Biochemistry, Primary Protein Structure

Fundamentals

There are 20 standard amino acids commonly found in proteins, each distinguished by a unique chemical structure and side chain, known as the R group, which determines its properties and functional role within proteins. The term protein derives from the Greek word proteios, meaning 'primary' or 'of first importance,' reflecting the essential role proteins play in the structure and function of all living organisms. First introduced in 1838 by Dutch chemist Gerardus Johannes Mulder and later popularized by Swedish chemist Jöns Jacob Berzelius, the term underscores the central role of proteins as the fundamental building blocks of life—critical to nearly every biological process, from catalyzing metabolic reactions to providing cellular structure and enabling intercellular communication.

The sequence of amino acids in a protein is called its primary structure. Longer amino acid chains are typically called polypeptides and are more commonly called proteins once they reach sufficient size and structural complexity. In contrast, shorter chains, usually fewer than 40 amino acids, are classified as peptides or oligopeptides. This linear sequence dictates how the protein folds into its 3-dimensional shape, ultimately determining its biological properties and function.

Polypeptides are synthesized through a condensation reaction in which the carboxyl group of one amino acid reacts with the amino group of another, releasing a molecule of water. The resulting amide linkage is a peptide bond, and the linked amino acids are residues. By convention, polypeptide chains are represented with the N-terminus (the free amino group) on the left and the C-terminus (the free carboxyl group) on the right.

Molecular Level

Except for glycine, each amino acid contains a central carbon atom (the α-carbon) bonded to a carboxyl group (–COOH), an amino group (–NH₂), a hydrogen atom, and a variable side chain, known as the R group. The chemical nature of the R group determines the properties of the amino acid and, by extension, the behavior of the protein it forms. Most amino acids exist as zwitterions at physiological pH (~7.4). In this state, the amino group (–NH2) gains a proton to become –NH3 (positively charged), while the carboxyl group (–COOH) loses a proton to become –COO (negatively charged). A zwitterion is a neutral molecule with a positive and a negative charge, with an overall net charge of 0.

Side chains can be acidic, basic, polar (uncharged), or nonpolar; these distinctions influence how a protein behaves in different environments. For instance, they affect whether the protein functions optimally in acidic or basic conditions, its solubility in water or lipids, stability at varying temperatures, and the placement of amino acids within the folded protein—hydrophobic residues tend to be buried in the protein core. In contrast, hydrophilic residues are typically exposed to the aqueous environment.

Certain amino acids also participate in key stabilizing interactions, such as ionic bonds (formed between charged side chains) and disulfide bridges (covalent bonds formed between cysteine residues). These interactions are critical for maintaining the protein’s 3-dimensional structure. Ultimately, the position and properties of specific amino acids in the primary structure guide the folding and organization of the protein’s secondary, tertiary, and quaternary structures, directly influencing its function and stability. 

Nonpolar Amino Acids

Aliphatic amino acids are the backbone molecules that form hydrogen bonds. They include:

• Glycine: Can cause a bend when used in an alpha helix chain (secondary structure) [1]
• Alanine
• Valine
• Leucine
• Isoleucine
• Methionine

Aromatic amino acids are the backbone molecules that form hydrogen bonds. They include:

• Phenylalanine
• Tryptophan
• Tyrosine

Polar Amino Acids

Uncharged amino acids are the backbone/side chain molecules that can be used to form hydrogen bonds (besides proline and cysteine). They include:

• Serine
• Threonine
• Asparagine
• Glutamine
• Proline: Causes a bend when used in an alpha helix chain (secondary structure) [2]
• Cysteine: The sulfur atoms from 2 cysteine side chains covalently bond together to form a disulfide bridge 

Acidic amino acids can form hydrogen bonds (backbone/side chain molecules) and salt bridges (side chain molecules only). They include:

• Aspartic acid/aspartate
• Glutamic acid/glutamate

Basic amino acids can form hydrogen bonds (backbone/side chain molecules) and salt bridges (side chain molecules only). They include:

• Lysine
• Arginine
• Histidine

Testing

Several methods are available for qualitative and quantitative analysis of polypeptide chains, with Edman degradation and mass spectrometry being 2 of the most widely used.

Edman Degradation

This method is used to find a protein's sequence of amino acids.[3] The amino-terminal residue is labeled and cleaved from the polypeptide chain without disturbing the peptide bonds that hold other amino acid residues together. This process repeats by cleaving off 1 amino acid until the entire chain is sequenced. Due to the tedious nature of this method, a protein sequenator can be used to perform the Edman degradation in an automated way.

