Cysteic acid also known as 3-sulfo--alanine is the organic compound with the formula HO<sub>3</sub>SCH<sub>2</sub>CH(NH<sub>2</sub>)CO<sub>2</sub>H. It is often referred to as cysteate, which near neutral pH takes the form <sup>âÂÂ</sup>O<sub>3</sub>SCH<sub>2</sub>CH(NH<sub>3</sub><sup>+</sup>)CO<sub>2</sub><sup>âÂÂ</sup>.
It is an amino acid generated by oxidation of cysteine, whereby a thiol group is fully oxidized to a sulfonic acid/sulfonate group. It is further metabolized via 3-sulfolactate, which converts to pyruvate and sulfite/bisulfite. The enzyme L-cysteate sulfo-lyase catalyzes this conversion. Cysteate is a biosynthetic precursor to taurine in microalgae. By contrast, most taurine in animals is made from cysteine sulfinate.