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From Proteopedia

This is . Fumarase is an enzyme of the citric acid cycle or Kreb's cycle. The function of fumarase is to stereospecifically convert fumarate into L-malate. This specific image (1FUO) is of Fumarase C from E. Coli. It is a dimer bound with 2 ligands; 2 citrate molecules and 2 malate molecules. The two citrate molecules are the two center and the malate molecules are the outer two ligands shown. The fumarate/L-malate conversion is reversible.

A moajority of fumarase's secondary structure is composed of shown in blue. The longer α-helicies are grouped together and appear to form short coiled coils which compse the mid section of the enzyme's structure. Fumarase contains 3 small sections of , each consisting of two strands in anti-parallel allignment connected by a short random coil. within the structure is shown by the thin black lines. The hydrogen bonding is responsible for the shape of th α-helicies and the bonding between the two strands of the β-sheets. H-bonding is also responsible for the general shape of the random coil sections as well. The of fumarase are shown as wire frame in red. Note that they tend to be located on the interior of the protein and tend to be less solvent accessible due to their lack of polarity. on the other hand, the tend to be located on the outer or solvent accessible edges of the protein as they are either polar or charged and would be stabilized by non-covalent interactions with the solvent. The solvent for fumarase is . Note that the water tends to be isolated to the outside of the protein and that there is a large pocket in the center that is completely solvent free. It is important to note the presence of water underneath the citrate molecule. This water is strategically bound and aids in the conversion of fumarate into L-malate. Water is also excluded from the space within the coiled coils created by the α-helicies.

The are represented by the white to blue stick and wire sections of this image and the ligands, both citrate and malate, are represented by the grey and red molecules. Blue residues indicate a cation and red, if present represent anionic residues. A majority of the ligand binding residues are cations which would bond well with the electronegative oxygens in citrate and L-malate.

The residues are shown in blue. Note that they are relatively far from the ligand binding site. Fumarase often exists in a tetrameric form in which these active site residue would likely line up closer to the ligand binding site on another subunit of the protein to create more of an active site pocket than is shown in this image.