They are similar to a non-competitive inhibitor, as they utilise a binding site away from the enzyme active site, but uncompetitive inhibitors only bind to an enzyme-substrate complex, rather than free enzymes.
Effect on Vmax and Km
Uncompetitive inhibitors decrease both Vmax and Km. The Vmax decreases as the inhibitor prevents the enzyme-substrate complex from completing the reaction and releasing products. Km decreases due to the enzyme-substrate complex being unable to dissociate, which increases the enzyme's affinity for the substrate.