Expression and purification of a recombinant avidin with a lowered isoelectric point in Pichia pastoris

Precision cellular agriculture: The future role of recombinantly expressed protein as food

Cellular agriculture is a rapidly emerging field, within which cultured meat has attracted the majority of media attention in recent years. An equally promising area of cellular agriculture, and one that has produced far more actual food ingredients that have been incorporated into commercially available products, is the use of cellular hosts to produce soluble proteins, herein referred to as precision cellular agriculture (PCAg). In PCAg, specific animal- or plant-sourced proteins are expressed recombinantly in unicellular hosts-the majority of which are yeast-and harvested for food use. The numerous advantages of PCAg over traditional agriculture, including a smaller carbon footprint and more consistent products, have led to extensive research on its utility. This review is the first to survey proteins currently being expressed using PCAg for food purposes. A growing number of viable expression hosts and recent advances for increased protein yields and process optimization have led to its application for producing milk, egg, and muscle proteins; plant hemoglobin; sweet-tasting plant proteins; and ice-binding proteins. Current knowledge gaps present research opportunities for optimizing expression hosts, tailoring posttranslational modifications, and expanding the scope of proteins produced. Considerations for the expansion of PCAg and its implications on food regulation, society, ethics, and the environment are also discussed. Considering the current trajectory of PCAg, food proteins from any biological source can likely be expressed recombinantly and used as purified food ingredients to create novel and tailored food products.

Ferulic acid production by metabolically engineered Escherichia coli

Ferulic acid (p-hydroxy-3-methoxycinnamic acid, FA) is a natural active substance present in plant cell walls, with antioxidant, anticancer, antithrombotic and other properties; it is widely used in medicine, food, and cosmetics. Production of FA by eco-friendly bioprocess is of great potential. In this study, FA was biosynthesized by metabolically engineered Escherichia coli. As the first step, the genes tal (encoding tyrosine ammonia-lyase, RsTAL) from Rhodobacter sphaeroides, sam5 (encoding p-coumarate 3-hydroxylase, SeSAM5) from Saccharothrix espanaensis and comt (encoding Caffeic acid O-methytransferase, TaCM) from Triticum aestivum were cloned in an operon on the pET plasmid backbone, E. coli strain containing this construction was proved to produce FA from L-tyrosine successfully, and confirmed the function of TaCM as caffeic acid O-methytransferase. Fermentation result revealed JM109(DE3) as a more suitable host cell for FA production than BL21(DE3). After that the genes expression strength of FA pathway were optimized by tuning of promoter strength (T7 promoter or T5 promoter) and copy number (pBR322 or p15A), and the combination p15a-T5 works best. To further improve FA production, E. coli native pntAB, encoding pyridine nucleotide transhydrogenase, was selected from five NADPH regeneration genes to supplement redox cofactor NADPH for converting p-coumaric acid into caffeic acid in FA biosynthesis process. Sequentially, to further convert caffeic acid into FA, a non-native methionine kinase (MetK from Streptomyces spectabilis) was also overexpressed. Based on the flask fermentation data which show that the engineered E. coli strain produced 212 mg/L of FA with 11.8 mg/L caffeic acid residue, it could be concluded that it is the highest yield of FA achieved by E. coli K-12 strains reported to the best of our knowledge.

Enhancement of a protocol purifying T1 lipase through molecular approach

T1 Lipase is a thermostable secretary protein of Geobacillus zalihae strain previously expressed in a prokaryotic system and purified using three-step purification: affinity 1, affinity 2, and ion exchange chromatography (IEX). This approach is time consuming and offers low purity and recovery yield. In order to enhance the purification strategy of T1 lipase, affinity 2 was removed so that after affinity 1, the cleaved Glutathione S-transferase (GST) and matured T1 lipase could be directly separated through IEX. Therefore, a rational design of GST isoelectric point (pI) was implemented by prediction using ExPASy software in order to enhance the differences of pI values between GST and matured T1 lipase. Site-directed mutagenesis at two locations flanking the downstream region of GST sequences (H215R and G213R) was successfully performed. Double point mutations changed the charge on GST from 6.10 to 6.53. The purified lipase from the new construct GST tag mutant-T1 was successfully purified using two steps of purification with 6,849 U/mg of lipase specific activity, 33% yield, and a 44-fold increase in purification. Hence, the increment of the pI values in the GST tag fusion T1 lipase resulted in a successful direct separation through IEX and lead to successful purification.

