Protein

Proteins are large molecules made up of chains of smaller building blocks called amino acids. They are essential for almost everything that happens inside living things. Proteins help carry out important jobs, such as speeding up chemical reactions, copying DNA, supporting the structure of cells, and moving molecules from one place to another.

Each protein is different because of the order of its amino acids, which is determined by the genetic code in genes. This order causes the protein to fold into a special shape that lets it do its job. Some proteins work together in groups to perform complex tasks.

Proteins are vital for life. Many act as enzymes, which are helpers that make chemical reactions happen faster. Others give cells their shape or help cells communicate with each other. In animals, eating proteins supplies the body with important building blocks it cannot make on its own.

History and etymology

Further information: History of molecular biology

Proteins have been known since the 1700s. Early scientists studied materials like albumin, gluten, and fibrin. In 1838, the term “protein” was created, coming from a Greek word meaning “primary” or “in the lead.”

Later, scientists discovered that proteins are made of chains of smaller parts called amino acids. In 1949, a scientist named Frederick Sanger showed the exact order of amino acids in a protein called insulin. This helped us understand how proteins are built. With new tools like X-ray crystallography and cryo-electron microscopy, scientists can now see the shapes of proteins in great detail.

Classification

Proteins can be grouped based on their structure and sequence. One common way is by using the EC number system, especially for enzymes. Another method is gene ontology, which sorts proteins by what they do and where they are found inside cells.

Scientists also classify proteins by looking at similar parts called domains, which help understand their function. Bigger proteins often have many of these domains.

Biochemistry

Main articles: Biochemistry, Amino acid, and Peptide bond

Proteins are made from long chains of building blocks called amino acids. These amino acids are connected by special bonds called peptide bonds, forming a chain. Each amino acid has a unique side chain that affects how the protein folds into its special shape. This shape is very important because it determines what the protein can do in the body.

Proteins are very important in our cells. A tiny bacteria cell might have around 50,000 proteins, while a human cell can have up to 3 billion! These proteins help with many jobs, like speeding up chemical reactions, copying DNA, and giving cells their shape. One of the most common proteins in nature is RuBisCO, which helps plants turn carbon dioxide into food through photosynthesis.

Synthesis

Main article: Peptide synthesis

Proteins are built from smaller units called amino acids. The instructions for making these amino acid chains are stored in our genes. Genes are made of nucleotides, and every three nucleotides form a code called a codon that tells cells which amino acid to add next. This process, called biosynthesis, starts in the cell’s nucleus and finishes in the cytoplasm, where tiny machines called ribosomes read the instructions and link the amino acids together.

Scientists can also make short proteins in laboratories using special chemical methods. These methods let them add special tags or unusual amino acids for research, but they work best for small proteins and can’t easily create the complex shapes that natural proteins usually have.

Structure

Proteins usually fold into special 3D shapes, which help them do their jobs. Scientists call this folded shape the protein's "native conformation." Some proteins fold on their own, while others need help from special helpers called molecular chaperones.

There are four main ways to describe a protein's shape:

• Primary structure: the chain of amino acids that make up the protein.
• Secondary structure: local patterns like spirals (α-helix) or flat sheets (β-sheet) held together by hydrogen bonds.
• Tertiary structure: the overall 3D shape of the protein, which is important for its function.
• Quaternary structure: the shape formed when several proteins work together as a group.

Proteins can be grouped into three main types based on their shapes: globular proteins, which are usually soluble and act as enzymes; fibrous proteins, which are often structural like collagen in connective tissue or keratin in hair and nails; and membrane proteins, which help molecules pass through cell membranes.

Cellular functions

Proteins are important workers inside cells, doing many jobs based on instructions from genes. They make up much of the cell's dry weight, more than DNA or RNA. Proteins can grab onto other molecules very well, which helps them do many different tasks.

One big job of proteins is to speed up chemical reactions, called enzymes. These help with processes like breaking down food and copying DNA. Proteins also help cells talk to each other and move things around the body. Some proteins give cells their shape, like the ones in hair and muscles, while others help cells move and stay organized.

Methods of study

Scientists use many ways to study how proteins work and look. Some important methods include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance, and mass spectrometry. These studies can be done inside cells, outside cells, or with computer models.

To study proteins outside cells, scientists first separate the protein from other parts of the cell. This is done using steps like spinning the mixture very fast to sort out different parts, or using special materials that stick to the protein. Once the protein is separated, scientists can test how it works and changes under different conditions.

Digestion

Proteins are broken down into smaller pieces called peptides and amino acids during digestion. This process, called proteolysis, helps our bodies absorb these important building blocks in the small intestine. Special proteins called proteases, such as pepsin in the stomach and trypsin from the pancreas, help break down proteins.

Proteins can also be broken down in factories to make amino acids from materials like feathers and blood meal. This is done using hot hydrochloric acid, which breaks the links between the building blocks of proteins.

Mechanical properties

The mechanical properties of proteins are very diverse and important for their functions in living things. For example, proteins like keratin and collagen help give structure and strength to materials such as hair, nails, and connective tissues. These properties also allow muscle tissue to stretch and contract.

The stiffness of a protein is measured by something called Young’s modulus. Proteins like collagen and keratin are much stiffer than proteins like elastin, which helps structures such as blood vessels stay flexible. Scientists study these properties using special computer simulations and experiments to understand how proteins behave under different forces.