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Proteins and Enzymes

Proteins and Enzymes

Proteins are large biological molecules made up of long chains of smaller molecules called amino acids. Organisms rely on proteins for a variety of functions—proteins transport molecules across cell membranes, replicate DNA, and catalyze metabolic reactions, to name only a few of their functions.

The properties of proteins are functions of the combination of amino acids that compose them and can vary greatly. Interactions between amino acid sequences in the chains of proteins result in the folding of the chain into specific, three-dimensional structures that determine the protein’s activity.

Amino acids are organic molecules that contain an amine functional group (–NH2), a carboxylic acid functional group (–COOH), and a side chain (that is specific to each individual amino acid). Most living things build proteins from the same 20 different amino acids. Amino acids connect by the formation of a peptide bond, which is a covalent bond formed between two amino acids when the carboxylic acid group of one amino acid reacts with the amine group of the other amino acid.

The formation of the bond results in the production of a molecule of water (in general, reactions that result in the production of water when two other molecules combine are referred to as condensation reactions). The resulting bond—between the carbonyl group carbon atom and the amine nitrogen atom is called a peptide link or peptide bond.

Since each of the original amino acids has an unreacted group (one has an unreacted amine and the other an unreacted carboxylic acid), more peptide bonds can form to other amino acids, extending the structure. (see the figure below) A chain of connected amino acids is called a polypeptide. Proteins contain at least one long polypeptide chain.

This figure shows two amino acid molecules. These molecules have two singly bonded carbon atoms to which an amino group is bonded on the left and the C atom to the right is a component of a carboxyl group. The C atom at the center has an R group bonded below and an H atom bonded above. The amino acid at the top left has an amino group identified and enclosed in a green dashed rectangle. This group is comprised of an N atom with two bonded H atoms. The amino acid at the right has a carboxyl group identified in a green dashed rectangle. This group has a C atom to which an O H group and a doubly bonded O atom are bonded. The amino acid to the left has the O H group to the lower right in red. The amino acid on the right has an H atom that is bonded to the N atom in red. An arrow points downward and is labeled condensation reaction. A curved arrow extends down and to the right off of the downward arrow, pointing to H subscript 2 O, which is in red. A single, larger molecule appears beneath the downward arrow. At the locations of the red O H group and H atom, the amino acid molecules are bonded together. This bond is labeled as a peptide bond and the larger molecule formed is labeled as a polypeptide chain.

This condensation reaction forms a dipeptide from two amino acids and leads to the formation of water.

Enzymes are large biological molecules, mostly composed of proteins, which are responsible for the thousands of metabolic processes that occur in living organisms. Enzymes are highly specific catalysts; they speed up the rates of certain reactions. Enzymes function by lowering the activation energy of the reaction they are catalyzing, which can dramatically increase the rate of the reaction.

Most reactions catalyzed by enzymes have rates that are millions of times faster than the noncatalyzed version. Like all catalysts, enzymes are not consumed during the reactions that they catalyze. Enzymes do differ from other catalysts in how specific they are for their substrates (the molecules that an enzyme will convert into a different product). Each enzyme is only capable of speeding up one or a few very specific reactions or types of reactions.

Since the function of enzymes is so specific, the lack or malfunctioning of an enzyme can lead to serious health consequences. One disease that is the result of an enzyme malfunction is phenylketonuria. In this disease, the enzyme that catalyzes the first step in the degradation of the amino acid phenylalanine is not functional (see the figure below). Untreated, this can lead to an accumulation of phenylalanine, which can lead to intellectual disabilities.

This figure includes a computer generated image of an enzyme molecule showing string and curled ribbon-like structural components in purple, green, and yellow hues.

A computer rendering shows the three-dimensional structure of the enzyme phenylalanine hydroxylase. In the disease phenylketonuria, a defect in the shape of phenylalanine hydroxylase causes it to lose its function in breaking down phenylalanine.

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