Human Nutrition » Proteases and Protein Digestion

What you'll learn this session

Study time: 30 minutes

The structure and function of proteins in the human body
How proteins are broken down during digestion
The role of proteases in protein digestion
Where proteases are produced and how they work
The products of protein digestion and how they're absorbed
How protein digestion is regulated in the digestive system

Introduction to Protein Digestion

Proteins are essential macromolecules that our bodies need for growth, repair and many vital functions. However, the proteins we eat in our food are too large to be absorbed directly into our bloodstream. They must first be broken down into smaller molecules through the process of digestion. This is where proteases come in – these special enzymes are responsible for breaking down proteins into forms our bodies can use.

Key Definitions:

Proteins: Large, complex molecules made up of amino acids that perform various essential functions in the body.
Proteases: Enzymes that break down proteins by catalysing the hydrolysis of peptide bonds.
Digestion: The process of breaking down food into smaller molecules that can be absorbed into the bloodstream.
Amino acids: The building blocks of proteins; there are 20 different amino acids that can combine in various ways to form proteins.

🍖 Protein Structure

Proteins are made up of chains of amino acids linked by peptide bonds. Each protein has a unique sequence of amino acids that determines its structure and function. The human body needs proteins for:

Building and repairing tissues
Making enzymes and hormones
Supporting immune function
Providing structure to cells
Transporting substances around the body

🔬 Why We Need to Digest Proteins

The proteins in our food can't be used directly by our bodies because:

They're too large to pass through the gut wall into the bloodstream
They have different structures from the proteins our bodies need
Our bodies need to break them down into amino acids to build our own specific proteins
Some food proteins could trigger immune responses if absorbed whole

Proteases: The Protein-Digesting Enzymes

Proteases are a group of enzymes specifically designed to break down proteins. They work by breaking the peptide bonds that hold amino acids together in protein chains. Different proteases work in different parts of the digestive system, each adapted to the specific conditions of their environment.

👅 Mouth

No protein digestion occurs here. The mouth is primarily for mechanical breakdown of food and the start of carbohydrate digestion.

🥞 Stomach

Protease: Pepsin

Produced as inactive pepsinogen by chief cells. Activated by stomach acid (HCl). Works best in acidic conditions (pH 1.5-3.5). Begins breaking proteins into smaller peptides.

🥔 Small Intestine

Proteases: Trypsin, Chymotrypsin, Carboxypeptidase

Produced by the pancreas and released into the small intestine. Work best in alkaline conditions (pH 7-8). Break down proteins and peptides into smaller peptides and amino acids.

The Process of Protein Digestion

Step 1: Stomach Digestion

Protein digestion begins in earnest in the stomach. Here's what happens:

Hydrochloric acid (HCl) in the stomach creates an acidic environment (pH 1.5-3.5)
HCl denatures proteins by breaking their tertiary structure, unfolding them to expose peptide bonds
HCl activates pepsinogen by converting it to pepsin
Pepsin breaks some peptide bonds, creating smaller polypeptides
About 10-20% of protein digestion occurs in the stomach

Interesting Fact: Protecting the Stomach

The stomach produces about 2 litres of gastric juice daily, containing strong acid and pepsin that could digest the stomach itself! The stomach is protected by a thick layer of mucus and by cells that produce bicarbonate to neutralise acid near the stomach lining. When these protective mechanisms fail, stomach ulcers can form.

Step 2: Small Intestine Digestion

Most protein digestion (about 80-90%) occurs in the small intestine:

🥑 Pancreatic Proteases

The pancreas produces several proteases that are secreted as inactive forms (zymogens) to prevent self-digestion:

Trypsinogen → activated to trypsin by enterokinase (an enzyme in the small intestine)
Chymotrypsinogen → activated to chymotrypsin by trypsin
Procarboxypeptidase → activated to carboxypeptidase by trypsin

These enzymes work together to break down proteins and polypeptides into smaller peptides and some amino acids.

🧠 Intestinal Proteases

The small intestine itself produces enzymes that complete protein digestion:

Aminopeptidases: Remove amino acids from the N-terminus (one end) of peptide chains
Dipeptidases: Break dipeptides (two amino acids joined together) into individual amino acids

These enzymes are found on the microvilli of the small intestine's epithelial cells, where they complete the final stages of protein digestion.

Products of Protein Digestion and Absorption

The end products of protein digestion are:

Individual amino acids (about 80% of absorption)
Dipeptides and tripeptides (about 20% of absorption)

These small molecules are absorbed through the epithelial cells lining the small intestine by several mechanisms:

Active transport: Amino acids are transported across the intestinal epithelium using specific carrier proteins that require energy (ATP)
Co-transport: Some amino acids are absorbed along with sodium ions
Facilitated diffusion: Some di- and tripeptides are absorbed using carrier proteins that don't require energy

Once absorbed, these products enter the bloodstream via the hepatic portal vein and are transported to the liver, which regulates their distribution to other body tissues.

Case Study Focus: Protein Digestion Disorders

Phenylketonuria (PKU) is a genetic disorder where the body can't properly process the amino acid phenylalanine. People with PKU must follow a special diet low in phenylalanine to prevent it from building up in the body and causing brain damage. This highlights the importance of proper protein digestion and metabolism. In the UK, all newborn babies are screened for PKU with a simple blood test, allowing early dietary intervention if needed.

Regulation of Protein Digestion

The digestive system carefully controls protein digestion through several mechanisms:

🔗 Hormonal Control

Several hormones regulate protein digestion:

Gastrin: Released when protein enters the stomach; stimulates HCl and pepsinogen secretion
Cholecystokinin (CCK): Released when partially digested proteins enter the small intestine; stimulates pancreatic enzyme secretion
Secretin: Released in response to acid in the small intestine; stimulates pancreatic bicarbonate secretion to neutralise stomach acid

🛡 Protective Mechanisms

The body has several ways to protect itself from its own digestive enzymes:

Proteases are produced as inactive zymogens (e.g., pepsinogen, trypsinogen)
Pancreatic juice contains protease inhibitors that prevent premature activation
The pancreas has specialised compartments to store enzymes safely
Mucus protects the digestive tract lining from enzyme damage

Summary: The Protein Digestion Journey

Let's recap the journey of a protein from food to amino acids:

Mouth: Mechanical breakdown through chewing, but no chemical digestion of proteins
Stomach: Proteins are denatured by acid and partially digested by pepsin into polypeptides
Small intestine: Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) break polypeptides into smaller peptides
Intestinal brush border: Aminopeptidases and dipeptidases complete digestion to amino acids and small peptides
Absorption: Amino acids and small peptides are absorbed into the bloodstream
Transport: Products travel to the liver via the hepatic portal vein
Utilisation: The body uses these building blocks to create its own proteins

Exam Tip 💡

When answering questions about protein digestion, remember to include:

The names of specific enzymes (pepsin, trypsin, etc.)
Where each enzyme works (stomach, small intestine)
The optimal pH for each enzyme
How enzymes are activated (e.g., pepsinogen → pepsin)
The end products (amino acids, dipeptides, tripeptides)

🧠 Test Your Knowledge!