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3.4 Proteins

Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. Each cell in a living system may contain thousands of different proteins, each with a unique function. Their structures, like their functions, vary greatly. However, they are all polymers of amino acids arranged in a linear sequence and connected by covalent bonds.

Amino acids are the monomers that make up proteins (Figure 3.4.1). Each amino acid has the same fundamental structure, which consists of a central carbon atom bonded to an amino group (NH2), a carboxyl group (COOH), and a hydrogen atom. Every amino acid also has another atom or group of atoms bonded to the central atom, known as the side chain or R group.  There are 20 common amino acids commonly found in proteins, each with a different R group that determines its chemical nature (whether it is acidic, basic, polar, or nonpolar).

Amino acid structure
Figure 3.4.1 General structure of amino acids. Image by OpenStax, CC-BY 4.0

The sequence and number of amino acids ultimately determine a protein’s shape, size, and function. Each amino acid is attached to another amino acid by a covalent bond, a peptide bond formed by a dehydration reaction. The carboxyl group of one amino acid and the amino group of a second amino acid combine, releasing a water molecule. The resulting bond is the peptide bond.

Molecule depiction of peptide bond formation
Figure 3.4.2 Peptide bond formation is a dehydration synthesis reaction. The carboxyl group of one amino acid is linked to the amino group of the incoming amino acid. In the process, a molecule of water is released. Image by OpenStax, CC-BY 4.0

The products formed by such a linkage are called polypeptides. While the terms polypeptide and protein are sometimes used interchangeably, a polypeptide is technically a polymer of amino acids. In contrast, the term protein is used for a polypeptide that has folded into a distinct shape and has a specific function.

Protein Function

The functions of proteins can be very diverse because the function depends on the protein’s shape. The order of the amino acids determines the shape of a protein. Proteins are often hundreds of amino acids long and can have very complex shapes because there are so many possible orders for the 20 amino acids.

Type Examples Functions
Enzymes Digestive enzymes (amylase, lipase) Help catalyze chemical reactions
Transport Hemoglobin Carry substances throughout the body
Structural Hair (keratin), ligaments (collagen) Construct and provide support for different structures
Hormones Insulin, adrenaline Coordinate the activity of different body systems
Defence Antibodies, immunoglobulins Protect the body from foreign pathogens
Contractile Actin, myosin Allow muscle contraction
Storage Legume storage proteins, egg white (albumin) Provide nourishment in the early development of the embryo and the seedling

Protein Structure

The shape of a protein is critical to its function. To understand how the protein gets its final shape or conformation, we need to understand the four levels of protein structure: primary, secondary, tertiary, and quaternary.

 

Primary
Secondary
Tertiary
Quaternary
The four structures of protein: Primary, secondary, tertiary and quaternary.
Figure 3.4.3 The four levels of protein structure. Image by OpenStax, CC BY 4.0 licence. A modification of work by the National Human Genome Research Institute, Public Domain.
Fried egg.
Figure 3.4.4 Albumin in the white denatures and then reconnects in an abnormal fashion. Image by Matthew Murdock, Public Domain

Each protein has a unique sequence and shape held together by chemical interactions. When exposed to changes in temperature, pH, or chemicals, the protein may lose its shape, a process known as denaturation. Denaturation is often reversible because the primary structure remains intact, so the protein can regain its function once the denaturing agent is removed. Sometimes denaturation is irreversible, resulting in a loss of function. An example of protein denaturation is seen when an egg is fried or boiled; the albumin protein in the liquid egg white denatures in the heat, changing from clear to opaque white.

 


Proteins” from Principles of Biology by Catherine Creech is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

2.3 Biological Molecules” from Biology and the Citizen by Colleen Jones is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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Biology Essentials 1 Copyright © 2025 by Kari Moreland is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.