Introduction

Proteins are the most diverse class of biological molecule.  They are composed of long chains of smaller subunits called ‘amino acid’ and proteins can be ‘folded’ into many different structural configurations, resulting in a vast array of possible functions provided to the cell.

Let’s take a closer look now…

Learning

If we consider that proteins are very long chains of amino acids joined together through a series of peptide bonds, it becomes apparent that the sequence of amino acids will differ between each and every type of protein.  It is estimated that there are over 100,000 different types of proteins found in the human body.

Primary Structure

The sequence of amino acids that occur in a protein, or polypeptide chain, is referred to as its primary structure.  This precise sequence of amino acids is determined by the organism’s genetic information, which we will cover in detail later in this course.

Secondary Structure

The different amino acids that make up the primary structure of the protein interact with one another to cause the polypeptide chain to form simple, three-dimensional structures.  It is these simple interactions that cause the protein to assume its ‘Secondary Structure’.  Examples of secondary structure include a simple helical formation or simple folded sheets.

Tertiary Structure

Further folding of the polypeptide chain can occur to form ‘Tertiary Structure’.  The Tertiary Structure of a protein can be influenced by the protein’s interaction with water, where the hydrophobic amino acids will orient themselves away from the water molecules and the hydrophilic amino acids will point outward to interact with water.  It is the complex tertiary structure of proteins which is essential for their overall function.  Heat is a key disruptor of protein tertiary structure and is the main reason protein-rich compounds, such as egg whites, will become altered when heat is applied (think of a boiled or fried egg compared to a raw egg…!)

Quaternary Structure

Finally, folded proteins which have assumed their tertiary structure can actually combine together to form larger protein molecules or enzymes.  When these multiple polypeptide chains are associated with one another, the protein assumes a ‘Quaternary Structure’ as shown below.  It is important to note that not all proteins have quaternary structure, but there are some very important ones that do. For example, the protein Hemoglobin consists of four individual polypeptide chains held together through a series of weaker bonds.  Each of the four polypeptides in the hemoglobin molecule is capable of binding to iron-containing molecules which carry oxygen throughout the body.

Protein Function

It is important to understand the relationship between protein structure and function.  As we just learned, it is the exact sequence and position of the amino acids occurring in a polypeptide chain, along with the chemical differences between the variable functional groups found in each amino acid, that determine the final three-dimensional structure of each and every protein molecule.  As it turns out, the precise three-dimensional structure of the protein molecule is primarily responsible for its overall function in the cell.  To reinforce this, let’s consider the wide range of functions carried out by proteins inside of cells…

Examples

The table outlines some of the important functions performed by proteins along with examples of each type.  Please note that these functions range from simple structural roles (such as those performed by keratin which strengthens our hair and skin) to complex enzymes which are used to carry out a variety of chemical reactions, including digestion, which involves the breakdown of larger macromolecules into their smaller subunits.

Summary

• Proteins are comprised of a string of amino acids- bonded through a series of peptide bonds (primary structure).
• Proteins can be folded up to achieve unique 3-dimensional shapes (secondary, tertiary structure) and even interact together to form larger molecules (quaternary structure).
• The amino acid sequence (primary structure) along with the environment which determines a protein’s overall 3-dimensional shape. and this overall shape then provides and determines the function of any given protein.
• Proteins play a variety of roles in biological systems, ranging from structural functions to complex movements and chemical reactions (e.g. performed by enzymes).

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Sources:

“Proteins.” By OpenStax Biology 2e. Retrieved from: https://openstax.org/books/biology-2e/pages/3-4-proteins/ Licensed under: CC-By: Attribution