Peptides vs Proteins
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What are the Differences?
Peptides and proteins, while similar in many regards, have several key differences that are important to understand. Oftentimes the terms "peptide" and "protein" are used synonymously, but differing characteristics and biological activities between the two compounds prevent the terms from being totally interchangeable. To fully appreciate the differences between proteins and peptides, it is important to understand amino acids, the building blocks of both, and how all three (amino acids, peptides, and proteins) relate to one another.
Peptides
Short chains of amino acids linked by peptide bonds. Generally refers to a compound made up of two or more amino acids (e.g., Dipeptide, Tripeptide). Further classified as Oligopeptides (few, typically <10) and Polypeptides (>10).
Proteins
Historically defined as polypeptides composed of more than 50 amino acids. Crucially, proteins fold into stable, fixed three-dimensional structures required for specific biological functions (e.g., hemoglobin).
Amino Acids: The Building Blocks
Amino acids are small but biologically vital compounds containing an amino group (NH2) and a carboxylic acid group (COOH) as well as a side-chain structure that varies between different amino acids. While hundreds of amino acids are known, only twenty are genetically combined into peptides (such as arginine, lysine, and glutamine), while others can be combined synthetically.
Importantly, amino acids make up the building blocks of peptides. When amine and carboxylic acid functional groups in amino acids join to form amide bonds, a peptide is formed.
Key Distinctions: Size and Structure
Scientists commonly differentiate between proteins and polypeptides based on two main factors:
- Size: A polypeptide composed of more than 50 amino acids is generally classified as a protein, though the threshold can range from 40-100 amino acids.
- Structure: Typically, polypeptides shorter than about 40-50 amino acids in length do not fold into a fixed structure. Proteins, however, form stable 3D structures.
Which Term to Use?
Importantly, all proteins are technically polypeptides. However, as a researcher, it can sometimes be useful to differentiate between the two and reserve the term "proteins" to refer to relatively long and structurally fixed amino acid chains. Accordingly, peptides will generally refer to shorter (sub-50) amino acid chains.