Protein

Introduction & Top Questions
General structure and properties of proteins
- The amino acid composition of proteins
  - Structures of common amino acids
  - Physicochemical properties of the amino acids
  - Amino acid sequence in protein molecules
- Levels of structural organization in proteins
  - Primary structure
  - Secondary structure
  - Tertiary structure
  - Quaternary structure
- The isolation and determination of proteins
- Physicochemical properties of proteins
  - The molecular weight of proteins
  - The shape of protein molecules
  - Hydration of proteins
  - Electrochemistry of proteins
    - Electrometric titration
    - Electrophoresis
- Conformation of globular proteins
  - Results of X-ray diffraction studies
  - Other approaches to the determination of protein structure
    - Spectrophotometric behavior
    - Optical activity
    - Chemical reactivity of proteins
  - Association of protein subunits
  - Protein denaturation
  - Conformation of proteins in interfaces
Classification of proteins
- Classification by solubility
- Classification by biological functions
Special structure and function of proteins
- Structural proteins
  - Scleroproteins
    - Collagen
    - Keratin
    - Others
  - The muscle proteins
  - Fibrinogen and fibrin
- Albumins, globulins, and other soluble proteins
  - Proteins of the blood serum
  - Milk proteins
  - Egg proteins
  - Protamines and histones
  - Plant proteins
- Conjugated proteins
  - Combination of proteins with prosthetic groups
    - Mucoproteins and glycoproteins
    - Lipoproteins and proteolipids
    - Metalloproteins
    - Heme proteins and other chromoproteins
    - Nucleoproteins
  - Respiratory proteins
    - Hemoglobin
    - Other respiratory proteins
- Protein hormones
  - Hormones of the thyroid gland
  - Hormones of the pancreas
  - Peptides with hormonelike activity
- Immunoglobulins and antibodies
Enzymes
- Role of enzymes in metabolism
- Other functions
- General properties
  - Classification and nomenclature
  - Chemical nature
  - Cofactors
- The nature of enzyme-catalyzed reactions
  - The nature of catalysis
  - The role of the active site
  - The specificity of enzymes
  - The mechanism of enzymatic action
- The rate of enzymatic reactions
  - The Michaelis-Menten hypothesis
  - Inhibition of enzymes
  - Effects of temperature
- Enzyme flexibility and allosteric control
  - The induced-fit theory
  - Types of allosteric control

References & Edit History Quick Facts & Related Topics

Images, Videos & Interactives

How Does Protein Help Build Muscles?

Illustration showing four stages of protein structure: a chain of spheres, a helix, a folded chain, and a complex arrangement of intertwined chains.

Diagram showing protein synthesis: The top section illustrates ribosomes moving along messenger RNA to build a protein chain. The bottom section details transfer RNA molecules matching codons on the mRNA within a ribosome, adding amino acids to a growing polypeptide chain.

Diagram of the hemoglobin tetramer showing two alpha and two beta subunits, each in a different color, with four heme groups labeled, each containing a central iron atom for oxygen binding.

Protein calculator

$Proteins and X-ray diffraction$

Diagram showing two horizontal tubes with rodlike macromolecules: in A, the molecules are randomly oriented, while in B, they are aligned parallel, illustrating flow birefringence.

A line graph shows the electrometric titration of glycine, plotting pH on the x-axis against the volume of acid or base added on the y-axis. Red dots mark experimental data points, with the curve showing sharp pH changes near the isoelectric point at pH 6.1 and buffer regions below pH 3 and above pH 8. Explanatory text is included on the right.

Diagram of an alpha-helix protein structure showing a chain of atoms connected by lines, with carbon atoms in black, nitrogen in green, oxygen in red, and hydrogen in blue.

At a Glance

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protein summary

Protein: Media

biochemistry

Videos

How Does Protein Help Build Muscles?

How Does Protein Help Build Muscles?

Are you seeing gains yet?

Video: © American Chemical Society (A Britannica Publishing Partner)

Illustration showing four stages of protein structure: a chain of spheres, a helix, a folded chain, and a complex arrangement of intertwined chains.

The Building Blocks of Protein

Protein is more than a marketing term—it's a complex substance found in all living...

Video: Encyclopædia Britannica, Inc.

Protein engineering

Protein engineering

How protein engineering helps scientists battle diseases.

Video: AbbVie (A Britannica Publishing Partner)

Why do lobsters change color when cooked?

Why do lobsters change color when cooked?

Using chemistry to understand the colors and diseases of lobster shells.

Video: © American Chemical Society (A Britannica Publishing Partner)

Close-up of a medical blood bag filled with red blood, showing attached tubes and connectors against a blurred background.

Why Is Blood Red?

Are you “red-y” to find out?

Video: Encyclopædia Britannica, Inc.

The chemistry of digestion: How food is broken down

The chemistry of digestion: How food is broken down

Learn about the chemical process the human body uses to digest carbohydrates, proteins,...

