In acid solution, water can add to the double bond of 2‑butenedioic acid to form 2‑hydroxysuccinic acid.
HOOCCH=CHCOOH+H2O <== H2SO4 ==> HOOCCH2CH(OH)COOH
This reaction is also an important reaction of the tricarboxylic acid cycle in cells, wherein the reaction occurs in neutral solution, so the acid groups are both ionized to the carboxylate form. The reaction is catalyzed by the stereospecific enzyme fumarase that utilizes only the trans form of 2‑butenedioate ion, also known as fumarate, and produces only the (S)‑2‑hydroxysuccinate enantiomer,also known as (s)-malate). Draw the correct stereochemical structures of these two compounds of the fumarase-catalyzed reaction.
fumarate (s)-malate
(trans-2-butenedioate) ((s)-2-hydroxysuccinate)
Fumarase
+ H2O <===========>

The stereochemical structures of the two compounds of the fumarase-catalyzed reaction are in the attachment below. The reaction been referred to is illustrated in the equation below

HOOCCH=CHCOOH + H₂O ==> HOOCCH₂CH(OH)COOH

The compounds attached are trans-2-butenedioate (which is one of the reactants) and (s)-2-hydroxysuccinate (which is the product formed)

Note that stereoisomers are isomers that differ in spatial orientation, thus there are other isomers that could contain the same atoms and have slightly but different spatial orientation such as cis-2-butenedioate and (r)-2-hydroxysuccinate