A Highly Sensitive Immunosorbent Assay Based on Biotinylated Graphene Oxide and the Quartz Crystal Microbalance

A high-sensitivity flow-based immunoassay is reported based on a gold-coated quartz crystal microbalance (QCM) chip functionalized directly in the QCM without requiring covalent conjugation steps. Specifically, the irreversible adsorption of a biotinylated graphene oxide-avidin complex followed by loading of a biotinylated capture antibody is applied to avoid more complex conventional surface modification chemistries and enable chip functionalization and sensing all within the QCM instrument. The resulting immunosensors exhibit significantly lower nonspecific protein adsorption and stronger signal for antigen sensing relative to simple avidin-coated sensors. Reproducible quantification of rabbit IgG concentrations ranging from 0.1 ng/mL to 10 μg/mL (6 orders of magnitude) can be achieved depending on the approach used to quantify the binding with simple mass changes used to detect higher concentrations and a horseradish peroxidase-linked detection antibody that converts its substrate to a measurable precipitate used to detect very low analyte concentrations. Sensor fabrication and assay performance take ∼5 h in total, which is on par with or faster than other techniques. Quantitative sensing is possible in the presence of complex protein mixtures, such as human plasma. Given the broad availability of biotinylated capture antibodies, this method offers both an easy and flexible platform for the quantitative sensing of a variety of biomolecule targets.

High-level expression of tamavidin 2 in human cells by codon-usage optimization

Tamavidin 2 is a fungal protein that binds to biotin with an extremely high affinity. Tamavidin 2 is superior to avidin or streptavidin in terms of its low-level non-specific binding and high-level thermal stability. However, the gene for tamavidin 2 is highly expressed in Escherichia coli but not in mammalian cells, restricting its application as an affinity tag in mammalian cells. Here, we optimized the codon usage of tamavidin 2 for human cells and found that the resultant mutant expressed tamavidin 2 at approximately 30-fold higher level compared with the native gene. The protein thus produced in human cells could be purified by iminobiotin affinity chromatography, bound tightly to biotin, and was stable at high temperature (82 °C). This powerful technology for high-level expression of tamavidin 2 in mammalian cells will be of value in evaluating various fusion proteins produced in mammalian cells for numerous applications.

High-level secretion of recombinant full-length streptavidin in Pichia pastoris and its application to enantioselective catalysis

Artificial metalloenzymes result from the incorporation of a catalytically competent biotinylated organometallic moiety into full-length (i.e. mature) streptavidin. With large-scale industrial biotechnology applications in mind, large quantities of recombinant streptavidin are required. Herein we report our efforts to produce wild-type mature and biotin-free streptavidin using the yeast Pichia pastoris expression system. The streptavidin gene was inserted into the expression vector pPICZαA in frame with the Saccharomyces cerevisiae α-mating factor secretion signal. In a fed-batch fermentation using a minimal medium supplemented with trace amounts of biotin, functional streptavidin was secreted at approximately 650mg/L of culture supernatant. This yield is approximately threefold higher than that from Escherichia coli, and although the overall expression process takes longer (ten days vs. two days), the downstream processing is simplified by eliminating denaturing/refolding steps. The purified streptavidin bound ∼3.2molecules of biotin per tetramer. Upon incorporation of a biotinylated piano-stool catalyst, the secreted streptavidin displayed identical properties to streptavidin produced in E. coli by showing activity as artificial imine reductase.

Reversible biofunctionalization of surfaces with a switchable mutant of avidin

Label-free biosensors detect binding of prey molecules (″analytes″) to immobile bait molecules on the sensing surface. Numerous methods are available for immobilization of bait molecules. A convenient option is binding of biotinylated bait molecules to streptavidin-functionalized surfaces, or to biotinylated surfaces via biotin-avidin-biotin bridges. The goal of this study was to find a rapid method for reversible immobilization of biotinylated bait molecules on biotinylated sensor chips. The task was to establish a biotin-avidin-biotin bridge which was easily cleaved when desired, yet perfectly stable under a wide range of measurement conditions. The problem was solved with the avidin mutant M96H which contains extra histidine residues at the subunit-subunit interfaces. This mutant was bound to a mixed self-assembled monolayer (SAM) containing biotin residues on 20% of the oligo(ethylene glycol)-terminated SAM components. Various biotinylated bait molecules were bound on top of the immobilized avidin mutant. The biotin-avidin-biotin bridge was stable at pH ≥3, and it was insensitive to sodium dodecyl sulfate (SDS) at neutral pH. Only the combination of citric acid (2.5%, pH 2) and SDS (0.25%) caused instantaneous cleavage of the biotin-avidin-biotin bridge. As a consequence, the biotinylated bait molecules could be immobilized and removed as often as desired, the only limit being the time span for reproducible chip function when kept in buffer (2-3 weeks at 25 °C). As expected, the high isolectric pH (pI) of the avidin mutant caused nonspecific adsorption of proteins. This problem was solved by acetylation of avidin (to pI < 5), or by optimization of SAM formation and passivation with biotin-BSA and BSA.