Video: © American Chemical Society (A Britannica Publishing Partner)

Large biological molecules: Carbs, lipids, proteins, and nucleic acids

Large biological molecules: Carbs, lipids, proteins, and nucleic acids

Overview of large biological molecules.

Video: Encyclopædia Britannica, Inc.

Why our bodies need proteins

Why our bodies need proteins

The structure and function of amino acids.

Video: Encyclopædia Britannica, Inc.

The science behind dyeing Easter eggs

The science behind dyeing Easter eggs

Successful Easter egg dyeing requires the interaction of calcium carbonate, protein,...

Video: © American Chemical Society (A Britannica Publishing Partner)

What really makes a nutritious and complete breakfast?

What really makes a nutritious and complete breakfast?

Discover what constitutes a complete breakfast.

Video: © American Chemical Society (A Britannica Publishing Partner)

Images

Peptide

Peptide

The molecular structure of a peptide (a small protein) consists of a sequence of...

© raimund14/stock.adob.eomc

Diagram showing protein synthesis: The top section illustrates ribosomes moving along messenger RNA to build a protein chain. The bottom section details transfer RNA molecules matching codons on the mRNA within a ribosome, adding amino acids to a growing polypeptide chain.

Protein synthesis

Ribosomes are key components of protein synthesis in cells.

Encyclopædia Britannica, Inc.

Diagram of the hemoglobin tetramer showing two alpha and two beta subunits, each in a different color, with four heme groups labeled, each containing a central iron atom for oxygen binding.

The structure of hemoglobin

Hemoglobin is a protein made up of four polypeptide chains (α₁, α₂,...

Encyclopædia Britannica, Inc.

$Proteins and X-ray diffraction$

Proteins and X-ray diffraction

X-ray diffraction pattern of a crystallized enzyme.

Jeff Dahl

Diagram showing two horizontal tubes with rodlike macromolecules: in A, the molecules are randomly oriented, while in B, they are aligned parallel, illustrating flow birefringence.

Macromolecules

Flow birefringence depicting the orientation of elongated, rodlike macromolecules...

Encyclopædia Britannica, Inc.

A line graph shows the electrometric titration of glycine, plotting pH on the x-axis against the volume of acid or base added on the y-axis. Red dots mark experimental data points, with the curve showing sharp pH changes near the isoelectric point at pH 6.1 and buffer regions below pH 3 and above pH 8. Explanatory text is included on the right.

Glycine and electrometric titration

Electrometric titration of glycine.

Encyclopædia Britannica, Inc.

Diagram of an alpha-helix protein structure showing a chain of atoms connected by lines, with carbon atoms in black, nitrogen in green, oxygen in red, and hydrogen in blue.

Protein structure

The α-helix in the structural arrangement of a protein.

Encyclopædia Britannica, Inc.

A simplified diagram of the lysozyme protein from hen's egg white, showing a single blue peptide chain with numbered red dots marking amino acid residues from the N-terminal (1) to the C-terminal (129).

Lysozyme and protein conformation

The simplified structure of lysozyme from hen's egg white has a single peptide chain...

Encyclopædia Britannica, Inc.

Space-filling model of a collagen molecule shown as a twisted helix with clustered spheres in orange, blue, and purple against a black background.

Collagen

A three-dimensional model of a collagen molecule.

© ynse/Shutterstock.com

Keratin

Keratin

Scanning electron micrograph showing strands of keratin in a feather, magnified 186×.

Janice Carr/Centers for Disease Control and Prevention (CDC) (ID # 8673)

Collagenous fibers

Collagenous fibers

Randomly oriented collagenous fibers of varying size in a thin spread of loose areolar...

Don W. Fawcett, M.D.

Diagram showing the structure and interaction of actin and myosin filaments in muscle, with labeled actin filaments, myosin filaments, tropomyosin molecules, troponin, and a myosin molecule with rod and heads.

The structure of actin and myosin filaments

Myosin proteins bind to actin filaments and use ATP hydrolysis to drive contraction...

Encyclopædia Britannica, Inc.

Fibrin in blood clotting

Fibrin in blood clotting

Red blood cells (erythrocytes) trapped in a mesh of fibrin threads. Fibrin, a tough,...

BSIP/age fotostock

Diagram of an antibody molecule showing two identical light chains and two identical heavy chains joined by disulfide bonds in a Y-shaped structure, with labeled regions for antigen binding and effector function.

Antibody structure

The four-chain structure of an antibody, or immunoglobulin, molecule. The basic unit...

Encyclopædia Britannica, Inc.

Morning Glory Pool hot spring in Yellowstone National Park, showing vivid rings of yellow, green, and blue created by cyanobacteria and algae thriving in the water.

Blue-green algae

The Morning Glory Pool at Yellowstone National Park in Wyoming is a brilliant display...

