What Are the Monomers of Proteins Called? 4 Key Facts

The monomers of proteins are called amino acids. These small molecules link together through peptide bonds to form polypeptide chains, which fold into functional proteins. There are 20 standard amino acids used in protein synthesis, each with a unique side chain that influences how the protein folds and functions.

Amino Acid Structure

Each amino acid has a central carbon atom bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable R-group. The R-group determines the identity and chemical behavior of the amino acid. For example, glycine has a hydrogen atom as its R-group, making it the smallest, while tryptophan has a large, ringed structure that makes it hydrophobic.

The properties of the R-group—such as polarity, charge, and size—affect how the amino acid interacts within a protein. Nonpolar R-groups tend to cluster in the protein’s interior, away from water, while polar or charged R-groups are often found on the surface, where they can interact with water or other molecules.

The 20 Standard Amino Acids

Of the hundreds of known amino acids, only 20 are genetically encoded and directly involved in ribosomal protein synthesis. These are known as the standard or canonical amino acids. They include alanine, serine, leucine, lysine, and others, each contributing specific chemical traits to the proteins they build.

Some amino acids are essential, meaning the human body cannot make them and they must come from food. There are nine essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The remaining 11 are non-essential, as the body can synthesize them from other compounds.

How Amino Acids Build Proteins

Amino acids connect via dehydration synthesis, forming a covalent bond between the carboxyl group of one amino acid and the amino group of the next. This bond is called a peptide bond, and the resulting chain is a polypeptide. The sequence of amino acids is determined by genetic instructions in DNA and translated by ribosomes Khan Academy.

As the chain grows, it begins to fold into secondary structures like alpha helices and beta sheets, stabilized by hydrogen bonds. Further folding creates tertiary and sometimes quaternary structures, forming the final functional protein. Chaperone proteins often assist in this process to prevent misfolding, which can lead to diseases such as Alzheimer’s or Parkinson’s Nature.

Example: Insulin

Insulin is a hormone that regulates blood glucose. It starts as a single polypeptide chain called preproinsulin. As it matures, the signal sequence is removed and the chain folds. Enzymes then cleave parts of the chain, leaving two separate chains (A and B) held together by disulfide bonds between cysteine amino acids. The final three-dimensional shape allows insulin to bind to its receptor and signal cells to absorb glucose. This shows how the sequence and chemistry of amino acids directly determine protein function.

Key Takeaways

• The monomers of proteins are amino acids, which link together to form polypeptide chains.
• There are 20 standard amino acids, each with a unique R-group that determines its chemical properties.
• Amino acids connect via peptide bonds in a sequence specified by DNA, and this sequence dictates how the protein folds and functions.
• Nine amino acids are essential for humans and must be obtained from the diet.
• Correct amino acid sequence and folding are critical for protein function—errors can result in disease.