The final step in digestion of dietary carbohydrates and proteins occurs right on the face of small intestinal enterocytes, in the immediate vicinity of the transporters which will ferry the resulting sugars and amino acids into the epithelial cells. The enzymes responsible for this terminal stage of digestion are not free in the intestinal lumen, but rather, tethered as integral membrane proteins in the plasma membrane of the enterocyte. The apical plasma membrane housing these enzymes is composed of numerous microvilli which extend from the cell and constitute the "brush border". Hence, the enzymes embedded in those microvilli are referred to as brush border enzymes.
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