Enzymes are proteins that are produced by living organisms (fungi or bacteria) and help in the process of metabolic reactions. They act as catalysts for every single biochemical process in the body, from digestion to tissue regeneration. BioMed Research International 2013 (1) 

As protein molecules, they are made up of amino acids. Most enzymes contain between 100 and 1,000 amino acids. These amino acids are joined together in a long chain,  which is folded to produce a unique 3D structure. How Cells Work 2015 (2)

     In this computer generated image of the enzyme alpha-glucosidase, the red beads highlight the important, "active site" of an enzyme molecule.

 

In this computer generated image of the enzyme alpha-glucosidase, the red beads highlight the important, "active site" of an enzyme molecule.

Enzymes are very specific about which reactions they catalyze. Enzymes have a specific structural arrangement known as the ACTIVE SITE. It will only fit a specific SUBSTRATE ( also called, REACTANT). Only SUBSTRATES with exactly the right shape will bind to the enzyme and react like a "lock and key". When enzymes align the chemical groups of the substrates with chemical groups on the surface of the enzyme, this triggers the movement of electrons that is the "reaction".Once they are bound together, the enzyme helps the reaction and the REACTANT breaks into two pieces, "PRODUCTS". Biochemistry. Wikibooks.org 2014 (3)

As a catalyst, an enzyme facilitates a given reaction by lowering the amount of energy required. Enzymes speed up reactions by lowering the activation energy (Ea) of a reaction. The activation energy is the energy needed to start a reaction. The enzyme is thought to shorten the "path" of the reaction. This "short cut" would require less energy for each molecule of REACTANT to convert into PRODUCT. Therefore, the kinetic energy of most enzyme molecules exceeds the activation energy required to convert the reactant into product. Enzyme catalyzed reactions are often from 100 million to more than 10 billion times faster than the same reaction in the absence of the enzyme. Mosaic 1983 (4)

As catalysts, enzymes require cofactors (metallic ions or organic molecules) to aid with catalytic activity and reaction rate. Activation of the enzyme occurs upon binding of an organic or inorganic cofactor. Organic cofactors are also called “coenzymes” and are non-protein organic molecules, mostly derivatives of water-soluble vitamins. Biochemistry, 5th Edition. Berg, J. 2002 (5) The coenxymes NAD (nicotinamide adenine dinucleotide) and NADPH (nicotinamide adenine dinucleotide phosphate) are electron donors for the electron transport chain (series of compounds that transfer electrons from electron donors to electron acceptors via redox reactions).  Like "pings and pongs" between oxidized and re­duced forms, they are two-electron carriers, transferring hydride anion (H-) to and from substrates. Enzymes react NADP+ with either free electrons and H+ to form NADPH. Biochemistry, 4th Edition, Garret, R. Grisham, C. 2010 (6) During one stage of photosynthesis, NADPH and ADP pick up electrons and phosphate and transfer them, via leaf stroma, to the Calvin cycle to create sugar from CO2, helping power the synthesis of adenosine triphosphate (ATP), or the generation of chemical energy. The oxidation of one molecule of NADH is enough energy to synthesize several APT molecules. Molecular Cell Biology, 4th Edition. 2000 (7)

The above illustration is the enzyme Nitrate Reductase (NaR), showing its amino acid se­quence (image courtesy of NECi). The cofactor-binding regions of NaR are laid out linearly in the backbone (Campbell and Kinghorn, 1990; Solomonson and Barber, 1990; Rouze and Caboche, 1992). NaR has two active sites, one where coenzyme NADH donates electrons to cofactor Flavin Adenosine dinucleotide (FAD) to begin the transport of electrons to the Mo I Mo-pterin in the second active site, where nitrate is reduced to nitrite. NaR has two-site, ping-pong, steady-state kinetics, where the enzyme "pings and pongs" between oxidized and re­duced forms, as NADH/NAD+ bind at the electron donor active site and nitrate/ nitrite bind at the electron acceptor site. This "drives the reaction forward" in the reduction of nitrate to nitrite.  Plant Psysiol. 1996 PDF (8)


References:

  1. Gurung N., Ray, S., Bose,S., Rai V. 2013. A Broader View: Microbial Enzymes and Their Relevance in Industries, Medicine, and Beyond. BioMed Research International, Volume 2013, Article ID 329121, 18 pages
  2. Brian, M. Science HowStuffWorks.Com. Enzymes: How Cells Work, http://science.howstuffworks.com/life/cellular-microscopic/cell2.htm     
  3. Price N.C., Stevens L., 1999. Fundamentals of Enzymology, 3rd Ed., Oxford University Press, New York. Wikibooks.org. Biochemistry. March 28, 2013
  4. Metzger, N. Making (like) an Enzyme. Mosaic, January/February 1983
  5. Berg, Jeremy M., et al. 2007. Biochemistry, 6th ed. W.H. Freeman and Company, NY
  6. Garrett, H., Reginald and Charles Grisham. 2008. Biochemistry. Boston: Twayne Publishers
  7.  Lodish H, Berk A, Zipursky SL, et al.  Molecular Cell Biology. 4th edition. 2000, Electron Transport and Oxidative Phosphorylation. W. H. Freeman and Company
  8. Campell, W. H. 1996. Nitrate Reductase Biochemistry Comes of Age. Phytotechnology Research Center and Department of Biological Sciences, Michigan Technological University, Houghton, Michigan 49931-1 295, Plant Physiol. (1996) 111: 355-361