Insecticidal activity of recombinant avidin produced in yeast

An expression construct encoding chicken (Gallus gallus) avidin was assembled from amplified fragments of genomic DNA. Recombinant, functional avidin was produced in Pichia pastoris, with yields of up to 80 mg/l of culture supernatant. The recombinant avidin had similar insecticidal activity to egg white avidin when assayed against larvae of a lepidopteran crop pest, cabbage moth (Mamestra brassicae), causing >90% reduction in growth and 100% mortality when fed in optimised diets at levels of 1.5 microM and 15 microM (100 ppm and 1000 ppm wet weight of recombinant protein). The recombinant protein was also highly toxic to a hemipteran pest, the pea aphid (Acyrthosiphon pisum), when fed in liquid artificial diet, causing 100% mortality after 4 days when present at concentrations > or = 3.8 microM (0.25 mg/ml, 250 ppm). Mortality was dose-dependent, with an estimated LC(50) of 2.1 microM. Toxicity to A. pisum was prevented by biotin supplementation of diet. In contrast, avidin had no significant effects on the survival of cereal aphid (Sitobion avenae) at concentrations up to 30 microM in liquid diet. Analysis of genomic DNA showed that symbionts from both aphid species lack the ability to synthesise biotin de novo. Cereal aphids appear to be less sensitive to recombinant avidin in the diet through proteolysis of the ingested protein, which would allow recovery of bound biotin.

Efficient expression of codon-adapted human acetaldehyde dehydrogenase 2 cDNA with 6xHis tag in Pichia pastoris

Human mitochondrial acetaldehyde dehydrogenase 2 (ALDH2) catalyzes the oxidation of acetaldehyde to acetic acid. Therefore, ALDH2 has therapeutic potential in detoxification of acetaldehyde. Furthermore, ALDH2 catalyzes nitroglycerin to nitrate and 1, 2-glyceryldinitrate during therapy for angina pectoris, myocardial infarction, and heart failure. Large quantities of ALDH2 will be needed for potential clinical practice. In this study, Pichia pastoris was used as a platform for expression of human ALDH2. Based on the ALDH2*1 cDNA sequence, we designed ALDH2 cDNA by choosing the P. pastoris preferred codons and by decreasing the G + C content level. The sequence was synthesized using the overlap extension PCR method. The cDNA and 6xHis tags were subcloned into the plasmid pPIC9K. The recombinant protein was expressed in P. pastoris GS115 and purified using Ni(2+)-Sepharose affinity chromatography. The amount of secreted protein in the culture was 80 mg/L in shake-flask cultivation and 260 mg/L in high-density bioreactor fermentation. Secreted ALDH2 was easily purified from the culture supernatant by using Ni(2+)-Sepharose affinity chromatography. After purification of the fermentation supernatant, the enzyme had a specific activity of 1.2 U/mg protein. The yield was about 16 mg/L in a shake flask culture of P. pastoris GS115 which contained the original human ALDH2*1 cDNA.

