Questions: Protein Digestion and Peptide Absorption

Question 1

A patient has Hartnup disorder, a genetic defect in the intestinal neutral amino acid transporter. Why doesn't this result in severe deficiency of all neutral amino acids?

• A. Neutral amino acids are synthesized de novo in the enterocyte from other substrates
• B. Gastric pepsin delivers neutral amino acids directly into the bloodstream, bypassing intestinal transport
• C. Neutral amino acids contained in dipeptides and tripeptides can still be absorbed intact via PepT1, partially compensating for the defective transporter
• D. The basic amino acid transporter has overlapping specificity and absorbs neutral amino acids as a backup

Answer: (C)

PepT1 transports di- and tripeptides intact regardless of their amino acid composition. Even though the specific neutral amino acid carrier is defective, the same amino acids can arrive at the enterocyte as small peptides and be absorbed via PepT1, then hydrolyzed intracellularly. This is the clinical demonstration of why the redundancy between free amino acid transporters and PepT1 matters — a defect in one pathway is partially buffered by the other. It also illustrates that absorption as peptides, not just as free amino acids, is physiologically significant.

Question 2

You consume two supplements providing identical amounts of the same amino acids: one as free-form amino acids, one as equivalent dipeptides and tripeptides. Which is typically absorbed faster, and why?

• A. The free amino acid supplement, because no further hydrolysis is needed before transport
• B. The peptide supplement, because PepT1 operates as a single high-capacity transporter handling all peptide sequences, whereas free amino acids must compete across multiple separate class-specific carrier systems
• C. They are absorbed at identical rates, since absorption depends only on the total nitrogen content
• D. The free amino acid supplement, because the intestinal lumen cannot transport intact peptide bonds

Answer: (B)

PepT1 is a broad-specificity, proton-coupled transporter with high capacity that absorbs di- and tripeptides regardless of their sequence. Free amino acids, by contrast, must use multiple separate, class-specific sodium-coupled transporters (neutral, basic, acidic, imino), which compete for capacity and can become saturated. This is why peptide-based supplements often produce faster post-absorptive amino acid rises than equivalent free amino acid mixtures — the transport kinetics favor intact peptides.

Question 3

Protein digestion is essentially complete after leaving the stomach; the small intestine's primary role is absorption, not further breakdown.

• T. True
• F. False

Answer: False

The stomach performs only preliminary protein digestion via pepsin, which cleaves at aromatic residues but leaves most of the protein as large peptide fragments. The majority of protein digestion occurs in the small intestine, driven by a battery of pancreatic proteases — trypsin, chymotrypsin, elastase, and carboxypeptidases — along with brush-border peptidases. The stomach is the 'preliminary chopper'; the small intestine is where proteolysis is essentially completed. Students who assume the stomach finishes digestion will misunderstand why pancreatic enzyme deficiency (e.g., in chronic pancreatitis) causes such profound protein malabsorption.

Question 4

Cooking plant foods improves protein digestibility both by denaturing the protein structure and by inactivating antinutritional factors such as trypsin inhibitors.

• T. True
• F. False

Answer: True

Both mechanisms are real and additive. Heat denatures proteins — unfolding their compact tertiary structure — which exposes peptide bonds to enzymatic attack, dramatically increasing proteolysis efficiency. Simultaneously, heat destroys antinutritional factors: trypsin inhibitors (present in legumes) that would otherwise inhibit pancreatic trypsin, phytates that impair mineral and protein absorption, and lectins that damage the intestinal epithelium. This explains why the DIAAS scores for cooked legumes significantly exceed those for raw legumes, even though the amino acid sequence is unchanged.

Question 5

Why are dipeptides and tripeptides sometimes absorbed more efficiently than an equivalent amount of free amino acids, and what transporter is responsible for their uptake?

Model answer: Di- and tripeptides are absorbed via PepT1 (peptide transporter 1), a proton-coupled transporter on the intestinal brush border that accepts virtually any two- or three-amino acid peptide. PepT1 has higher transport capacity than the multiple separate amino acid carriers (which are divided by amino acid class and can become saturated), so small peptides often traverse the intestinal wall faster than free amino acids do. Inside the enterocyte, cytosolic peptidases complete hydrolysis before the amino acids enter the portal blood.

This is the key counterintuitive fact: the body has not optimized absorption around first fully breaking proteins down to free amino acids. Instead, a parallel and sometimes faster route exists for small peptides. This is why food scientists and sports nutritionists are interested in protein hydrolysates — partially pre-digested proteins that enter the bloodstream as small peptides via PepT1 rather than competing for the multiple amino acid transporters.