Protein structure

Proteins are complex molecules that serve as the building blocks of life. They are polymers, which means they are large molecules made of repeating smaller units. These smaller units are called amino acids, or residues. When amino acids join together, they undergo a condensation reaction. In this process, each amino acid loses one water molecule to form a peptide bond with another. A chain containing fewer than 30 amino acids is typically called a peptide. Once the chain grows larger, it is considered a protein.

To understand how proteins function, scientists must study their three-dimensional arrangement of atoms. This field of study is known as structural biology. Researchers use advanced techniques like X-ray crystallography, NMR spectroscopy, and cryo-electron microscopy to see these shapes. Proteins vary greatly in size. They can range from tens to several thousand amino acids. By physical size, proteins are classified as nanoparticles, measuring between 1 and 100 nanometers.

Protein structure is organized into four distinct levels. The first level is the primary structure. This is the specific sequence of amino acids in the polypeptide chain. This sequence is determined by a gene. DNA is transcribed into mRNA, which a ribosome then reads during translation. This process creates the unique order of amino acids. Frederick Sanger famously discovered the amino acid sequence of insulin. This work proved that proteins have defining, unique sequences.

The second level is the secondary structure. These are regular, local shapes formed along the polypeptide backbone. In 1951, Linus Pauling suggested two main types: the α-helix and the β-strand, also called β-sheets. These structures are held together by patterns of hydrogen bonds. These bonds saturate the hydrogen bond donors and acceptors in the peptide backbone. Some parts of a protein may be ordered without forming these specific regular shapes.

The third level is the tertiary structure. This refers to the full three-dimensional shape of a single polypeptide chain. This shape is often a compact, globular structure. Folding is driven by hydrophobic interactions, where hydrophobic residues are buried away from water. The structure is stabilized by specific interactions like salt bridges and hydrogen bonds. Disulfide bonds can also help lock the structure in place. However, these bonds are rare in cytosolic proteins because the cytosol is a reducing environment.

The fourth level is the quaternary structure. This occurs when two or more polypeptide chains, called subunits, aggregate into a single unit. This functional unit is called a multimer. If there are two subunits, it is a dimer. Three subunits form a trimer, and four form a tetramer. Proteins made of identical subunits are called homomers. Proteins made of different subunits are called heteromers. A common example is hemoglobin, which is a heterotetramer.

Proteins often contain smaller functional units called domains. A structural domain is a part of the protein that is self-stabilizing. It can often fold independently of the rest of the chain. Many domains are not unique to just one protein. They can appear in many different types of proteins. Because they are stable, scientists can use genetic engineering to swap domains between proteins. This can create new, hybrid proteins known as chimeras.

Proteins are not static, frozen objects. They are dynamic and move constantly. They exist in different shapes called conformations. When a protein shifts between these shapes, it is called a conformational change. These movements allow proteins to act like nanoscale biological machines. For example, myosin is a motor protein that helps muscles contract. Kinesin moves cargo inside cells. Even the beating of cilia is powered by protein movement.

Some proteins are different because they lack a stable shape. These are called intrinsically disordered proteins. They exist in a flexible state rather than a single fixed structure. To study them, scientists use conformational ensembles. This means they look at a collection of many possible shapes the protein might take. Researchers use computational algorithms to model these shapes. They use methods like molecular dynamics to understand how these flexible proteins behave.