A quantitative analysis of the benefits of mixed feeds of sorbitol and methanol for the production of recombinant avidin with Pichia pastoris

The advantages of mixed feeds of sorbitol and methanol for the production of recombinant proteins with Pichia pastoris were analyzed quantitatively. The influence of the methanol-sorbitol ratio in the feed medium was investigated on growth stoichiometry and recombinant protein productivity with a P. pastoris Mut(+) strain secreting avidin by performing a transient nutrient gradient in continuous cultivation at a dilution rate of 0.03h(-1). Results showed that mixed feeds of sorbitol and methanol instead of methanol as sole carbon source can improve the productivity in recombinant avidin due to increased biomass yields during mixed substrate growth. The highest volumetric avidin productivity was achieved at a methanol fraction of 43%C-molC-mol(-1) in the feed medium: the volumetric avidin productivity was 1.3-fold higher than during chemostat culture on methanol. The heat production and the oxygen consumption rates were reduced by about 38% for a given dry cell weight concentration at this methanol fraction, features which are very useful for high cell density cultures. Results were in good agreement with a high cell density fed-batch culture performed with a mixed feed of 43% methanol and 57% sorbitol C-molC-mol(-1) at a specific growth rate of 0.03h(-1) during the induction phase. Moreover, it was confirmed that sorbitol accumulation in the culture medium does not affect recombinant avidin productivity, which can especially be advantageous during large-scale cultures where transient substrate accumulation can result from imperfect mixing.

Regulation of alcohol oxidase of a recombinant Pichia pastoris Mut+ strain in transient continuous cultures

In the methylotrophic yeast Pichia pastoris, alcohol oxidase (AOX) is a key enzyme involved in the dissimilation of methanol. Heterologous proteins are usually expressed under the control of the AOX1 promoter, which drives the expression of alcohol oxidase 1 in the wild-type strain. This study investigates the regulation of the alcohol oxidase enzyme of a recombinant P. pastoris Mut+ strain in cultures on glycerol and methanol as sole carbon sources and in mixed substrate cultures on both substrates. The aim was to have a better insight in the transition from growth on glycerol to growth on methanol, which is a key step in standard high cell density P. pastoris cultures for the production of foreign proteins. Nutrient shifts in chemostat cultures showed that after growth on glycerol use of mixed feeds of glycerol and methanol allowed faster induction of alcohol oxidase and faster adaptation of cellular metabolism than with a feed containing methanol as sole carbon source. The results of this study showed also how critical it is to avoid transient methanol accumulation during P. pastoris cultures operated at low residual methanol concentrations. Indeed, pulse experiments during chemostat cultures showed that sudden increase in methanol concentrations in cultures performed under methanol-limited or dual methanol and glycerol-limited growth conditions leads to wash-out of the culture because of too high consumption rate of methanol, which leads to excretion of toxic intermediates. High rate of methanol consumption was due to high specific AOX activities observed at low residual methanol concentrations.

Brave new (strept)avidins in biotechnology

Avidin and streptavidin are widely used in (strept)avidin-biotin technology, which is based on their tight biotin-binding capability. These techniques are exceptionally diverse, ranging from simple purification and labeling methods to sophisticated drug pre-targeting and nanostructure-building approaches. Improvements in protein engineering have provided new possibilities to develop tailored protein tools. The (strept)avidin scaffold has been engineered to extend the existing range of applications and to develop new ones. Modifications to (strept)avidins--such as simple amino acid substitutions to reduce biotin binding and alter physico-chemical characters--have recently developed into more sophisticated changes, including chimeric (strept)avidins, topology rearrangements and stitching of non-natural amino acids into the active sites. In this review, we highlight the current status in genetically engineered (strept)avidins and illustrate their versatility as advanced tools in the multiple fields of modern bioscience, medicine and nanotechnology.

Mixed feeds of glycerol and methanol can improve the performance of Pichia pastoris cultures: A quantitative study based on concentration gradients in transient continuous cultures

Transient continuous cultures constitute a means to speed up strain characterization, by avoiding the need for many time-consuming steady-state experiments. In this study, mixed substrate growth on glycerol and methanol of a Pichia pastoris strain expressing and secreting recombinant avidin was characterized quantitatively by performing a nutrient gradient with linear increase of the methanol fraction in the feed medium from 0.5 to 0.93 C-mol C-mol(-1) at a dilution rate of 0.06 h(-1). The influence of the methanol fraction in the feed medium on recombinant avidin productivity and on specific alcohol oxidase activity were also examined. Results showed that, compared with cultures on methanol as sole carbon source, the specific recombinant avidin production rate was the same provided the methanol fraction in the feed medium was higher than 0.6 C-mol C-mol(-1). The volumetric avidin production rate was even 1.1-fold higher with a methanol fraction in the feed medium of 0.62 C-mol C-mol(-1) as a result of the higher biomass yield on mixed substrate growth compared with methanol alone. Moreover, since heat production and oxygen uptake rates are lower during mixed substrate growth on glycerol and methanol, mixed substrate cultures present technical advantages for the performance of high cell density P. pastoris cultures. Results obtained in a high cell density fed-batch culture with a mixed feed of 0.65 C-mol C-mol(-1) methanol and 0.35 C-mol C-mol(-1) glycerol were in agreement with results obtained during the transient nutrient gradient.

