Structural Basis of the Substrate Range and Enantioselectivity of Two (S)-Selective ω-Transaminases.

ω-Transaminases are enzymes that can introduce an amino group in industrially interesting compounds. We determined crystal structures of two (S)-selective ω-transaminases, one from Arthrobacter sp. (Ars-ωTA) and one from Bacillus megaterium (BM-ωTA), which have 95% identical sequences but somewhat different activity profiles. Substrate profiling measurements using a range of (R)- and (S)-substrates showed that both enzymes have a preference for substrates with large, flat cyclic side groups, for which the activity of BM-ωTA is generally somewhat higher. BM-ωTA has a preference for (S)-3,3-dimethyl-2-butylamine significantly stronger than that of Ars-ωTA, as well as a weaker enantiopreference for 1-cyclopropylethylamine. The crystal structures showed that, as expected for (S)-selective transaminases, both enzymes have the typical transaminase type I fold and have spacious active sites to accommodate largish substrates. A structure of BM-ωTA with bound (R)-α-methylbenzylamine explains the enzymes' preference for (S)-substrates. Site-directed mutagenesis experiments revealed that the presence of a tyrosine, instead of a cysteine, at position 60 increases the relative activities on several small substrates. A structure of Ars-ωTA with bound l-Ala revealed that the Arg442 side chain has been repositioned to bind the l-Ala carboxylate. Compared to the arginine switch residue in other transaminases, Arg442 is shifted by six residues in the amino acid sequence, which appears to be a consequence of extra loops near the active site that narrow the entrance to the active site.

Structural basis of Na(+)-independent and cooperative substrate/product antiport in CaiT.

Transport of solutes across biological membranes is performed by specialized secondary transport proteins in the lipid bilayer, and is essential for life. Here we report the structures of the sodium-independent carnitine/butyrobetaine antiporter CaiT from Proteus mirabilis (PmCaiT) at 2.3-A and from Escherichia coli (EcCaiT) at 3.5-A resolution. CaiT belongs to the family of betaine/carnitine/choline transporters (BCCT), which are mostly Na(+) or H(+) dependent, whereas EcCaiT is Na(+) and H(+) independent. The three-dimensional architecture of CaiT resembles that of the Na(+)-dependent transporters LeuT and BetP, but in CaiT a methionine sulphur takes the place of the Na(+) ion to coordinate the substrate in the central transport site, accounting for Na(+)-independent transport. Both CaiT structures show the fully open, inward-facing conformation, and thus complete the set of functional states that describe the alternating access mechanism. EcCaiT contains two bound butyrobetaine substrate molecules, one in the central transport site, the other in an extracellular binding pocket. In the structure of PmCaiT, a tryptophan side chain occupies the transport site, and access to the extracellular site is blocked. Binding of both substrates to CaiT reconstituted into proteoliposomes is cooperative, with Hill coefficients up to 1.7, indicating that the extracellular site is regulatory. We propose a mechanism whereby the occupied regulatory site increases the binding affinity of the transport site and initiates substrate translocation.

Furoates and thenoates inhibit pyruvate dehydrogenase kinase 2 allosterically by binding to its pyruvate regulatory site.

The last decade has witnessed the reawakening of cancer metabolism as a therapeutic target. In particular, inhibition of pyruvate dehydrogenase kinase (PDK) holds remarkable promise. Dichloroacetic acid (DCA), currently undergoing clinical trials, is a unique PDK inhibitor in which it binds to the allosteric pyruvate site of the enzyme. However, the safety of DCA as a drug is compromised by its neurotoxicity, whereas its usefulness as an investigative tool is limited by the high concentrations required to exert observable effects in cell culture. Herein, we report the identification - by making use of saturation-transfer difference NMR spectroscopy, enzymatic assays and computational methods - of furoate and thenoate derivatives as allosteric pyruvate-site-binding PDK2 inhibitors. This work substantiates the pyruvate regulatory pocket as a druggable target.

X-ray crystallographic validation of structure predictions used in computational design for protein stabilization.

