Structure and functional characterization of a bile acid 7α dehydratase BaiE in secondary bile acid synthesis

Conversion of the primary bile acids cholic acid (CA) and chenodeoxycholic acid (CDCA) to the secondary bile acids deoxycholic acid (DCA) and lithocholic acid (LCA) is performed by a few species of intestinal bacteria in the genus Clostridium through a multistep biochemical pathway that removes a 7α-hydroxyl group. The rate-determining enzyme in this pathway is bile acid 7α-dehydratase (baiE). In this study, crystal structures of apo-BaiE and its putative product-bound [3-oxo-Δ(4,6) -lithocholyl-Coenzyme A (CoA)] complex are reported. BaiE is a trimer with a twisted α + β barrel fold with similarity to the Nuclear Transport Factor 2 (NTF2) superfamily. Tyr30, Asp35, and His83 form a catalytic triad that is conserved across this family. Site-directed mutagenesis of BaiE from Clostridium scindens VPI 12708 confirm that these residues are essential for catalysis and also the importance of other conserved residues, Tyr54 and Arg146, which are involved in substrate binding and affect catalytic turnover. Steady-state kinetic studies reveal that the BaiE homologs are able to turn over 3-oxo-Δ(4) -bile acid and CoA-conjugated 3-oxo-Δ(4) -bile acid substrates with comparable efficiency questioning the role of CoA-conjugation in the bile acid metabolism pathway.

Efficient Preparation of Site-Specific Antibody-Drug Conjugates Using Phosphopantetheinyl Transferases

Post-translational modification catalyzed by phosphopantetheinyl transferases (PPTases) has previously been used to site-specifically label proteins with structurally diverse molecules. PPTase catalysis results in covalent modification of a serine residue in acyl/peptidyl carrier proteins and their surrogate substrates which are typically fused to the N- or C-terminus. To test the utility of PPTases for preparing antibody-drug conjugates (ADCs), we inserted 11 and 12-mer PPTase substrate sequences at 110 constant region loop positions of trastuzumab. Using Sfp-PPTase, 63 sites could be efficiently labeled with an auristatin toxin, resulting in 95 homogeneous ADCs. ADCs labeled in the CH1 domain displayed in general excellent pharmacokinetic profiles and negligible drug loss. A subset of CH2 domain conjugates underwent rapid clearance in mouse pharmacokinetic studies. Rapid clearance correlated with lower thermal stability of the particular antibodies. Independent of conjugation site, almost all ADCs exhibited subnanomolar in vitro cytotoxicity against HER2-positive cell lines. One selected ADC was shown to induce tumor regression in a xenograft model at a single dose of 3 mg/kg, demonstrating that PPTase-mediated conjugation is suitable for the production of highly efficacious and homogeneous ADCs.

Structural and functional characterization of BaiA, an enzyme involved in secondary bile acid synthesis in human gut microbe

Despite significant influence of secondary bile acids on human health and disease, limited structural and biochemical information is available for the key gut microbial enzymes catalyzing its synthesis. Herein, we report apo- and cofactor bound crystal structures of BaiA2, a short chain dehydrogenase/reductase from Clostridium scindens VPI 12708 that represent the first protein structure of this pathway. The structures elucidated the basis of cofactor specificity and mechanism of proton relay. A conformational restriction involving Glu42 located in the cofactor binding site seems crucial in determining cofactor specificity. Limited flexibility of Glu42 results in imminent steric and electrostatic hindrance with 2'-phosphate group of NADP(H). Consistent with crystal structures, steady state kinetic characterization performed with both BaiA2 and BaiA1, a close homolog with 92% sequence identity, revealed specificity constant (kcat /KM ) of NADP(+) at least an order of magnitude lower than NAD(+) . Substitution of Glu42 with Ala improved specificity toward NADP(+) by 10-fold compared to wild type. The cofactor bound structure uncovered a novel nicotinamide-hydroxyl ion (NAD(+) -OH(-) ) adduct contraposing previously reported adducts. The OH(-) of the adduct in BaiA2 is distal to C4 atom of nicotinamide and proximal to 2'-hydroxyl group of the ribose moiety. Moreover, it is located at intermediary distances between terminal functional groups of active site residues Tyr157 (2.7 Å) and Lys161 (4.5 Å). Based on these observations, we propose an involvement of NAD(+) -OH(-) adduct in proton relay instead of hydride transfer as noted for previous adducts.

Simultaneous purification and site-specific modification of pyrroline-carboxy-lysine proteins

Sticky residue: Pyrroline-carboxy-lysine (Pcl) can be readily incorporated into proteins expressed in E. coli and mammalian cells by using the pyrrolysyl tRNA/tRNA synthetase pair. Pcl can be used as a single amino acid purification tag and can be site-specifically modified with functional probes during the elution process.

Helix Propensity of Highly Fluorinated Amino Acids

Highly fluorinated amino acids have been used to stabilize helical proteins for potential application in various protein-based biotechnologies. To gain further insight into the effect of these highly fluorinated amino acids on helix formation exclusively, we measured the helix propensity of three highly fluorinated amino acids: (S)-5,5,5,5',5',5'-hexafluoroleucine (Hfl), (S)-2-amino-4,4,4-trifluorobutyric acid (Atb), and (S)-pentafluorophenylalanine (Pff). We have developed a short chemoenzymatic synthesis of Hfl with extremely high enantioselectivity (>99%). To measure the helix propensity (w) of the amino acids, alanine-based peptides were synthesized, purified, and investigated by circular dichroism spectroscopy (CD). On the basis of the CD data, the helix propensity of hydrocarbon amino acids can decrease up to 24-fold (1.72 kcal.mol-1.residue-1) upon fluorination. This difference in helix propensity has previously been overlooked in estimating the magnitude of the fluoro-stabilization effect (which has been estimated to be 0.32-0.83 kcal.mol-1.residue-1 for Hfl), resulting in a gross underestimation. Therefore, the full potential of the fluoro-stabilization effect should provide even more stable proteins than the fluoro-stabilized proteins to date.

Drug-induced akathisia revisited

Despite the fact that the neuroleptic drugs have been widely used for more than 40 years, one of their most common side-effects, akathisia, has been relatively neglected. There are still no universally agreed diagnostic criteria for akathisia, particularly chronic akathisia, and in this review article, we discuss the controversies surrounding the voluntary nature of its motor features and the importance of the dysphoric component. We also review the published epidemiological studies to show the great variation in frequency of occurrence. Finally, we discuss the possible neurotransmitters involved in the pathophysiology and treatment of this condition.