Protein Digestion
The hydrolysis of protein molecules occurs in two main stages, during which long, folded amino acid chains are converted into small fragments suitable for assimilation.
Initial Stage of Hydrolysis At this stage, the initial substrate (a large protein molecule) is split into shorter fragments by the cleavage of a portion of peptide bonds. The reaction catalysts include:
- Pepsins — the main proteolytic enzymes of the stomach.
- Carboxypeptidases — exopeptidases that cleave amino acids from the C-terminus of the protein chain.
The result of the initial cleavage is the formation of polypeptides.
Further Breakdown (Deep Hydrolysis) These polypeptides are exposed to proteases — a broad group of enzymes that catalyze the hydrolysis of peptide bonds. This yields low-molecular-weight peptides:
- Oligopeptides — short chains consisting of a small number of amino acids.
- Tripeptides — molecules consisting of exactly 3 amino acid residues.
- Dipeptides — molecules consisting of 2 amino acids.
Lipid Preparation and Hydrolysis
The absorption rate of fats depends directly on their preliminary emulsification and subsequent hydrolysis.
Emulsification The main emulsifying agent is bile. It breaks large lipid droplets down to a size of 1–2 µm. This repeatedly increases the interfacial surface area ("fat/water"), which is critical for facilitating access for hydrolytic enzymes—lipases.
Stages and Localization of Hydrolysis The sources of lipases in the body include the oral cavity, stomach, and pancreas. The breakdown process is distributed across segments of the gastrointestinal tract:
- Stomach: 10% to 30% of ingested dietary fat is hydrolyzed here.
- Duodenum and proximal small intestine: The bulk of the work occurs here, hydrolyzing 70–90% of lipids.
Biochemistry of the Process Digestion proceeds stepwise. Pancreatic lipase acts on triglycerides (TGs), converting them into diglycerides (DGs), and then into monoglycerides (MGs) and fatty acids (FAs). The hydrolysis process is finalized by intestinal lipase. The resulting products are readily soluble in solutions of bile acid salts.
Formation of Transport Forms
For lipid hydrolysis products to be absorbed, specialized transport structures are formed.
Micelles These are spherical complexes approximately 20–100 nm in diameter.
- Composition: Monoglycerides, fatty acids, bile acid salts, phospholipids, and cholesterol.
- Structure: Organized such that polar hydrophilic components (including bile acids and monoglycerides) are located on the exterior. This outer shell ensures contact between the lipid core and the aqueous intestinal environment.
Further Transport Following micelle formation, chylomicrons are assembled within enterocytes. At the final stage, free fatty acids (FFAs) and chylomicrons leave the enterocytes and enter the blood and lymph.