Posted on 01 Oct 2012 14:51
A coenzyme is a small organic molecule that combines with an enzyme and causes that enzyme to become active or which facilitate its activity. In general, molecules that combine with enzymes in this way are called cofactors, but when the molecules are organic, rather than simple ions of elements, they are called coenzymes. Even though they are organic molecules, coenzymes are not proteins, as enzymes are, and they are not catalytically active themselves, which perhaps makes cosubstrate or cofactor a less confusing name for them. In many reactions catalyzed by enzymes, electrons or groups of atoms are transferred from one compound to another, and this usually involves a coenzyme, which temporarily accepts the group being transferred.
Without an enzyme to latch onto them, a coenzyme would have no specificity, and would be very unlikely to collide with a substrate at the right orientation at exactly the right place to react with it. Enzymes provide this proximity and orientation, and also activates the coenzyme by removing a proton, polarizing the substrate, etc. Active enzymes with their nonprotein coenzyme component are called holoenzymes. Inactive enzymes, without their coenzyme component, are called apoenzymes.
Most coenzymes are obtained from vitamins, and many of them have B vitamins as part of their structure. Some B vitamins act as coenzymes to enzymes that act in metabolic pathways to release energy from glucose, glycerol, fatty acids, and amino acids. This action is the foundation of the myth that B vitamins can give you energy, just by consuming more of them. Some action of B vitamins as coenzymes are:
- Vitamin B6: Assists enzymes that metabolize amino acids.
- Vitmain B12: Assists cells in multiplying, such as red blood cells and cells lining the GI tract.
- Folate: Also assists cells in multiplying, along with B12
- Thiamin: Part of the coenzyme TPP, which helps with the conversion of pyruvate to acetyl CoA. 221
- Riboflavin: Part of the coenzymes FMN and FAD, which accept and donate hydrogen atoms during energy metabolism.
- Niacin: Part of the coenzymes NAD and NADP, which are central in numerous metabolic activities, assisting in energy-transfer reactions in glucose, fat, and alcohol metabolism, among others. NAD and NADP, similar to FMN and FAD, can carry hydrogen atoms.
- Biotin: As a coenzyme, biotin acts as a carbon dioxide carrier, making it critical in the TCA cycle, contributing a carbon to 3-carbon pyruvate to replenish it to a 4-carbon so that it can combine with actyl CoA in the TCA cycle.
- Pantothenic Acid: Part of Coenzyme A, which forms acetyl CoA, present in several metabolic reactions. Coenzyme A helps carry acetyl CoA and other molecules in pathways of glucose, fatty acid, and energy metabolism.
- Vitamin C (ascorbic acid): A coenzyme for hydroxylation reactions, such as converting proline to hydroxyproline; and lysine to hydroxylysine in the formation of procollagen.
- Vitamin K (quinone or hydroquinone in reduced form): Acts with carboxlase enzyme to activate prothrombin by carboxylation of a series of glutamate amino acid residues, which is a step in the activation of platelets during blood clotting.
Coenzyme A (CoA)
- Coenzyme A (CoA): As above, coenzyme A is formed from pantothenic acid. It is the "CoA" part of acetyl CoA and it is therefore of central importance in the metabolism of nutrients for energy.
- Coenzyme Q (ubiquinone): Part of oxidative phosphorylation or the 'electron transport chain,' Coenzyme Q recieves eelctrons from a flavoprotein and passes the electrons to cytochrome b.
- NAD+(nicotinamide adenine dinucleotide): The coenzyme form of niacin, NAD is a pyridine nucleotides which transport hydride ions. This means it can pick up hydrogen atoms liberated during glucose metabolism and transport these unbound H+ protons to other sites of reaction. When this happens, electrons, which are attracted to the positive charge of the protons, are transported as well. When carrying a proton, NAD+ is called NADH.
- NADP+ (nicotinamide adenine dinucleotide phosphate): The reduced phosphate form of NAD also transfers protons and electrons in certain degradative pathways, such as fatty acid formation. When carrying protons and electrons it is abbreviated NADPH.
- FMN: A flavin coenzyme that can accept a maximum of two electrons and two protons during reduction, similar to NAD.
- FAD (flavin adenine dinucleotide): Formed when FMN bonds with AMP, it is functionally similar to FMN. The reduced form, when actively transporting, is called FADH2.
For more in-depth information see Biochemistry by by Richard A. Harvey, Denise R. Ferrier or Introduction to Enzyme and Coenzyme Chemistry by T. D. H. Bugg.
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