Substrate interaction with extracellular enzymes

Lipases act only at a lipid-water interface and have very little activity on soluble substrates [17]. For extracellular lipases to become active at an oil/water interface, it has been suggested that the lipase s hydrophobic head is bound to the oil droplet by hydrophobic interactions, while the enzyme s active site aligns with, and binds to the substrate molecule [19]. A similar mechanism could be valid for lipase activity on polyester surfaces. 

In the body of high mammals, extracellular cathepsin D probably is bound by a2-macroglobulin, once it reaches the circulation, and thus inactivated and eliminated. Barrett and Starkey (23) showed that labeled cathepsin D is bound by a2-macroglobulin at pH 6, and that the interaction depends upon the activity of the enzyme, since it was partially blocked by pepstatin (a weak inhibitor at this pH value). Complexes of proteinases with a2-macroglobulin are inactive against protein substrates, and are rapidly cleared from the circulation (23). ... 

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