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Escherichia coli fumarase C is a fumarate hydratase of Escherichia coli, commonly designated FumC. It catalyzes the reversible conversion between fumarate plus water and L-malate in the citric acid cycle. The name identifies a particular enzyme, while the broader activity name can cover related catalysts.^[ref]

The 1995 structure revealed a four-subunit assembly at $2.0\text{A˚}$ resolution, with the active site formed using atoms from three chains.^[ref] Further structures in 1996 used bound inhibitors and poor substrates to distinguish this catalytic A-site from a nearby B-site about $12\text{A˚}$ away. The second site was formed by one subunit rather than three.^[ref]

This gives a concrete interpretation problem for a binding experiment. Detecting a compound somewhere on the protein does not establish that it occupies the catalytic site. Comparing its position with substrate analogues, inhibitory behaviour and the multisubunit geometry supplies additional evidence. Similarly, a change in assembly could affect catalytic activity while retaining the same substrate-contacting amino acid residues. Structure and reaction-rate measurements describe complementary aspects of the working enzyme.

Fumarase C, often designated FumC, is a classII form of fumarate hydratase. It catalyzes reversible hydration of fumarate to L-malate. The protein designation should be tied to an organism: Escherichia coli fumarase C is a well-studied species-specific example, while the broader enzyme-activity name does not specify one protein sequence.^[ref]

The net substrate inventory makes the chemistry explicit:Four carbon atoms remain four; adding water changes hydrogen and oxygen content without changing the total charge. This hydration is chemically distinct from oxidation of L-malate to oxaloacetate. The enzyme accelerates a reversible reaction rather than making its net direction independent of conditions.^[ref]

The1995 X-ray crystallography study of the E.coli enzyme resolved an assembly of four identical protein subunits at2.0Å. Its proposed active site involved contributions from three of the four protein subunits; a local catalytic environment therefore need not lie inside just one polypeptide chain. A1996 follow-up with bound compounds distinguished the catalytic A-site from a nearby second binding site. These experiments also show why observing ligand binding does not by itself identify a catalytic site.^[ref][ref]

Fumarate hydratase, also called fumarase, is the enzyme that reversibly interconverts fumarate plus water with L-malate. The activity is EC4.2.1.2:In the hydration direction, water adds across the carbon-carbon double bond of fumarate. In the reverse direction, water is eliminated. Its lyase classification describes this transformation; the mere presence of water does not make the reaction hydrolysis.^[ref]

For an invented inventory, begin with ten micromoles of fumarate and let six micromoles undergo hydration. Four micromoles of fumarate remain and six micromoles of L-malate form. Each retains a four-carbon skeleton, so the counted carbon inventory remains $4(4+6)=40$ micromoles of carbon atoms. The chosen extent does not determine chemical equilibrium or enzyme kinetics.

A 1995 X-ray crystallography study of Escherichia coli fumarase C found a four-subunit assembly at $2.0\text{A˚}$ resolution. A competitive tungstate derivative helped locate the active site, which included atoms from three of the four protein subunits.^[ref] The example shows why counting subunits and locating a catalytic site are different tasks: several chains can cooperate to build one local reaction environment.

T. M. Weaver is the published author name credited on the 1995 structural study of Fumarase C from Escherichia coli. The paper lists a University of Minnesota biochemistry affiliation. Its initials are retained here as the verified scientific byline.^[ref]

The study determined the four-subunit protein complex at $2.0$ ångströms crystallographic resolution. A tungstate derivative, acting as a competitive inhibitor, helped locate an active site containing contributions from three of the four protein subunits. Thus, the site was not simply a pocket belonging to one isolated chain.^[ref]

This gives an illuminating reason to distinguish folding from assembly. A purified protein subunit could retain its own folded shape yet fail to form a complete active site if a neighboring contribution is missing. Conversely, identifying a conserved catalytic group in a sequence does not establish that the required protein complex has assembled.

An original probability model illustrates the possible amplification. Suppose an idealized site requires three independently functional subunit contributions, each present with probability $p$. Its probability of being complete is $p^3$. With $p=0.9$, this is $0.729$, rather than $0.9$. In that model, losing $10\%$ of individual contributions leaves $27.1\%$ of sites incomplete. This is not a measured activity law for Fumarase C: independent availability, random assembly and all-or-nothing site function are explicit assumptions. A real defect could affect assembly, chemical turnover or both.

The structural observation motivates complementary measurements: assess protein oligomerization separately from reaction rate, then ask whether altered assembly accounts for altered catalysis. A crystal structure supplies a concrete spatial hypothesis about these relationships, while functional measurements test its consequences.

Tungstate commonly denotes the tungsten oxoanion $\mathrm{W}{\mathrm{O}}_4^{2-}$ and salts containing it. The formula specifies one tungsten and four oxygen atoms with a net charge of $-2$. The Protein Data Bank chemical-component dictionary identifies its WO4 ligand as tungstate(VI), while the corresponding PubChem record gives the dianion formula.^[ref]

Taking each oxygen to have oxidation state $-2$, charge balance gives $x+4(-2)=-2$, hence $x=+6$ for tungsten. Oxidation state and net ionic charge therefore answer different questions. A neutral salt needs two singly charged cations per tungstate anion, or one doubly charged cation. That count balances charge without requiring the ions to form a discrete isolated molecule in a crystal.

Tungstate also provides an experimental structural probe. In a 1995 study of Escherichia coli fumarase C, a competitive tungstate derivative helped locate an active site containing contributions from three protein subunits.^[ref] Binding a detectable inorganic ion can reveal where a molecular interaction occurs. It does not by itself show that the ion is the enzyme's physiological substrate or that every binding site catalyzes a reaction.