Penta- and hexa-coordinate ferric hemoglobins display distinct pH titration profiles measured by Soret peak shifts

Arginine-Modified Hemin Enhances G-Quadruplex DNAzyme Peroxidase Activity for High Sensitivity Detection

Hemin/G-quadruplex (hG4) complexes are frequently used as artificial peroxidase-like enzymatic systems (termed G4 DNAzymes) in many biosensing applications, in spite of a rather low efficiency, notably in terms of detection limits. To tackle this issue, we report herein a strategy in which hemin is chemically modified with the amino acids found in the active site of parent horseradish peroxidase (HRP), with the aim of recreating an environment conducive to high catalytic activity. When hemin is conjugated with a single arginine, it associates with G4 to create an arginine-hemin/G4 (R-hG4) DNAzyme that exhibits improved catalytic performances, characterized by kinetic analysis and DFT calculations. The practical relevance of this system was demonstrated with the implementation of biosensing assays enabling the chemiluminescent detection of G4-containing DNA and colorimetry detection of the flap endonuclease 1 (FEN1) enzyme with a high efficiency and sensitivity. Our results thus provide a guide for future enzyme engineering campaigns to create ever more efficient peroxidase-mimicking DNA-based systems.

Sphaerotilus natans hemoglobins have an NADH oxidation activity and promote the yield of limonene in an engineered E. coli strain

Bacterial hemoglobins play important roles inside the cell. Phylogenetically, they belong to three different families: the single domain hemoglobin, flavohemoglobin and truncated hemoglobin. Vitreoscilla hemoglobin (VHb) is the first characterized bacterial hemoglobin, and belongs to the single domain hemoglobin family. Heterologous expression of VHb promotes the growth of host cells under microaerobic conditions, and enhances the yield of products during fermentation. Although VHb has been widely applied in the biotechnology field, other bacterial hemoglobins have not demonstrated similar applications. In this study, we identified four bacterial hemoglobins from the microaerobic growing bacterium Sphaerotilus natans, including one flavohemoglobins (FHB) and three truncated hemoglobins (THB1, THB2 and THB3). Absorption spectrum studies validate the existent of the Soret peak and Q-band characteristic to heme and suggest heme groups in FHB and THB1 are hexa- or penta-coordinated, respectively. Our studies demonstrate that FHB and all three truncated hemoglobins have NADH oxidation and radical production activities, which is surprising since truncated hemoglobins do not have a reductase domain that could bind NADH. However, the M. tuberculosis HbN does not show these activities, indicating they are not universal among truncated hemoglobins. Docking studies suggest the nicotinamide ring of NADH may bind to the distal heme pocket of THB1, suggesting the direct electron transfer from NADH to heme might be possible. Our truncated hemoglobins also show peroxidase activities that in THB2 and THB3 could be inhibited by FdR, indicating possible interactions between FdR and truncate hemoglobins. Expression of FHB and THB1 in E. coli could promote cell growth. THB1 also enhances the production of limonene in an engineered E. coli strain, while VHb does not have this effect, which suggests that studies on truncated hemoglobins may lead to the discovery of new and more powerful tools that could have profound impact on biotechnology.

Structure, mechanism, and inhibition of Hedgehog acyltransferase

The Sonic Hedgehog (SHH) morphogen pathway is fundamental for embryonic development and stem cell maintenance and is implicated in various cancers. A key step in signaling is transfer of a palmitate group to the SHH N terminus, catalyzed by the multi-pass transmembrane enzyme Hedgehog acyltransferase (HHAT). We present the high-resolution cryo-EM structure of HHAT bound to substrate analog palmityl-coenzyme A and a SHH-mimetic megabody, revealing a heme group bound to HHAT that is essential for HHAT function. A structure of HHAT bound to potent small-molecule inhibitor IMP-1575 revealed conformational changes in the active site that occlude substrate binding. Our multidisciplinary analysis provides a detailed view of the mechanism by which HHAT adapts the membrane environment to transfer an acyl chain across the endoplasmic reticulum membrane. This structure of a membrane-bound O-acyltransferase (MBOAT) superfamily member provides a blueprint for other protein-substrate MBOATs and a template for future drug discovery.

Stability and Folding of the Unusually Stable Hemoglobin from Synechocystis is Subtly Optimized and Dependent on the Key Heme Pocket Residues

AIMS

The aim of our study is to understand the biophysical traits that govern the stability and folding of Synechocystis hemoglobin, a unique cyanobacterial globin that displays unusual traits not observed in any of the other globins discovered so far.

BACKGROUND

For the past few decades, classical hemoglobins such as vertebrate hemoglobin and myoglobin have been extensively studied to unravel the stability and folding mechanisms of hemoglobins. However, the expanding wealth of hemoglobins identified in all life forms with novel properties, like heme coordination chemistry and globin fold, have added complexity and challenges to the understanding of hemoglobin stability, which has not been adequately addressed. Here, we explored the unique truncated and hexacoordinate hemoglobin from the freshwater cyanobacterium Synechocystis sp. PCC 6803 known as "Synechocystis hemoglobin (SynHb)". The "three histidines" linkages to heme are novel to this cyanobacterial hemoglobin.

OBJECTIVE

Mutational studies were employed to decipher the residues within the heme pocket that dictate the stability and folding of SynHb.

METHODS

Site-directed mutants of SynHb were generated and analyzed using a repertoire of spectroscopic and calorimetric tools.

RESULTS

The results revealed that the heme was stably associated to the protein under all denaturing conditions with His117 playing the anchoring role. The studies also highlighted the possibility of existence of a "molten globule" like intermediate at acidic pH in this exceptionally thermostable globin. His117 and other key residues in the heme pocket play an indispensable role in imparting significant polypeptide stability.

CONCLUSION

Synechocystis hemoglobin presents an important model system for investigations of protein folding and stability in general. The heme pocket residues influenced the folding and stability of SynHb in a very subtle and specific manner and may have been optimized to make this Hb the most stable known as of date. Other: The knowledge gained hereby about the influence of heme pocket amino acid side chains on stability and expression is currently being utilized to improve the stability of recombinant human Hbs for efficient use as oxygen delivery vehicles.