Insights into the stability and substrate specificity of the E. coli aerobic β-oxidation trifunctional enzyme complex

Abstract Degradation of fatty acids by the β-oxidation pathway results in the formation of acetyl-CoA which enters the TCA cycle for the production of ATP. In E. coli, the last three steps of the β-oxidation are catalyzed by two heterotetrameric α₂β₂ enzymes namely the aerobic trifunctional enzyme (EcTFE) and the anaerobic TFE (anEcTFE). The α-subunit of TFE has 2E-enoyl-CoA hydratase (ECH) and 3S-hydroxyacyl-CoA dehydrogenase (HAD) activities whereas the β-subunit is a thiolase with 3-ketoacyl-CoA thiolase (KAT) activity. Recently, it has been shown that the two TFEs have complementary substrate specificities allowing for the complete degradation of long chain fatty acyl-CoAs into acetyl-CoA under aerobic conditions. Also, it has been shown that the tetrameric EcTFE and anEcTFE assemblies are similar to the TFEs of Pseudomans fragi and human, respectively. Here the properties of the EcTFE subunits are further characterized. Strikingly, it is observed that when expressed separately, EcTFE-α is a catalytically active monomer whereas EcTFE-β is inactive. However, when mixed together active EcTFE tetramer is reconstituted. The crystal structure of the EcTFE-α chain is also reported, complexed with ATP, bound in its HAD active site. Structural comparisons show that the EcTFE hydratase active site has a relatively small fatty acyl tail binding pocket when compared to other TFEs in good agreement with its preferred specificity for short chain 2E-enoyl-CoA substrates. Furthermore, it is observed that millimolar concentrations of ATP destabilize the EcTFE complex, and this may have implications for the ATP-mediated regulation of β-oxidation in E. coli

Structural enzymology binding studies of the peptide‐substrate‐binding domain of human collagen prolyl 4‐hydroxylase (type‐II):high affinity peptides have a PxGP sequence motif

Abstract The peptide‐substrate‐binding (PSB) domain of collagen prolyl 4‐hydroxylase (C‐P4H, an α2β2 tetramer) binds proline‐rich procollagen peptides. This helical domain (the middle domain of the α subunit) has an important role concerning the substrate binding properties of C‐P4H, although it is not known how the PSB domain influences the hydroxylation properties of the catalytic domain (the C‐terminal domain of the α subunit). The crystal structures of the PSB domain of the human C‐P4H isoform II (PSB‐II) complexed with and without various short proline‐rich peptides are described. The comparison with the previously determined PSB‐I peptide complex structures shows that the C‐P4H‐I substrate peptide (PPG)3, has at most very weak affinity for PSB‐II, although it binds with high affinity to PSB‐I. The replacement of the middle PPG triplet of (PPG)3 to the nonhydroxylatable PAG, PRG, or PEG triplet, increases greatly the affinity of PSB‐II for these peptides, leading to a deeper mode of binding, as compared to the previously determined PSB‐I peptide complexes. In these PSB‐II complexes, the two peptidyl prolines of its central P(A/R/E)GP region bind in the Pro5 and Pro8 binding pockets of the PSB peptide‐binding groove, and direct hydrogen bonds are formed between the peptide and the side chains of the highly conserved residues Tyr158, Arg223, and Asn227, replacing water mediated interactions in the corresponding PSB‐I complex. These results suggest that PxGP (where x is not a proline) is the common motif of proline‐rich peptide sequences that bind with high affinity to PSB‐II

Structural insights into the substrate-binding proteins Mce1A and Mce4A from Mycobacterium tuberculosis

Abstract Mycobacterium tuberculosis (Mtb), which is responsible for more than a million deaths annually, uses lipids as the source of carbon and energy for its survival in the latent phase of infection. Mtb cannot synthesize all of the lipid molecules required for its growth and pathogenicity. Therefore, it relies on transporters such as the mammalian cell entry (Mce) complexes to import lipids from the host across the cell wall. Despite their importance for the survival and pathogenicity of Mtb, information on the structural properties of these proteins is not yet available. Each of the four Mce complexes in Mtb (Mce1–4) comprises six substrate-binding proteins (SBPs; MceA–F), each of which contains four conserved domains (N-terminal transmembrane, MCE, helical and C-terminal unstructured tail domains). Here, the properties of the various domains of Mtb Mce1A and Mce4A, which are involved in the import of mycolic/fatty acids and cholesterol, respectively, are reported. In the crystal structure of the MCE domain of Mce4A (MtMce4A39–140) a domain-swapped conformation is observed, whereas solution studies, including small-angle X-ray scattering (SAXS), indicate that all Mce1A and Mce4A domains are predominantly monomeric. Further, structural comparisons show interesting differences from the bacterial homologs MlaD, PqiB and LetB, which form homohexamers when assembled as functional transporter complexes. These data, and the fact that there are six SBPs in each Mtb mce operon, suggest that the MceA–F SBPs from Mce1–4 may form heterohexamers. Also, interestingly, the purification and SAXS analysis showed that the helical domains interact with the detergent micelle, suggesting that when assembled the helical domains of MceA–F may form a hydrophobic pore for lipid transport, as observed in EcPqiB. Overall, these data highlight the unique structural properties of the Mtb Mce SBPs

Crystal structure of the collagen prolyl 4-hydroxylase (C-P4H) catalytic domain complexed with PDI:toward a model of the C-P4H α₂β₂ tetramer

Abstract Collagen prolyl 4-hydroxylases (C-P4H) are α₂β₂ tetramers, which catalyze the prolyl 4-hydroxylation of procollagen, allowing for the formation of the stable triple-helical collagen structure in the endoplasmic reticulum. The C-P4H α-subunit provides the N-terminal dimerization domain, the middle peptide-substrate-binding (PSB) domain, and the C-terminal catalytic (CAT) domain, whereas the β-subunit is identical to the enzyme protein disulfide isomerase (PDI). The structure of the N-terminal part of the α-subunit (N-terminal region and PSB domain) is known, but the structures of the PSB-CAT linker region and the CAT domain as well as its mode of assembly with the β/PDI subunit, are unknown. Here, we report the crystal structure of the CAT domain of human C-P4H-II complexed with the intact β/PDI subunit, at 3.8 Å resolution. The CAT domain interacts with the a, b’, and a’ domains of the β/PDI subunit, such that the CAT active site is facing bulk solvent. The structure also shows that the C-P4H-II CAT domain has a unique N-terminal extension, consisting of α-helices and a β-strand, which is the edge strand of its major antiparallel β-sheet. This extra region of the CAT domain interacts tightly with the β/PDI subunit, showing that the CAT-PDI interface includes an intersubunit disulfide bridge with the a’ domain and tight hydrophobic interactions with the b’ domain. Using this new information, the structure of the mature C-P4H-II α₂β₂ tetramer is predicted. The model suggests that the CAT active-site properties are modulated by α-helices of the N-terminal dimerization domains of both subunits of the α₂-dimer