Catalysis of the reaction: 2-O-(4-deoxy-beta-L-threo-hex-4-enopyranuronosyl)-alpha-L-rhamnopyranose(1-) + H2O = (4S,5S)-4,5-dihydroxy-2,6-dioxohexanoate + alpha-L-rhamnopyranose.
unsaturated rhamnogalacturonyl hydrolase activity