitutions that Increase the Stability and Activity of Aspergillus niger Glucose Oxidase

Glucose oxidase is one of the most conspicuous commercial enzymes due to its many different applications in diverse industries such as food, chemical, energy and textile. Among these applications, the most remarkable is the manufacture of glucose biosensors and in particular sensor strips used to measure glucose levels in serum. The generation of ameliorated versions of glucose oxidase is therefore a significant biotechnological objective. We have used a strategy that combined random and rational approaches to isolate uncharacterized mutations of Aspergillus niger glucose oxidase with improved properties. As a result, we have identified two changes that increase significantly the enzymes thermal stability. One (T554M) generates a sulfur-pi interaction and the other (Q90R/Y509E) introduces a new salt bridge near the interphase of the dimeric protein structure. An additional double substitution (Q124R/L569E) has no significant effect on stability but causes a twofold increase of the enzymes specific activity. Our results disclose structural motifs of the protein which are critical for its stability. The combination of mutations in the Q90R/Y509E/T554M triple mutant yielded a version of A. niger glucose oxidase with higher stability than those previously described.

Global estimation of mRNA stability in yeast

Turnover of mRNA is an important level of gene regulation. Individual mRNAs have different intrinsic stabilities. Moreover, mRNA stability changes dynamically with conditions such as hormonal stimulation or cellular stress. While accurate methods exist to measure the half-life of an individual transcript, global methods to estimate mRNA turnover have limitations in terms of resolution in time and precision. We describe and compare two complementary approaches to estimating global transcript stability: (1) direct measurement of decay rates; (2) indirect estimation of turnover from determination of mRNA synthesis rates and steady-state levels. Since the two approaches have distinct strengths yet confer different cellular perturbations, it is valuable to consider results obtained with both methods. The practical aspects of the chapter are written from a yeast perspective; the general considerations hold true for all eukaryotes, however.

Enzyme Engineering for Oligosaccharide Biosynthesis

Oligosaccharides are compounds of great interest in the food and pharmaceutical industry, mainly due to their prebiotic action and other health-promoting effects, in addition to other properties, acting as low-caloric sweeteners, antioxidants, or antimicrobial agents. Oligosaccharide functionality depends on their chemical structure, which relies on the nature of the building sugar residues, the linkage type, and the degree of polymerization. Their production is based either in the partial hydrolysis of polysaccharides or in the synthesis from other sugars with lower degree of polymerization. This methodology is described along the first part of this chapter, making special emphasis on the enzymatic-based technology, which shows clear advantages over chemical methods. Glycoside hydrolases and glycosyltransferases are the two enzyme classes involved in oligosaccharide production. They operate as highly specialized molecular tools for the generation of the different types of oligosaccharides. In the second part of the chapter, the strategies employed for improving the properties of these enzymes are explained. Enzyme engineering, through site-directed mutagenesis or directed evolution, has succeeded in delivering enzymatic variants with enhanced oligosaccharide yields, a different product profile or a wider substrate specificity. Moreover, immobilization of enzymes on solid supports through different methods has allowed their reutilization during repeated batches or their implementation in continuous systems, and, in some instances, it was also accompanied by an increased activity or stability. Overall, this chapter provides an overview about the facts and potentials of oligosaccharide production methodologies.