Influence of specific growth rate on specific productivity and glycosylation of a recombinant avidin produced by aPichia pastoris Mut+ strain

A recombinant avidin-producing Mut+ Pichia pastoris strain was used as a model organism to study the influence of the methanol feeding strategy on the specific product productivity (q(p)) and protein glycosylation. Fed-batch cultivations performed at various specific growth rates (micro) and residual methanol concentrations showed that the specific avidin productivity is growth-dependent. The specific productivity increases strongly with the specific growth rate for micro ranging from 0 to 0.02 h(-1), and increases only slightly with the specific growth rate above this limit. N-terminal glycosylation was also found to be influenced by the specific growth rate, since 9-mannose glycans were the most abundant form at low growth rates, whereas 10-mannose carbohydrate chains were favored at higher micro. These results show that culture parameters, such as the specific growth rate, may significantly affect the activity of glycoproteins produced in Pichia pastoris. In terms of process optimization, this suggests that a compromise on the specific growth rate may have to be found, in certain cases, to work with an acceptable productivity while avoiding the addition of many mannoses.

Designed evolution of artificial metalloenzymes: protein catalysts made to order

Artificial metalloenzymes based on biotin-streptavidin technology, a "fusion" of chemistry and biology, illustrate how asymmetric catalysts can be improved and evolved using chemogenetic approaches.

Artificial metalloenzymes for enantioselective catalysis based on the noncovalent incorporation of organometallic moieties in a host protein

Enzymatic and homogeneous catalysis offer complementary means to produce enantiopure products. Incorporation of achiral, biotinylated aminodiphosphine-rhodium complexes in (strept)avidin affords enantioselective hydrogenation catalysts. A combined chemogenetic procedure allows the optimization of the activity and the selectivity of such artificial metalloenzymes: the reduction of acetamidoacrylate proceeds to produce N-acetamidoalanine in either 96 % ee (R) or 80 % ee (S). In addition to providing a chiral second coordination sphere and, thus, selectivity to the catalyst, the phenomenon of protein-accelerated catalysis (e.g., increased activity) was unraveled. Such artificial metalloenzymes based on the biotin-avidin technology display features that are reminiscent of both homogeneous and of enzymatic catalysis.

Optimisation of culture conditions with respect to biotin requirement for the production of recombinant avidin in Pichia pastoris

Due to its very high affinity to biotin, avidin is one of the most widely exploited proteins in modern biotechnological and biomedical applications. Since biotin is an essential vitamin for the growth of many microorganisms, we examined the effect of biotin deficiency on growth for a recombinant Pichia pastoris strain expressing and secreting a recombinant glycosylated avidin. The results showed that biotin deficiency lowers growth rate and biomass yield for P. pastoris. Substitution of biotin in the medium by the two structurally unrelated compounds, aspartic acid and oleic acid, which do not bind to recombinant avidin was analyzed quantitatively. These two compounds had a growth promoting effect in biotin-deficient medium, but did not replace biotin completely. Indeed, in chemostat culture, wash-out occurred after about six liquid residence times and recombinant avidin productivity was lowered. However, addition of low amounts of biotin (20 microg L(-1) of biotin for a cell density of 8 g L(-1)) resulted in stable chemostat cultures on methanol with the production of recombinant biotin-free avidin. The specific avidin production rate was 22 microg g(-1) h(-1) at a dilution rate of 0.06 h(-1).

Tetravalent single-chain avidin: from subunits to protein domains via circularly permuted avidins

scAvd (single-chain avidin, where two dcAvd are joined in a single polypeptide chain), having four biotin-binding domains, was constructed by fusion of topologically modified avidin units. scAvd showed similar biotin binding and thermal stability properties as chicken avidin. The DNA construct encoding scAvd contains four circularly permuted avidin domains, plus short linkers connecting the four domains into a single polypeptide chain. In contrast with wild-type avidin, which contains four identical avidin monomers, scAvd enables each one of the four avidin domains to be independently modified by protein engineering. Therefore the scAvd scaffold can be used to construct spatially and stoichiometrically defined pseudotetrameric avidin molecules showing different domain characteristics. In addition, unmodified scAvd could be used as a fusion partner, since it provides a unique non-oligomeric structure, which is fully functional with four high-affinity biotin-binding sites. Furthermore, the subunit-to-domain strategy described in the present study could be applied to other proteins and protein complexes, facilitating the development of sophisticated protein tools for applications in nanotechnology and life sciences.