© Yoyo_slc/Shutterstock.com

Tobacco mosaic virus

Tobacco mosaic virus

Schematic structure of the tobacco mosaic virus. The cutaway section shows the helical...

Encyclopædia Britannica, Inc.

Diagram of an IgG immunoglobulin showing two heavy chains and two light chains connected by red and green dots representing intra-chain and inter-chain disulfide bonds, with labeled Fab and Fc regions and a note explaining the structure and cleavage points.

IgG immunoglobulin

Diagram of an IgG immunoglobulin.

Encyclopædia Britannica, Inc.

Diagram showing how B-vitamin coenzymes, such as vitamin B6, folic acid, and biotin, assist in metabolic pathways involving carbohydrates, fats, and proteins, highlighting their roles in energy production and the tricarboxylic acid cycle.

B-vitamin coenzymes in metabolism

Functions of B-vitamin coenzymes in metabolism.

Encyclopædia Britannica, Inc.

Diagram illustrating the lock-and-key model of enzyme action, showing a substrate fitting precisely into the enzyme's active site, while non-substrate shapes do not fit. The enzyme is depicted with a complex outline and labeled binding site, emphasizing specificity.

Enzymes and their active sites

The role of the active site in the lock-and-key fit of a substrate (the key) to an...

Encyclopædia Britannica, Inc.

Diagram showing how enzymes catalyze reactions: enzymes bind to substrates, form intermediate complexes, and release products. The lower section details specific mechanisms for acetylcholinesterase, sucrose phosphorylase, and maltose phosphorylase, with labeled steps and components.

The actions of enzymes

Mechanisms of enzymatic action.

Encyclopædia Britannica, Inc.

Two graphs illustrating enzyme action: the left graph shows a Michaelis-Menten curve with reaction velocity rising and leveling off as substrate concentration increases; the right graph compares cooperative curves, with positive cooperativity showing a steep rise, negative cooperativity a gradual rise, and both labeled accordingly.

Diagram of enzyme action

Curves representing enzyme action.

Encyclopædia Britannica, Inc.

A multi-panel diagram illustrates how a substrate binds to an enzyme's active site through induced fit, leading to product formation, and shows how non-substrates and allosteric regulators can alter enzyme activity by binding to different sites on the enzyme.

Induced-fit theory

Various anomalous properties of enzymes are explained by the induced-fit binding...

Encyclopædia Britannica, Inc.

Assortment of protein-rich foods on a wooden table, including salmon, beef, chicken breast, eggs, milk, cheese, beans, and almonds.

High-protein foods

Certain fad diets focus on the consumption of protein. An example is the carnivore...

© bit245—iStock/Getty Images

Diagram showing various proteins of the immunoglobulin superfamily embedded in a cell membrane, including CD4, CD8, CD28, Fc receptor, Thy-1, class I and II MHC proteins, T-cell receptor, and IgM, with disulfide bonds highlighted.

schematic representation of proteins of the immunoglobulin superfamily

Schematic representation of some proteins of the immunoglobulin (Ig) superfamily....

Encyclopædia Britannica, Inc.

Diagram of a cell showing the process of protein processing and secretion, with proteins assembled by ribosomes, modified in the rough endoplasmic reticulum, transported in vesicles to the Golgi complex for further processing, and then released through the cell membrane.

Protein biosynthesis and the endoplasmic reticulum

The endoplasmic reticulum (ER) plays a major role in the biosynthesis of proteins....

Encyclopædia Britannica, Inc.

Diagram illustrating the process of protein synthesis: DNA unzips in the nucleus, messenger RNA (mRNA) forms on the DNA strands, and then mRNA moves to a ribosome where transfer RNA (tRNA) helps assemble amino acids into a protein chain.

DNA and protein synthesis

DNA in the cell nucleus carries a genetic code, which consists of sequences of adenine...

Encyclopædia Britannica, Inc.

major histocompatibility complex

major histocompatibility complex

Protein images comparing the MHC I (left) and MHC II (right) molecules. The orange...

David S. Goodsell/Protein Data Bank/Research Collaboratory for Structural Bioinformatics (RCSB)

A flowchart illustrating protein and amino acid metabolism, showing how dietary proteins are digested and absorbed into the body's amino acid pool, which is then used for building tissue and plasma proteins, creating amino acid derivatives, energy metabolism, and excretion of waste products like urea.

protein and amino acid metabolism

General scheme of protein and amino acid metabolism.

Encyclopædia Britannica, Inc.

Diagram showing the basic structure and expression of a gene, with DNA made of exons and introns, transcription to RNA, splicing to form mRNA, translation into an amino acid chain, and posttranslational modification to form a protein.

gene; intron and exon

Genes are made up of promoter regions and alternating regions of introns (noncoding...

Encyclopædia Britannica, Inc.

Interactives

Protein calculator

Protein calculator

Find out how much protein the human body needs, based on height, weight, age, sex,...

Encyclopædia Britannica, Inc.

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