Protein breakdown
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 broken down into shorter fragments due to the cleavage of some peptide bonds. The reaction catalysts are:
- 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 breakdown is polypeptides.
Further breakdown (deep hydrolysis) The resulting polypeptides are exposed to proteases — a general group of enzymes that carry out the hydrolysis of peptide bonds. This results in low-molecular-weight peptides:
- Oligopeptides — short chains composed 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 rate of fat absorption directly depends on their preliminary emulsification and subsequent hydrolysis.
Emulsification Bile is the main emulsifying agent. It breaks down large lipid droplets to a size of 1–2 µm. This repeatedly increases the surface area of the phase boundary ("fat/water"), which is extremely important for facilitating access to hydrolytic enzymes — lipases.
Stages and localization of hydrolysis The sources of lipases in the body are the oral cavity, stomach, and pancreas. The breakdown process is distributed across the segments of the gastrointestinal tract:
- Stomach: from 10% to 30% of ingested dietary fat is hydrolyzed here.
- Duodenum and initial segments of the small intestine: this is where the bulk of the work takes place — 70–90% of lipids are hydrolyzed.
Biochemistry of the process Breakdown occurs in stages. Pancreatic lipase acts on triglycerides (TGs), converting them into diglycerides (DGs), and then into monoglycerides (MGs) and fatty acids (FAs). Intestinal lipase ultimately completes the hydrolysis process. The resulting products are highly soluble in bile acid salt solutions.
Formation of transport forms
For lipid hydrolysis products to be assimilated, special transport structures are formed from them.
Micelles These are spherical complexes with a diameter of about 20–100 nm.
- Composition: monoglycerides, fatty acids, bile acid salts, phospholipids, and cholesterol.
- Structure: organized in such a way that polar hydrophilic components (including bile acids and monoglycerides) are located on the outside. This shell ensures contact of the lipid core with the aqueous environment of the intestine.
Further transport Following the formation of micelles, chylomicrons are formed within enterocytes. At the final stage, free fatty acids (FFAs) and chylomicrons leave the enterocytes and enter the blood and lymph.
Briefly: answers to topical questions
Which enzymes ensure the final breakdown of short peptides into free amino acids?
The final breakdown of peptides into free amino acids is provided by exopeptidases and small intestine peptidases.
These include:
- aminopeptidases — cleave the N-terminal amino acid;
- carboxypeptidases — cleave the C-terminal amino acid; carboxypeptidase A cleaves hydrophobic amino acids, carboxypeptidase B — lysine and arginine;
- dipeptidases — hydrolyze dipeptides into two free amino acids;
- tripeptidases — break down tripeptides into individual amino acids.
Aminopeptidases operate on the surface of the small intestinal epithelium; di- and tripeptidases are located within the intestinal epithelium, i.e., in enterocytes.
Which lipases, other than pancreatic and intestinal, participate in fat digestion?
In addition to pancreatic and intestinal lipases, fat digestion involves lipases originating from the oral cavity and stomach.
Gastric lipase is part of gastric juice and breaks down emulsified fats into glycerols and fatty acids. 10–30% of dietary fat is hydrolyzed in the stomach.
Which bile acids are part of micelles?
Micelles contain bile acid salts.
Micelles also contain:
- monoglycerides;
- fatty acids;
- phospholipids;
- cholesterol.
Polar hydrophilic components, including bile acids and monoglycerides, are located on the outside of micelles; this ensures contact with the aqueous environment of the intestine.
Class and exam questions
What does the rate of fat absorption depend on?
The rate of absorption directly depends on the quality of lipid emulsification by bile and the activity of subsequent enzymatic hydrolysis.
Where does the main lipid digestion take place?
The majority of fats (70–90%) are hydrolyzed in the duodenum and the initial segments of hallucinatory small intestine. Only 10–30% are broken down in the stomach.
What is the difference between initial and deep protein hydrolysis?
At the initial stage, large folded amino acid chains are broken down by pepsins and carboxypeptidases into polypeptides. During deep hydrolysis, proteases break down these polypeptides into di-, tri-, and oligopeptides.
What is the function of micelles?
Micelles act as transport forms. Due to their external polar hydrophilic components, they can exist in the aqueous environment of the intestine and deliver fatty acids and monoglycerides to enterocytes.
What else to study on this topic
- Mechanism of action of pepsins and carboxypeptidases
- Specifics of pancreatic and intestinal lipase activity
- The process of chylomicron formation inside enterocytes
- Transport of free fatty acids and chylomicrons into the blood and lymph
- Effect of bile acid salts on the solubility of monoglycerides
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