Protein engineering aimed at enhancing enzyme stability is increasingly supported by computational methods for calculation of mutant folding energies and for the design of disulfide bonds. To examine the accuracy of mutant structure predictions underlying these computational methods, crystal structures of thermostable limonene epoxide hydrolase variants obtained by computational library design were determined. Four different predicted effects indeed contributed to the obtained stabilization: (i) enhanced interactions between a flexible loop close to the N-terminus and the rest of the protein; (ii) improved interactions at the dimer interface; (iii) removal of unsatisfied hydrogen bonding groups; and (iv) introduction of additional positively charged groups at the surface. The structures of an eightfold and an elevenfold mutant showed that most mutations introduced the intended stabilizing interactions, and side-chain conformations were correctly predicted for 72-88% of the point mutations. However, mutations that introduced a disulfide bond in a flexible region had a larger influence on the backbone conformation than predicted. The enzyme active sites were unaltered, in agreement with the observed preservation of catalytic activities. The structures also revealed how a c-Myc tag, which was introduced for facile detection and purification, can reduce access to the active site and thereby lower the catalytic activity. Finally, sequence analysis showed that comprehensive mutant energy calculations discovered stabilizing mutations that are not proposed by the consensus or B-FIT methods.

Molecular basis of transport and regulation in the Na(+)/betaine symporter BetP.

Osmoregulated transporters sense intracellular osmotic pressure and respond to hyperosmotic stress by accumulation of osmolytes to restore normal hydration levels. Here we report the determination of the X-ray structure of a member of the family of betaine/choline/carnitine transporters, the Na(+)-coupled symporter BetP from Corynebacterium glutamicum, which is a highly effective osmoregulated uptake system for glycine betaine. Glycine betaine is bound in a tryptophan box occluded from both sides of the membrane with aromatic side chains lining the transport pathway. BetP has the same overall fold as three unrelated Na(+)-coupled symporters. Whereas these are crystallized in either the outward-facing or the inward-facing conformation, the BetP structure reveals a unique intermediate conformation in the Na(+)-coupled transport cycle. The trimeric architecture of BetP and the break in three-fold symmetry by the osmosensing C-terminal helices suggest a regulatory mechanism of Na(+)-coupled osmolyte transport to counteract osmotic stress.

High-resolution x-ray structure of human aquaporin 5.

Human aquaporin 5 (HsAQP5) facilitates the transport of water across plasma membranes and has been identified within cells of the stomach, duodenum, pancreas, airways, lungs, salivary glands, sweat glands, eyes, lacrimal glands, and the inner ear. AQP5, like AQP2, is subject to posttranslational regulation by phosphorylation, at which point it is trafficked between intracellular storage compartments and the plasma membrane. Details concerning the molecular mechanism of membrane trafficking are unknown. Here we report the x-ray structure of HsAQP5 to 2.0-A resolution and highlight structural similarities and differences relative to other eukaryotic aquaporins. A lipid occludes the putative central pore, preventing the passage of gas or ions through the center of the tetramer. Multiple consensus phosphorylation sites are observed in the structure and their potential regulatory role is discussed. We postulate that a change in the conformation of the C terminus may arise from the phosphorylation of AQP5 and thereby signal trafficking.

Yeast chitin synthase 2 activity is modulated by proteolysis and phosphorylation.

Saccharomyces cerevisiae Chs2 (chitin synthase 2) synthesizes the primary septum after mitosis is completed. It is essential for proper cell separation and is expected to be highly regulated. We have expressed Chs2 and a mutant lacking the N-terminal region in Pichia pastoris in an active form at high levels. Both constructs show a pH and cation dependence similar to the wild-type enzyme, as well as increased activity after trypsin treatment. Using further biochemical analysis, we have identified two mechanisms of chitin synthase regulation. First, it is hyperactivated by a soluble yeast protease. This protease is expressed during exponential growth phase, when budding cells require Chs2 activity. Secondly, LC-MS/MS (liquid chromatography tandem MS) experiments on purified Chs2 identify 12 phosphorylation sites, all in the N-terminal domain. Four of them show the perfect sequence motif for phosphorylation by the cyclin-dependent kinase Cdk1. As we also show that phosphorylation of the N-terminal domain is important for Chs2 stability, these sites might play an important role in the cell cycle-dependent degradation of the enzyme, and thus in cell division.

The structural basis of cephalosporin formation in a mononuclear ferrous enzyme.

Deacetoxycephalosporin-C synthase (DAOCS) is a mononuclear ferrous enzyme that transforms penicillins into cephalosporins by inserting a carbon atom into the penicillin nucleus. In the first half-reaction, dioxygen and 2-oxoglutarate produce a reactive iron-oxygen species, succinate and CO2. The oxidizing iron species subsequently reacts with penicillin to give cephalosporin and water. Here we describe high-resolution structures for ferrous DAOCS in complex with penicillins, the cephalosporin product, the cosubstrate and the coproduct. Steady-state kinetic data, quantum-chemical calculations and the new structures indicate a reaction sequence in which a 'booby-trapped' oxidizing species is formed. This species is stabilized by the negative charge of succinate on the iron. The binding sites of succinate and penicillin overlap, and when penicillin replaces succinate, it removes the stabilizing charge, eliciting oxidative attack on itself. Requisite groups of penicillin are within 1 A of the expected position of a ferryl oxygen in the enzyme-penicillin complex.