Mass Spectrometry

Peptide amino acid sequences can also be determined using mass spectrometry.[4] While this method provides limited information when applied to intact proteins, it is highly effective for peptide analysis. A common approach, peptide mass fingerprinting, is widely employed to identify single purified proteins, though it is unsuitable for heterogeneous protein mixtures.[5] A generalized procedure of this method is as follows: 

1. Break up the protein sample into smaller peptide fragments using proteolytic enzymes. 
2. Extract the fragments using acetonitrile.
3. Dry the fragments in a vacuum. 
4. Insert the peptides into the vacuum chamber of a mass spectrometer, such as matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) or electrospray ionization/time-of-flight (ESI-TOF). MALDI is an ionization technique that requires a laser-absorbing matrix to create ions from organic molecules with low amounts of fragmentation. ESI produces ions by applying a high voltage to a liquid to form an aerosol. TOF is a measurement of time used to measure the velocity of the ions through the vacuum chamber.
5. The mass spectrometer creates a list of molecular weights, called the peak list, from the sample. This list is then compared against databases (eg, GenBank or SwissProt) for relevant matches. 
6. Appropriate software performs a simulated chemical cleavage reaction with the relevant protein sequences found in a database. The mass of these simulated peptide fragments is calculated and then compared to the peak list of the experimental peptide masses. These results are statistically analyzed, and possible matches are shown.

Pathophysiology

Many diseases result from abnormalities in the amino acid composition or sequence of proteins. These mutations can alter protein folding, function, and stability, leading to various pathological outcomes. Below are 3 well-known examples:

Huntington Disease

Huntington disease is caused by a CAG trinucleotide repeat expansion in the HTT gene on chromosome 4. Normally, the CAG codon (which encodes glutamine) is repeated 10 to 35 times [6], but in affected individuals, it is repeated 36 or more times. The longer the repeat, the earlier the onset, and the more severe the symptoms. The abnormal protein causes degeneration of the caudate nucleus and putamen in the basal ganglia, leading to motor dysfunction (chorea), psychiatric symptoms, and progressive dementia.[7]

Sickle Cell Anemia

Sickle cell anemia is caused by a point mutation in the beta-globin gene, where a single base substitution changes the sixth codon from GAG (glutamic acid) to GTG (valine). This single amino acid substitution significantly alters the structure of hemoglobin.[8] The mutated hemoglobin causes red blood cells to deform into sickle shapes, leading to reduced oxygen transport, blockages in blood vessels, and symptoms such as anemia, pain episodes, organ damage, and growth delays.[9][10]

Cystic Fibrosis

Cystic fibrosis is caused by mutations in the CFTR gene on chromosome 7, which encodes a chloride ion channel. Over 1000 mutations have been identified, but the most common are: ΔF508, a deletion of 3 base pairs that removes phenylalanine at position 508. G551D, a point mutation that substitutes aspartate for glycine at position 551.[11] These mutations impair chloride ion transport, producing thick, sticky mucus in the lungs, pancreas, and other organs. Symptoms include chronic respiratory infections, pancreatic insufficiency, salty-tasting skin, infertility, and progressive lung disease.[12][13]

p53 Mutation and Tumorigenesis

One of the most common genetic alterations in human cancers involves mutations in the TP53 gene, which encodes the p53 protein—a critical tumor suppressor in regulating the cell cycle, DNA repair, and apoptosis. In its normal form, p53 responds to DNA damage by halting cell division and initiating repair mechanisms or triggering programmed cell death. However, mutations in TP53, especially missense mutations that substitute 1 amino acid for another in the DNA-binding domain of the protein, can disrupt its function. For example, a common mutation substitutes arginine with histidine at codon 273 (R273H), impairing p53’s ability to bind DNA and activate target genes. Without functional p53, cells accumulate mutations unchecked, promoting uncontrolled cell proliferation and tumor development.[14][15]

Clinical Significance

Identifying specific base-pair mutations within a gene allows healthcare professionals to better understand a patient’s disease phenotype. The clinical severity of a mutation depends on several factors, including the normal function of the affected protein, the number of amino acids altered, and the type of mutation involved. For instance, a base-pair substitution that results in a silent mutation produces no change in the amino acid sequence and therefore has little to no effect on protein function.

In contrast, inserting or deleting 1 or 2 base pairs causes a frameshift mutation, altering the reading frame and leading to a downstream amino acid sequence. This usually results in a nonfunctional protein. By comparison, insertions or deletions in multiples of three base pairs preserve the reading frame. Still, they may add or remove amino acids, affecting protein stability and activity.