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FadL
Long-Chain Fatty Acid Transporter

Secondary Structure showing the transmembrane beta barrels which serve as porins. This protein is found in Escherichia coli (E. coli) and serves as a lipid transporter for exogenous long-chain fatty acids. A mutation in fadL results in an inability of E. coli cells to grow. FadL protein is a member of the outer membrane (OM) protein family involved in the uptake of hydrophobic compounds.
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FadL
Protein Structure

The secondary structure displayed shows the long (~50 Å) barrel composed of 14 antiparallel transmembrane beta (ß) barrels in blue. The barrel allows long chain fatty acids to pass through the plasma and/or outer membrane (OM) of E. coli. The alpha are helicies shown in pink.
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FadL
Order of synthesizing the protein

The blue sections of the structure represent the amino end of the peptide which is synthesized first starting with the 5' end of the mRNA. The red sections of the protein represent the carboxyl end, which are synthesized last and are on the 3' end of the mRNA. The progression of colors in between are synthesized from blue/green to yellow/red.
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FadL
NPA cap

One of the helicies is capped by the conserved sequence asparagine-proline-analine (NPA): ASN33, PRO34, ALA35. NPA marks the turn of a short helix inside the membrane of an E. coli cell.
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FadL
High-affinity binding site

Three charged residues, ARG157, LYS317, and GLU319 are shown in the binding site. They are positioned to interact with the negatively charged head group on incoming fatty acids. This head group binding site is located close to the ligand, lauryldimethylamine-N-oxide (LDAO). Two residues, ARG157 and LYS317, are positively charged and interact with the head group of a fatty acid at the binding site. The other residue, GLU319, is negatively charged and used to stablilize the binding site by slightly repulsing the negatively charged fatty acid head group. Other amino acids lining the head group binding site are hydrophobic.
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FadL
Backbone and ligands in the homodimer

The ligand lauryl dimethylamine-N-oxide (LDAO), C₁₄H₃₁NO, is shown here at the site where the fatty acid would bind. It is composed of a long hydrocarbon chain with a nitrogen bonded to an oxygen and two methyl groups, instead of having a carboxyl group.
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FadL
Residues surrounding LDAO

The amino acid residues surrounding LDAO are shown here. The residues shown are ILE, GLY, PHE, ALA, TYR, LYS, and ASN.
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FadL
Regulation by mutagenesis

Mutagenesis of HIS83 to ALA83 inhibits the binding of fatty acids. This residue forms hydrogen bonds with carbonyl atoms on the backbone. Its mutagenesis disturbs the high-affinity binding pocket, allowing for regulation of fatty acid binding to the protein.
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FadL
Regulation by mutagenesis

Replacing GLY212 with GLU results in low fatty acid binding with a moderate effect on transport of fatty acids through the barrel. This suggests that the GLU mutant has an open channel allowing the fatty acids to diffuse throught the plasma membrane.
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FadL
Conformational kink disrupts beta (ß) barrel

The conformational kink that points inward disrupts the interstrand beta (ß) sheet hydrogen-bonding pattern from THR99 to ALA105, resulting in a gap in the barrel wall. This kink is stabilized by a short antiparallel beta (ß) strand between ASN101 to GLY103 (shown in green). Hydrogen bonding between the backbond and GLY103 may play a role in this stability.
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