1.55 A structure of the ectoine binding protein TeaA of the osmoregulated TRAP-transporter TeaABC from Halomonas elongata.

TeaABC from the moderate halophilic bacterium Halomonas elongata belongs to the tripartite ATP-independent periplasmic transporters (TRAP-T), a family of secondary transporters functioning in conjunction with periplasmic substrate binding proteins. TeaABC facilitates the uptake of the compatible solutes ectoine and hydroxyectoine that are accumulated in the cytoplasm under hyperosmotic stress to protect the cell from dehydration. TeaABC is the only known TRAP-T activated by osmotic stress. Currently, our knowledge on the osmoregulated compatible solute transporter is limited to ABC transporters or conventional secondary transporters. Therefore, this study presents the first detailed analysis of the molecular mechanisms underlying substrate recognition of the substrate binding protein of an osmoregulated TRAP-T. In the present study we were able to demonstrate by isothermal titration calorimetry measurements that TeaA is a high-affinity ectoine binding protein ( K d = 0.19 microM) that also has a significant but somewhat lower affinity to hydroxyectoine ( K d = 3.8 microM). Furthermore, we present the structure of TeaA in complex with ectoine at a resolution of 1.55 A and hydroxyectoine at a resolution of 1.80 A. Analysis of the TeaA binding pocket and comparison of its structure to other compatible solute binding proteins from ABC transporters reveal common principles in compatible solute binding but also significant differences like the solvent-mediated specific binding of ectoine to TeaA.

A robust cosolvent-compatible halohydrin dehalogenase by computational library design.

To improve the applicability of halohydrin dehalogenase as a catalyst for reactions in the presence of organic cosolvents, we explored a computational library design strategy (Framework for Rapid Enzyme Stabilization by Computational libraries) that involves discovery and in silico evaluation of stabilizing mutations. Energy calculations, disulfide bond predictions and molecular dynamics simulations identified 218 point mutations and 35 disulfide bonds with predicted stabilizing effects. Experiments confirmed 29 stabilizing point mutations, most of which were located in two distinct regions, whereas introduction of disulfide bonds was not effective. Combining the best mutations resulted in a 12-fold mutant (HheC-H12) with a 28°C higher apparent melting temperature and a remarkable increase in resistance to cosolvents. This variant also showed a higher optimum temperature for catalysis while activity at low temperature was preserved. Mutant H12 was used as a template for the introduction of mutations that enhance enantioselectivity or activity. Crystal structures showed that the structural changes in the H12 mutant mostly agreed with the computational predictions and that the enhanced stability was mainly due to mutations that redistributed surface charges and improved interactions between subunits, the latter including better interactions of water molecules at the subunit interfaces.

Mechanisms of photoprotection and nonphotochemical quenching in pea light-harvesting complex at 2.5 A resolution.

The plant light-harvesting complex of photosystem II (LHC-II) collects and transmits solar energy for photosynthesis in chloroplast membranes and has essential roles in regulation of photosynthesis and in photoprotection. The 2.5 A structure of pea LHC-II determined by X-ray crystallography of stacked two-dimensional crystals shows how membranes interact to form chloroplast grana, and reveals the mutual arrangement of 42 chlorophylls a and b, 12 carotenoids and six lipids in the LHC-II trimer. Spectral assignment of individual chlorophylls indicates the flow of energy in the complex and the mechanism of photoprotection in two close chlorophyll a-lutein pairs. We propose a simple mechanism for the xanthophyll-related, slow component of nonphotochemical quenching in LHC-II, by which excess energy is transferred to a zeaxanthin replacing violaxanthin in its binding site, and dissipated as heat. Our structure shows the complex in a quenched state, which may be relevant for the rapid, pH-induced component of nonphotochemical quenching.

MIR phasing using merohedrally twinned crystals.

Merohedral twinning is a crystal-growth disorder that seriously hinders the determination of macromolecular crystal structures by isomorphous replacement. The strategies used in the structures solved so far are discussed. Several methods can be used to determine the extent of twinning, the twin fraction and to detwin the data. Accurate determination of the twin fraction by analysing heavy-atom refinement statistics is possible, but only influences the resulting phases slightly. It seems more crucial to restrict the variation in twin fractions between data sets, either by making the twin fractions of some data sets artificially higher or by screening crystals to obtain data with a low twin fraction.