fumarate + L-cysteine <=> S-[(2R)-succino]-L-cysteine
fumarate + l-cysteine <=> s-[(2r)-succino]-l-cysteine
fumarate + L-cysteine <=> S-[(2R)-succino]-L-cysteine
fumarate + l-cysteine <=> s-[(2r)-succino]-l-cysteine