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Protein Basics | Overview


What is a protein?

A protein is a large, organic molecule made up of smaller molecules of amino acids. Although small proteins can contain as few as 50 amino acids, they are typically much larger, consisting of hundreds or even thousands of amino acids.

Proteins are sometimes intentionally cut into smaller segments. These segments are called peptides. A peptide can consist of anywhere between two and 50 amino acids.

In a protein, amino acids are linked one after another, forming a long chain. These are held together by peptide bonds, which are chemical bonds between the carbon and nitrogen atoms of the amino acids. In addition to carbon and nitrogen, amino acids contain hydrogen, oxygen and sometimes sulfur.

Proteins contain 20 types of amino acids. With the exception of leucine and isoleucine, which share the same atomic mass of 113 Daltons, each amino acid has a unique mass. (Dalton is the unit of measurement for atomic mass. One Dalton is equal to 1/12th the mass of one atom of carbon 12.)

What do proteins do?

Making up 12 to 18 percent of a lean, adult body, proteins play many crucial roles in human physiology. Here are a few:

  • Proteins form the structural framework of tissues and other structures in the body. For example, collagen helps make up tendons, ligaments, cartilage and other connective tissue.
  • Proteins carry substances throughout the body. For instance, hemoglobin is a carrier of oxygen and carbon dioxide.
  • Proteins work as enzymes to facilitate chemical reactions. There are over 1,000 known types of enzymes in the human body.
  • The proteins myosin and actin, allow muscles to contract.
  • Proteins regulate physiological processes and control growth.
  • Proteins are an integral part of the immune system. Antibodies, for example, consist of a type of protein used by the immune system.

How do genes code for proteins?

You can think of the human genome as one long string of DNA that is separated into 46 pieces called chromosomes. Within each chromosome are thousands of genes. The DNA sequence that makes up a gene is like software code that instructs the cell how to make proteins.

Four types of bases make up the DNA strand -- adenine (A), cytosine (C), guanine (G) and thymine (T). Each base has a complement -- another base that it usually pairs off with. A's pair off with T's, and C's pair off with G's. In all, the human genome is 3 billion base pairs long.

Three successive bases in the DNA strand, called a codon, can act as an instruction for the cell to select a specific type of amino acid. A series of codons instruct the cell to piece together a string of amino acids to form a protein. The cell knows when to begin and stop coding for proteins by recognizing special start and stop codons that are within the DNA strand.

Genetic instructions are carried out by various molecules and cell structures, including transfer RNA (tRNA), messenger RNA (mRNA) and ribosomes. In other words, the cell is able to read pieces of DNA code and use the information to put a protein together.

Amino acids come in an assortment of shapes. The cell, instructed by genes, takes advantage of this property and builds proteins that have vastly different sizes and shapes.