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The image covers Specificity of Pepsin — a classic biochemistry topic from gastric digestion. Here's a full breakdown:
Specificity of Pepsin
Pepsin is a gastric endopeptidase (protease) secreted as the zymogen pepsinogen by chief cells of the stomach. It is activated by the low pH of gastric acid (optimal activity ~pH 1.5–2.5). Its specificity refers to which peptide bonds it preferentially cleaves.
1. Bond Specificity (Primary Action)
Pepsin cleaves peptide bonds with a precise chemical preference:
- The –COOH (carboxyl) side of the bond must come from an aromatic amino acid: Phenylalanine (Phe), Tyrosine (Tyr), or Tryptophan (Trp).
- The –NH (amino) side of the bond must come from either:
- An acidic amino acid: Aspartate (Asp) or Glutamate (Glu), OR
- Another aromatic amino acid (Phe, Tyr, Trp).
In other words, pepsin cuts the bond after an aromatic residue, especially when the adjacent residue is acidic or also aromatic. This makes it an aromatic-residue-specific protease, unlike trypsin (cuts after Lys/Arg) or chymotrypsin (cuts after large hydrophobics).
Why aromatic residues? Pepsin's active site contains two aspartyl residues (Asp32 and Asp215) forming a hydrophobic binding pocket that accommodates the bulky aromatic side chains of Phe, Tyr, and Trp.
2. Broader Activity (Prolonged Exposure)
If proteins are exposed to pepsin for a longer duration, it can also hydrolyze peptide bonds formed by non-aromatic amino acids. The end result is the liberation of free amino acids — essentially complete protein digestion given enough time.
3. Inability (Proteins Pepsin Cannot Digest)
Pepsin cannot hydrolyze certain proteins:
| Protein | Reason for Resistance |
|---|---|
| Keratins | Heavily cross-linked by disulfide bonds; highly insoluble fibrous structure |
| Silk fibroin | Repetitive Gly-Ala-Gly-Ala sequence; few aromatic/acidic bonds to target |
| Mucoproteins | Glycosylated proteins; carbohydrate moieties sterically hinder access |
| Mucoids | Similar glycoprotein structure to mucoproteins |
| Protamines | Small, highly basic (Arg-rich) proteins lacking the aromatic/acidic bonds pepsin targets |
These proteins either lack the specific peptide bonds pepsin targets, or their structure is physically inaccessible to the enzyme.
4. Action on Milk
Pepsin acts on the phosphoprotein casein in milk through a two-step process:
- Casein → Paracasein + Proteose (whey protein)
- Pepsin cleaves a specific bond in casein, releasing the whey fraction (proteose).
- Paracasein + Ca²⁺ → Calcium paracaseinate (Ca-paracaseinate)
- Paracasein combines with calcium ions in milk to form an insoluble curd (Ca-paracaseinate), which is then further degraded to peptones.
This milk-curdling action of pepsin is similar to, but less potent than, that of rennin (chymosin) — the enzyme primarily responsible for milk curdling in infants.
Summary Table
| Feature | Details |
|---|---|
| Enzyme type | Aspartyl protease (endopeptidase) |
| Optimal pH | 1.5–2.5 |
| Primary cleavage | –COOH of Phe/Tyr/Trp → –NH of Asp/Glu or aromatic AA |
| Prolonged action | Liberates free amino acids |
| Cannot digest | Keratins, silk fibroin, mucoproteins, mucoids, protamines |
| Action on milk | Casein → Paracasein + Proteose → Ca-paracaseinate → Peptones |
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