Enzymes and Digestion of Carbs, Proteins, Fats Practice Questions

Correct answer: Salivary amylase

Salivary amylase begins the digestion of starch in the mouth. It breaks down complex carbohydrates into smaller polysaccharides.

Correct answer: Pepsin

Pepsin is activated from pepsinogen in the acidic environment of the stomach. It breaks proteins into smaller polypeptides.

Correct answer: Pancreatic lipase

Pancreatic lipase hydrolyzes triglycerides into fatty acids and monoglycerides. It is the most important enzyme for fat digestion.

Correct answer: Bile salts

Bile salts emulsify fats, increasing surface area for lipase action. This is crucial for efficient fat digestion.

Correct answer: Pancreatic amylase

Pancreatic amylase continues starch digestion in the small intestine. It produces maltose and dextrins.

Correct answer: Lactase

Lactase hydrolyzes lactose into glucose and galactose. Deficiency of this enzyme leads to lactose intolerance.

Correct answer: Enterokinase

Enterokinase converts trypsinogen into active trypsin. Trypsin then activates other pancreatic proteases.

Correct answer: Dipeptides and tripeptides

Dipeptides and tripeptides are absorbed efficiently via peptide transporters. They are further hydrolyzed into amino acids inside cells.

Correct answer: Nuclease

Pancreatic nucleases digest DNA and RNA into nucleotides. These are further processed before absorption.

Correct answer: Cellulose

Humans lack the enzyme cellulase. Therefore cellulose cannot be digested and acts as dietary fiber.

Correct answer: Chymotrypsin

Chymotrypsin preferentially cleaves peptide bonds adjacent to aromatic amino acids. It plays a major role in protein digestion.

Correct answer: Maltase

Maltase is a brush border enzyme. It hydrolyzes maltose into two glucose molecules.

Correct answer: Bicarbonate

Bicarbonate neutralizes gastric acid in the duodenum. This protects the mucosa and provides optimal pH for enzymes.

Correct answer: Aminopeptidase

Aminopeptidases are brush border enzymes. They cleave terminal amino acids from peptides.

Correct answer: Chylomicrons

Long-chain fatty acids are re-esterified and packaged into chylomicrons. These enter the lymphatic circulation.

Correct answer: Lactase

Lactase deficiency prevents proper digestion of lactose. This leads to osmotic diarrhea and gas formation.

Correct answer: Cholecystokinin

Cholecystokinin stimulates pancreatic enzyme secretion. It is released in response to fats and proteins.

Correct answer: Pepsin

Pepsin is active at low pH. This allows effective protein digestion in the stomach.

Correct answer: Galactose

Galactose is absorbed via sodium-dependent co-transport. This mechanism also enhances water absorption.

Correct answer: Isomaltase

Isomaltase digests isomaltose and alpha-limit dextrins. It is essential for complete starch digestion.

Correct answer: The enzyme is denatured by the low pH of gastric juice

Salivary amylase has an optimal pH of approximately 6.7 to 7.0. Once the bolus mixes with highly acidic gastric juice (pH 1.0 to 3.0), the protein structure of the enzyme is denatured, rendering it inactive.

Correct answer: Alpha-1,6-glycosidic bonds

Pancreatic amylase is an endosaccharidase that only targets internal alpha-1,4 bonds. The alpha-1,6 bonds at the branch points of amylopectin remain intact, resulting in the formation of 'limit dextrins' that must be further digested by brush border enzymes.

Correct answer: Alpha-dextrinase (Isomaltase)

Alpha-dextrinase, also known as isomaltase, is a brush border enzyme specifically designed to cleave the alpha-1,6-glycosidic bonds that amylase cannot touch. This allows for the complete conversion of branched starch into absorbable monosaccharides.

Correct answer: Spontaneous acid-catalyzed cleavage at pH below 5

In the presence of hydrochloric acid (pH < 5), pepsinogen undergoes a conformational change that allows it to cleave itself (autocatalysis) to form active pepsin. Once some pepsin is formed, it further accelerates the activation of more pepsinogen.

