Protein

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Molecular model of myoglobin, a protein that stores oxygen in muscle tissue.

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Illustration of a peptide bond formed between two amino acids.

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Historic model of myoglobin structure as determined by John Kendrew, the first protein structure solved by X-ray crystallography.

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Revised figure showing the chemical structure of a peptide with labeled components.

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Diagram showing resonance structures in a peptide group, contributing to its planarity.

Proteins are large, complex molecules that play many critical roles in the body. They are essential for the structure, function, and regulation of the body's tissues and organs. Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached in long chains. There are 20 different types of amino acids that can be combined to make a protein.

Structure

Proteins are made up of one or more long chains of amino acids, which are called polypeptides. The sequence of amino acids determines each protein’s unique 3-dimensional structure and its specific function. Proteins can be classified into four levels of structure:

Primary Structure

The primary structure of a protein is its unique sequence of amino acids. This sequence is determined by the gene corresponding to the protein.

Secondary Structure

The secondary structure refers to local folded structures that form within a polypeptide due to interactions between atoms of the backbone. The most common types of secondary structures are the alpha helix and the beta pleated sheet.

Tertiary Structure

The tertiary structure is the overall three-dimensional structure of a polypeptide. It is determined by interactions among various side chains (R groups) of the amino acids.

Quaternary Structure

Some proteins are made up of multiple polypeptide chains, also known as subunits. The quaternary structure is the arrangement of these subunits in a multi-subunit complex.

Function

Proteins perform a vast array of functions within organisms, including:

• Enzymatic activity: Proteins that act as enzymes speed up chemical reactions in the body.
• Structural support: Proteins like collagen provide structural support to cells and tissues.
• Transport and storage: Proteins such as hemoglobin transport oxygen in the blood.
• Signaling: Proteins are involved in cell signaling and communication.
• Immune response: Antibodies are proteins that help protect the body from pathogens.

Synthesis

Protein synthesis is a complex process that involves transcription and translation. During transcription, a segment of DNA is copied into mRNA, which is then translated into a polypeptide chain at the ribosome.

Dietary Proteins

Proteins are an essential part of the diet. They provide the amino acids that the body cannot synthesize on its own. Dietary proteins can be found in both animal and plant sources, such as meat, fish, eggs, dairy products, legumes, and nuts.

Gallery of Protein and Peptide Structures

• Schematic of mRNA translation by the ribosome, showing how peptides are synthesized in cells.
  Schematic of mRNA translation by the ribosome, showing how peptides are synthesized in cells.
• Table of the genetic code showing codon-amino acid correspondence.
  Table of the genetic code showing codon-amino acid correspondence.
• Diagram of peptide synthesis, illustrating step-by-step chemical formation of peptide bonds.
  Diagram of peptide synthesis, illustrating step-by-step chemical formation of peptide bonds.
• Structure of a chaperonin protein that assists in the proper folding of other proteins.
  Structure of a chaperonin protein that assists in the proper folding of other proteins.
• Visualization of a protein’s tertiary structure, using PDB entry 1TIM as an example.
  Visualization of a protein’s tertiary structure, using PDB entry 1TIM as an example.
• Composite image showing a variety of protein structural motifs and folding patterns.
  Composite image showing a variety of protein structural motifs and folding patterns.
• Schematic showing domain organization in EVH proteins, involved in actin regulation.
  Schematic showing domain organization in EVH proteins, involved in actin regulation.

Related pages

• Amino acid
• Enzyme
• Gene expression
• Metabolism