Proteins

Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are required 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 to one another in long chains. There are 20 different types of amino acids that can be combined to make a protein. The sequence of amino acids determines each protein's unique 3-dimensional structure and its specific function.

Structure

Proteins are made up of polypeptides, which are unique sequences of amino acids. Most proteins fold into unique 3-dimensional structures. The shape into which a protein naturally folds is known as its native conformation. The "side chains" of the amino acids in a protein interact with each other and with the environment, causing the chain to fold into a specific shape, which determines the protein's activity.

Function

Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, and transporting molecules from one location to another. Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in protein folding into a specific three-dimensional structure that determines its activity.

Classification

Proteins can be classified into three general types: fibrous, globular, and membrane. Fibrous proteins are often structural, such as collagen, which is connective tissue, and keratin, which is found in hair and nails. Globular proteins are generally compact, soluble, and spherical in shape. Membrane proteins often serve as receptors or provide channels for polar or charged molecules to pass through the cell membrane.

Synthesis

Proteins are assembled from amino acids using information encoded in genes. Each protein has its own unique amino acid sequence that is specified by the nucleotide sequence of the gene encoding this protein. The genetic code is a set of three-nucleotide sets called codons and each three-nucleotide combination designates an amino acid, for example AUG (adenine-uracil-guanine) is the code for methionine. Because the genetic code is degenerate, most amino acids are encoded by more than one codon.

See also

• Amino acid
• Enzyme
• Gene
• Nucleotide
• DNA

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