Correct answer: Enteropeptidase (enterokinase)

Enteropeptidase is a brush border enzyme that cleaves a hexapeptide from the N-terminus of trypsinogen to create active trypsin. Active trypsin then goes on to activate all other pancreatic proteases, including more trypsinogen.

Correct answer: Carboxypeptidase B

Trypsin, chymotrypsin, and elastase are endopeptidases that break internal peptide bonds. Carboxypeptidases A and B are exopeptidases that specifically remove amino acids one by one from the carboxyl (C-terminal) end of the peptide.

Correct answer: By secreting colipase, which anchors lipase to the droplet

Colipase is a small protein secreted by the pancreas in an inactive form (procolipase) and activated by trypsin. It binds to both the bile-salt-coated lipid droplet and pancreatic lipase, acting as a bridge that allows lipase to access the underlying triglycerides.

Correct answer: Two free fatty acids and one 2-monoglyceride

Pancreatic lipase specifically targets the 1 and 3 positions of the triglyceride molecule. This leaves the middle fatty acid attached to the glycerol backbone, resulting in two free fatty acids and a 2-monoglyceride.

Correct answer: Lingual lipase

Lingual and gastric lipases (acid lipases) are particularly important in neonates because their pancreatic function and bile salt pools are not yet fully developed. These enzymes can digest up to 50% of dietary fat in infants, primarily targeting the medium-chain fatty acids found in breast milk.

Correct answer: Sucrase

Sucrase breaks down sucrose (table sugar). While other enzymes like maltase and lactase handle different sugars, sucrase is the specific hydrolase required to yield glucose and fructose.

Correct answer: Dipeptides and tripeptides are hydrolyzed inside the enterocyte

While many amino acids are absorbed as singles, a significant portion of protein is absorbed as dipeptides and tripeptides. Once inside the enterocyte, cytosolic peptidases complete the digestion into free amino acids before they enter the blood.

Correct answer: Proteins primarily, but also fats

Without enterokinase, trypsinogen cannot be activated. Since trypsin is required to activate other proteases and procolipase (needed for fat digestion), the patient would fail to digest both proteins and fats effectively.

Correct answer: It cleaves the fatty acid from the second position of a phospholipid

Phospholipase A2 is a pancreatic enzyme activated by trypsin. It targets phospholipids such as lecithin, removing a fatty acid to produce lysophospholipids, which are important components of micelles.

Correct answer: To carry long-chain fatty acids across the unstirred water layer to the brush border

The end-products of fat digestion are poorly soluble in water. Micelles are small aggregates of bile salts and lipids that shield the hydrophobic fatty acids, allowing them to move through the aqueous environment of the intestinal lumen to reach the enterocytes for absorption.

Correct answer: Cellulose

Cellulose contains beta-1,4-glycosidic bonds. Humans lack the enzyme cellulase required to break these bonds, so cellulose passes through the small intestine undigested as dietary fiber.

Correct answer: It is an endopeptidase that digests elastin and other proteins

Elastase is a pancreatic endopeptidase secreted as proelastase and activated by trypsin. It is unique in its ability to digest elastin fibers found in meat.

Correct answer: Pancreatic enzymes are denatured or inactive at the acidic pH of gastric chyme

Pancreatic enzymes (lipase, amylase, proteases) have an optimal pH near neutral (7.0-8.0). Bicarbonate neutralizes the acidic chyme entering from the stomach, creating the necessary environment for these enzymes to function.

Correct answer: The presence of Pancreatic Secretory Trypsin Inhibitor (PSTI)

The pancreas produces PSTI (also called SPINK1), which binds to and inactivates any trypsin that is accidentally activated within the gland. This prevents a premature cascade of protease activation that would lead to pancreatitis.

Correct answer: Trehalase

Trehalase is a brush border enzyme that specifically hydrolyzes trehalose. Deficiency in this enzyme can lead to GI symptoms after eating mushrooms, similar to lactose intolerance.

Correct answer: It covalently binds to and inhibits pancreatic lipase, blocking fat absorption

Orlistat is a pharmacological inhibitor of gastric and pancreatic lipases. By preventing the breakdown of triglycerides into absorbable fatty acids, it reduces the total caloric intake from dietary fats, often used in weight management.