= L-malate
{c}
\b[L-malate] is the <malate> <enantiomer> with $S$ configuration at its hydroxyl-bearing carbon. In the dianionic form specified here, its formula is $\mathrm{C_4H_4O_5^{2-}}$. The stereochemical label and the charge state answer separate questions: one specifies the spatial arrangement around a stereogenic carbon, while the other specifies loss of the two carboxyl protons.<L-malate in ChEBI>[CHEBI:15589, definition, formula, charge and conjugate-acid relationship]{ref}
The connectivity can be written schematically as
$$
\mathrm{^-OOC-CH(OH)-CH_2-COO^-},
$$
but that flat <condensed structural formula> does not by itself distinguish the two <enantiomers>. An opposite configuration has the same elemental formula and charge. Likewise, changing <protonation> of a carboxyl group is not the same operation as reversing the stereochemical arrangement at the carbon bearing OH.
A metabolic transformation makes this distinction useful. The NAD-dependent oxaloacetate-forming reaction EC1.1.1.37 oxidizes <L-malate> to <oxaloacetate> while reducing <NAD+> to <NADH>. Conversion of the hydroxyl-bearing carbon to a carbonyl removes that stereogenic center in the oxaloacetate framework. This is a covalent oxidation, rather than simply another protonation state of malate.<Enzyme Nomenclature by the IUBMB>[EC1.1.1.37, accepted name, Reaction and Comments: NAD-dependent oxaloacetate formation]{ref}
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