Efficient production of active chicken avidin using a bacterial signal peptide in Escherichia coli

Chicken avidin is a highly popular tool with countless applications in the life sciences. In the present study, an efficient method for producing avidin protein in the periplasmic space of Escherichia coli in the active form is described. Avidin was produced by replacing the native signal sequence of the protein with a bacterial OmpA secretion signal. The yield after a single 2-iminobiotin-agarose affinity purification step was approx. 10 mg/l of virtually pure avidin. Purified avidin had 3.7 free biotin-binding sites per tetramer and showed the same biotin-binding affinity and thermal stability as egg-white avidin. Avidin crystallized under various conditions, which will enable X-ray crystallographic studies. Avidin produced in E. coli lacks the carbohydrate chains of chicken avidin and the absence of glycosylation should decrease the non-specific binding that avidin exhibits towards many materials [Rosebrough and Hartley (1996) J. Nucl. Med. 37, 1380-1384]. The present method provides a feasible and inexpensive alternative for the production of recombinant avidin, avidin mutants and avidin fusion proteins for novel avidin-biotin technology applications.

Artificial metalloenzymes based on biotin-avidin technology for the enantioselective reduction of ketones by transfer hydrogenation

Most physiological and biotechnological processes rely on molecular recognition between chiral (handed) molecules. Manmade homogeneous catalysts and enzymes offer complementary means for producing enantiopure (single-handed) compounds. As the subtle details that govern chiral discrimination are difficult to predict, improving the performance of such catalysts often relies on trial-and-error procedures. Homogeneous catalysts are optimized by chemical modification of the chiral environment around the metal center. Enzymes can be improved by modification of gene encoding the protein. Incorporation of a biotinylated organometallic catalyst into a host protein (avidin or streptavidin) affords versatile artificial metalloenzymes for the reduction of ketones by transfer hydrogenation. The boric acid.formate mixture was identified as a hydrogen source compatible with these artificial metalloenzymes. A combined chemo-genetic procedure allows us to optimize the activity and selectivity of these hybrid catalysts: up to 94% (R) enantiomeric excess for the reduction of p-methylacetophenone. These artificial metalloenzymes display features reminiscent of both homogeneous catalysts and enzymes.

Electrophoretic behavior of streptavidin complexed to a biotinylated probe: A functional screening assay for biotin-binding proteins

The biotin-binding protein streptavidin exhibits a high stability against thermal denaturation, especially when complexed to biotin. Herein we show that, in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), streptavidin is stabilized at high temperature in the presence of biotinylated fluorescent probes, such as biotin-4-fluorescein, which is incorporated within the binding pocket. In nondenaturing SDS-PAGE, streptavidin is detectable when complexed with biotin-4-fluorescein using a UV-transilluminator. Using biotin-4-fluorescein, the detection limit of streptavidin lies in the same range as with Coomassie blue staining. The functionality of streptavidin mutants can readily be assessed from crude bacterial extracts using biotin-4-fluorescein as a probe in nondenaturing SDS-PAGE.

Artificial Metalloenzymes: (Strept)avidin as Host for Enantioselective Hydrogenation by Achiral Biotinylated Rhodium−Diphosphine Complexes

We report on the generation of artificial metalloenzymes based on the noncovalent incorporation of biotinylated rhodium-diphosphine complexes in (strept)avidin as host proteins. A chemogenetic optimization procedure allows one to optimize the enantioselectivity for the reduction of acetamidoacrylic acid (up to 96% ee (R) in streptavidin S112G and up to 80% ee (S) in WT avidin). The association constant between a prototypical cationic biotinylated rhodium-diphosphine catalyst precursor and the host proteins was determined at neutral pH: log K(a) = 7.7 for avidin (pI = 10.4) and log K(a) = 7.1 for streptavidin (pI = 6.4). It is shown that the optimal operating conditions for the enantioselective reduction are 5 bar at 30 degrees C with a 1% catalyst loading.