Protein Structure

Proteins are built from amino acids and form polypeptides. Amino acids are the small building blocks, and a polypeptide is a chain of amino acids joined together. This basic relationship explains how proteins are made and why their structure matters.

Proteins have several important roles in living things. Some proteins help build body structures, while others carry out work inside cells. The transcript also connects proteins to enzymes, which are proteins that speed up chemical reactions.

Protein organization describes how a protein is arranged at several levels. These levels help explain how a chain of amino acids becomes a working molecule. The main stages are primary, secondary, tertiary, and quaternary structure.

Primary structure is the linear order of amino acids in the protein chain. Secondary structure is the first step of folding. In this stage, the chain can form patterns such as helices and sheets. Tertiary structure is the overall three-dimensional shape of one...

Protein folding is the process that gives a protein its final three-dimensional shape. That shape is essential because a protein’s structure helps determine how it works in the cell.

A folded protein is stabilized by several interactions. These include hydrogen bonds, van der Waals interactions, and disulfide bridges. Each one helps hold the protein in the correct form.

When a protein does not fold properly, its structure can change. That can affect its function and can interfere with normal...

Argonaute proteins have conserved domains that help small RNAs guide gene silencing. These proteins are found across organisms, but the number of Argonaute family members can differ from one species to another.

Protein domains are small, structurally independent parts of a single amino acid chain. They can keep their shape on their own, but they may still work together with other parts of the protein. During evolution, domains can duplicate, recombine, and form new combinations. This process,...

Proteins can be grouped by shape into globular proteins and fibrous proteins. These two subtypes also differ in how they dissolve in water and in the jobs they do in cells and tissues.

Globular proteins, also called spheroproteins, are usually round in shape. They contain a mix of amino acid types and varied primary sequences. Their shapes support many functions, including enzymes, cellular messengers, and molecular transporters. These roles often need the proteins to stay soluble in the...

Intrinsically disordered proteins are proteins that do not fold into one fixed three-dimensional shape. Their flexible structure helps them work with ordered proteins to do jobs that rigid proteins cannot easily do. They are found more often in eukaryotes than in prokaryotes, and they can be fully disordered or made of both ordered and disordered regions.

This lack of a rigid shape is linked to amino acid makeup. These proteins usually have many polar and charged amino acids and fewer...

Proteins can self-assemble into working complexes inside cells. Some complexes are homomeric, which means they contain more than one copy of the same protein. Others are heteromeric, which means they are built from different protein types.

Most protein complexes form through ordered self-assembly. In some cases, assembly factors help guide the proteins into the correct structure. Even in the crowded space inside a cell, proteins usually find the right partners and form functional complexes.

Conjugated proteins are proteins that bond with a non-protein part. Unlike simple proteins, which contain only amino acids, these protein complexes include another chemical moiety attached covalently. That extra part can change how the protein works in the body.

Nucleoproteins are conjugated proteins that contain nucleic acids. They are grouped as deoxyribonucleoproteins (DNPs) or ribonucleoproteins (RNPs). A nucleosome is a common DNP example, where nuclear DNA is associated with histone...

Amyloid fibrils are clumps of misfolded proteins linked to brain disease and other disorders. In normal cells, misfolded proteins are usually refolded by chaperone proteins or broken down by the proteasome. But in some mutations or diseases, these proteins can build up into large clusters and then assemble into long fibers called fibrils.

Amyloid deposits were seen as early as 1639 in the liver and spleen. In 1854, Rudolph Virchow used iodine staining, a method normally used to identify...

Antibodies are built from heavy and light chains that work together to form a Y-shaped molecule. These chains give the antibody its basic structure and help it bind to specific targets.

The two upper arms of the Y contain the variable regions, which differ from one antibody to another. These variable regions help determine which antigen, or foreign molecule, the antibody can recognize.

The lower part of the antibody is the constant region. This region is more similar across antibodies and...

Protein families are groups of homologous proteins with similar amino acid sequences and three-dimensional shapes. They often appear after gene duplication, when an extra copy of a gene is added to an organism’s genome. Mutations can then change the amino acids, as long as the protein can still be made correctly.

If the new proteins keep similar amino acids in key places, their domains and overall three-dimensional structure can stay similar too. Members of the same family can share as little...