Membranous-Phase Digestive Enzymes Are a Structural Part of the Intestinal Surface Membrane

Membranous-phase digestion, as with its luminal counter­part, occurs because of the hydrolytic action of enzymes. The difference between the two phases is that membranous-phase enzymes are chemically bound to the surface membrane of the intestine; thus the enzyme substrates must be in con­tact with the epithelium before hydrolysis can occur. These membrane-bound digestive enzymes are synthesized within the enterocytes and are subsequently transported to the luminal surface of the apical membrane. They remain attached to the surface by a short anchor segment while the large, catalytic portion of the enzyme molecule projects away from the surface, toward the gut lumen.