Proteins: Composition, Functions, and Structure

Proteins

Composition and Functions

• Proteins make up approximately 20% of the human body and are present in every single cell.
• They are necessary for proper immune system function, digestion, and hair and nail growth, and are involved in numerous other body functions.
• There are over 100,000 different proteins exist within the human body.

Definition and Structure

• Proteins are macromolecules composed of amino acids, which are their building blocks.
• Each amino acid consists of a central carbon atom connected to a side chain, hydrogen, nitrogen-containing amino group, and carboxylic acid.
• Proteins contain the elements carbon, hydrogen, oxygen, and nitrogen.

Sources

• Main sources of proteins include meats, chicken, eggs, breast milk, beans, ground nuts, lentils, fish, cheese, and milk.
• Animal foods contain more protein than plants and are usually better sources of body-building foods.
• Plant proteins can become more effective nutritionally when mixed with animal proteins.

Protein Digestion and Absorption

Mechanical Breakdown

• The first step in protein digestion involves chewing, which breaks down large pieces of proteins into smaller pieces.
• The stomach releases gastric juices containing hydrochloric acid (HCL) and the enzyme pepsin, which initiate the breakdown of the protein.

Chemical Breakdown

• Pepsin dismantles protein chains into smaller fragments.
• The stomach contractions churn the partially digested protein into a uniform mixture called chyme.
• Protein digestion in the stomach takes a longer time than carbohydrate digestion but a shorter time than fat digestion.

Absorption

• The chyme enters the small intestine, where the majority of protein digestion occurs.
• The pancreas secretes digestive juice containing enzymes that further break down protein fragments.
• Amino acids are transported from the intestinal lumen through the intestinal cells to the blood.

Transport and Excretion

• Amino acids are transported to the liver, where they are further catabolized and ammonia is released.
• The liver transforms ammonia into urea, which is then transported to the kidney and excreted in the urine.

Denaturation of Proteins

Definition

• Denaturation refers to the physical changes that take place in a protein exposed to abnormal conditions in the environment.

Causes

• Denaturation can be caused by heat, acid, high salt concentrations, alcohol, and mechanical agitation.

Effects

• Denaturation causes proteins to vibrate, destroying weak bonds and unravelling protein strands, which then stick together, forming a network.

Important Peptides

Glutathione

• Glutathione is a substance made from three amino acids: cysteine, glutamate, and glycine.
• It acts as an important antioxidant in the body, combating free radicals and helping to detoxify chemicals.

Vasopressin (Antidiuretic Hormone)

• Vasopressin is a peptide hormone secreted in the hypothalamus, responsible for regulating the amount of water present in extracellular fluid.
• It causes the kidneys to absorb water and is inhibited by alcohol consumption, leading to increased urination.