Digestive Enzymes

There are quite a few to know for the MCAT, but make it easier on yourself with two main tips:

  1. Learn them with respect to the function of each section of the gut.
  2. The site of action is determined by the site of production:
  1. Salivary Gland Enzymes - act in the mouth, because the stomach’s whole job is to chop proteins into big blocks (oligopeptides) like a meat cleaver, so they don’t survive past the esophagus.
  2. Stomach Enzyme (pepsin) - requires a really low pH (optimal: 1) to function, so it will unfold at the wrong pH (i.e., neutral). (See the Acid-Base chapter sections on Isoelectric Point and Buffer Systems to understand this better).
  3. Brush Border Enzymes - They are integral proteins on the extracellular face of the plasma membrane of the microvilli (brush border) of enterocytes (intestinal epithelial cells). As a result, they do their job where they are.
  4. Pancreatic Enzymes - Usually secreted as zymogens (inactive forms that need to be cleaved). Trypsin has to get to the small intestinal lumen before getting activated by enterokinase. At that point, trypsin activates itself and all the other pancreatic zymogens by cleaving off the inhibitory domains. Thus, the pancreatic enzymes get activated in the duodenum, where they can finally act.

If it seems like a lot of enzymes are acting in the duodenum, that is correct. Remember, the mnemonic “Dow Jones Industrial.”

Section of Small Intestine

Overall Function

Duodenum

Digestion. Remember, “D is for Digestion!”

Jejunum

Absorption

Ileum

Absorption, esp. LIPIDS in the terminal ileum

This should make sense to you. You can’t absorb macromolecules. They must be chemically broken down (digestion) into smaller units to get absorbed, as follows:

Biomolecules

What level of polymerization the gut can absorb

Sugars

Monomers = Monosaccharides

Peptides

Monomers - Trimers = Amino Acids, Dipeptides, and Tripeptides

Fats (TAG)

COOHs and Monoesters = Fatty Acids & Monoacylglycerol (MAG)

Pancreas (don’t forget that it has 2 JOBS!!!)

Exocrine Job: Base (HCO3-) & Enzymes

Endocrine Job: ∝ - Glucagon, β - Insulin, ∆ - Somatostatin

ENZYME

PRODUCTION site

Site of ACTION

FUNCTION

Amylase

Mouth(Salivary Glands)/ Pancreas

Mouth/Duodenum

Polysaccharides (Amylopectin, a.k.a Starch) → Dextrins (Oligosaccharides) → Maltose (Disaccharide)

Pepsin(-ogen)

Stomach - Chief Cells

Stomach

Peptide bond cleavage (Protein → Oligopeptide)

Maltase

Intestinal Mucosal Cell (a.k.a Intestinal Epithelial Cells, Enterocytes)

Duodenum (Fixed to the Brush Border of the Enterocytes → act where they are)

Maltose (glu-glu disaccharide → glucose)

Lactase

Enterocytes

Brush Border

Lactose → glu + galactose

Aminopeptidase

Intestinal Cells

Brush Border

Breaks Peptide Bonds from N-terminus (amino end)

Enterokinase

Intestinal Cells

Brush Border

Activates Pancreatic Enzymes (Trypsin)

Trypsin(-ogen)

Pancreatic Acinar Cells

Duodenum (In the Lumen)

Digest Proteins, Cleaves other Pancreatic Zymogens (propeptidases)

Chymotrypsin(-ogen)

Pancreatic Acinar Cells

Duodenum

Break Peptide Bonds

(Pro)-Carboxypeptidase (A & B)

Pancreatic Acinar Cells

Duodenum

Break Peptides from the C-terminus

Lipase

Pancreas

Duodenum

Fat Breakdown (Esterase: Triacylglycerol → glycerol & free FAs)