Enzymes

Enzymes

Enzyme: A protein (or protein-based molecule) that speeds up a chemical reaction in a living organism. An enzyme acts as catalyst for specific chemical reactions, converting a specific set of reactants (called substrates) into specific products. Without enzymes, life as we know it would not exist.
Enzymes are nonetheless subject to error. In 1902 Sir Archibald Garrod was the first to attribute a disease to an enzyme defect, to what Garrod called an "inborn error of metabolism." Today, newborns are routinely screened for certain enzyme defects such as PKU (phenylketonuria) and galactosemia, an error in the handling (metabolism) of the sugar galactose.

Enzymes are biological catalysts. They speed up reactions by lowering the activation energy (the energy required for a reaction to begin). This is accomplished by increasing the local concentration of reactants (the compounds used in the reaction) and bringing the reactants into proper orientation for the reaction to begin. All enzymes are made of protein and their production is controlled by DNA. Enzymes can be reused many times. The standard suffix for enzymes is "ase."
Practically all of the numerous and complex biochemical reactions that take place in animals, plants, and microorganisms are regulated by enzymes. These catalytic proteins are efficient and specific—that is, they accelerate the rate of one kind of chemical reaction of one type of compound, and they do so in a far more efficient manner than man-made catalysts. They are controlled by activators and inhibitors that initiate or block reactions. All cells contain enzymes, which usually vary in number and composition, depending on the cell type; an average mammalian cell,

Classification of enzyme
Enzymes are classified according to the reactions they catalyze. In some cases, the terms used are fairly clear; in others, less so. Examples:

Oxidoreductases:

These are enzymes which catalyze the reduction or oxidation of a molecule. Remember that oxidation is the reverse of reduction and that an enzyme has to catalyze the forward and reverse reactions to the same degree. Any enzyme which catalyzes a reduction has to also catalyze the reverse (oxidation) reaction, thus the double-barreled name "oxidoreductase."
Transferases:

These enzymes catalyze the transfer of a group of atoms from one molecule to another. A common example involves transfer of a phosphate between ATP and a sugar molecule.
Hydrolases:

As the name suggests, these enzymes catalyze hydrolysis reactions (and their reverse reactions). The hydrolysis of an ester would be an example of such a reaction.
Isomerases:

These enzymes catalyze the conversion of a molecule into an isomer. The cis-trans interconversion of maleate and fumarate is an example.
Lyases:

Reactions which add a small molecule such as water or ammonia to a double bond (and the reverse, elimination, reactions) are catalyzed by lyases.
Ligases:

These enzymes catalyze reactions which make bonds to join together (ligate) smaller molecules to make larger ones.


Activity of Enzyme
Enzymes serve a wide variety of functions inside living organisms. They are indispensable for signal transduction and cell regulation, often via kinases and phosphatases.[68] They also generate movement, with myosin hydrolysing ATP to generate muscle contraction and also moving cargo around the cell as part of the cytoskeleton.[69] Other ATPases in the cell membrane are ion pumps involved in active transport. Enzymes are also involved in more exotic functions, such as luciferase generating light in fireflies.[70] Viruses can also contain enzymes for infecting cells, such as the HIV integrase and reverse transcriptase, or for viral release from cells, like the influenza virus neuraminidase.
An important function of enzymes is in the digestive systems of animals. Enzymes such as amylases and proteases break down large molecules (starch or proteins, respectively) into smaller ones, so they can be absorbed by the intestines. Starch molecules,
Several enzymes can work together in a specific order, creating metabolic pathways.
Enzymes determine what steps occur in these pathways. Without enzymes, metabolism would neither progress through the same steps, nor be fast enough to serve the needs of the cell.

Enzyme activity
Enzyme activity = moles of substrate converted per unit time = rate × reaction volume. Enzyme activity is a measure of the quantity of active enzyme present and is thus dependent on conditions, which should be specified. The SI unit is the katal, 1 katal = 1 mol s-1, but this is an excessively large unit. A more practical and commonly-used value is 1 enzyme unit (U) = 1 μmol min-1. 1 U corresponds to 16.67 nanokatals.[1]

Specific activity
The specific activity of an enzyme is another common unit. This is the activity of an enzyme per milligram of total protein (expressed in μmol min-1mg-1). Specific activity gives a measurement of the purity of the enzyme. It is the amount of product formed by an enzyme in a given amount of time under given conditions per milligram of enzyme. Specific activity is equal to the rate of reaction multiplied by the volume of reaction divided by the mass of enzyme. The SI unit is katal kg-1, but a more practical unit is μmol mg-1 min-1. Specific activity is a measure of enzyme processivity, usually constant for a pure enzyme.