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  • Proteins play countless roles throughout the biological world, from catalyzing chemical

  • reactions to building the structures of all living things.

  • Despite this wide range of functions, all proteins are made out of the same twenty building

  • blocks, called amino acids. The way these twenty amino acids are arranged

  • dictates the folding of the protein into its unique final shape and its function.

  • Amino acids are made of carbon, oxygen, nitrogen, hydrogen, and sulfur atoms.

  • These atoms form an amino group, a carboxyl group and a side chain attached to a central

  • carbon atom. The side chain is the only part that varies from amino acid to amino acid

  • and determines its properties. Hydrophobic amino acids such as leucine and

  • isoleucine have carbon rich side chains, which don’t interact well with water.

  • Hydrophilic amino acids such as serine, or threonine interact well with water. Charged amino acids like

  • glutamic acid or arginine interact with oppositely charged amino acids or with water.

  • The primary structure of the protein is the linear sequence of amino acids as encoded

  • by DNA. The amino acids are joined by peptide bonds,

  • which link an amino group and a carboxyl group. A water molecule is released each time a bond

  • is formed. Specific amino acid sequences give proteins

  • their distinct shapes and chemical characteristics. These protein chains often folds into two

  • types of secondary structures stabilized by hydrogen bonds.

  • A protein chain can fold into a rigid alpha helix, forming regular patterns of hydrogen

  • bonds between the backbone atoms of nearby amino acids.

  • Backbone atoms of the chain can interact side-by-side to form beta sheets.

  • Many proteins fold into a compact globular shape, with hydrophobic side chains sheltered

  • inside away from the surrounding water. The functions of many proteins rely on this

  • folded structure. For instance, hemoglobin forms a pocket to hold heme, a small molecule

  • with an iron atom in the center that binds oxygen.

  • Two or more polypeptide chains can come together to form one functional molecule with several

  • subunits. The four subunits of hemoglobin cooperate so that the complex can pick up

  • more oxygen in the lungs and release it in the body.

  • Many proteins rely on the ability to recognize the shape of specific molecules in order to

  • function correctly. The flexible arms of antibodies protect the

  • body from disease by recognizing and binding to foreign molecules and thus preventing the

  • viral RNA or DNA to enter the cell. Collagen forms a strong triple helix that

  • is used throughout the body for structural support.

  • The calcium pump moves ions across cell membranes allowing the synchronized contraction of muscle

  • cells. The hormone insulin is a small, stable protein

  • that can easily maintain its shape while traveling through the blood to regulate blood sugar

  • levels. Alpha amylase is an enzyme with a catalytic

  • site that begins the breakdown of carbohydrates in our saliva.

  • Ferritin forms a hollow shell that stores iron from our food.

  • Learn more about the functions and 3D structures of nucleic acids, proteins, and molecular

  • machines at the RCSB Protein Data Bank.

Proteins play countless roles throughout the biological world, from catalyzing chemical

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