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Monday, December 13, 2010

ATP

Monday, December 13, 2010 - 0 Comments

ATP: (Adenosine Tri-phosphate):

The cell’s Energy Currency:      Major energy currency of all cells is nucleotide called Adenosine Tri-phosphate (ATP). As ATP plays central role as the energy currency in all organisms, it must have appeared early in the history of life.
The ability of ATP to store and release energy is because of the structure of ATP molecule. Each ATP molecule has three subunits.
(1) Adenine an organic molecule composed of two carbon nitrogen rings.
(2) Ribose, fine carbon sugar.
(3) Three phosphate groups in a linear chain.
The covalent bond connecting these phosphates is indicated by “tilde” symbol (–) and is high energy bond. The energy is not localized in the bond itself, it is property of entire molecule and is simply released as phosphate bond breaks. These bonds have low activation energy and breaks easily. The breaking of one bond releases about 7.3 K. Cal: (7300 calories) per mole of ATP.
The energy from ATP is sufficient to derive most of the cells endergonic reactions in a typical energy reaction only the outermost of the two high energy bonds, break. When this happens, ATP becomes ADP (Adenosine di-phosphate). In some cases ADP is further hydrolyzed to AMP (Adenosine mono-phosphate) as follows:
ATP ------------- > ADP + Pi + 7.7 K. Cal:
ADP ------------ > AMP + Pi + 7.3 K. Cal:

Cells contain reservoir of ADP and phosphate (Pi). As long as a cell is living, ATP is constantly being converted into ADP plus phosphate to drive the cells many energy requiring processes enabling the animal to perform biological work. ATP can not be stored for long, as ATP lasts only few seconds before it is used to perform biological work. Thus cells constantly recycle ADP, with the energy derived from food stuffs, stored fats and starches. ADP and phosphate recombine to form ATP, with 7.3 K. Cal: of energy per mole contributed to each newly formed high energy phosphate bond.

Coenzymes

Some enzymes have the ability to act independently while other can be functional in presence of some helpers. These helpers are of different chemical nature and two of these are important. They are cofactors and coenzymes.

Coenzymes:

They are non protein molecules that participate in enzymes catalyzed reaction, often by transporting electrons in the form of hydrogen atoms from one enzyme to another. Many vitamins like niacin and riboflavin function as coenzymes or are used to make coenzymes. The main function of coenzyme is to transport energy in the form of hydrogen atoms from one enzyme to another. One of the most important coenzyme in the cell is hydrogen acceptor micotinamide adenine dinucleotide (NAD+), which is made from vitamin B. Since animals have lost the ability to synthesize the vitamin components of coenzymes, it should be provided along with diet to prevent vitamin deficiency. In the body vitamins are recovered in their original form and are used repeatedly. When NAD+ acquires hydrogen atom from an enzyme, it reduces to NADH. The electron of hydrogen atom contains energy that the NADH molecule then carries. When various foods are oxidized in the cells, the electrons are removed from the food molecules and are transferred to these coenzymes, as NAD+ is reduced to NADH.

Cofactors

Some enzymes have the ability to act independently while other can be functional in presence of some helpers. These helpers are of different chemical nature and two of these are important. They are cofactors and coenzymes.
Cofactors:

Many enzymes require certain metal ions to change non functioning active site to a functioning one. These metal ions that help enzyme in their catalytic activity are known as cofactors. Some of common cofactors are Ca2+, Mg2+, Mn2+, Cu2+ and Zn2+. The attachment of a cofactor with main enzyme (apoenzyme) changes the shape of protein and allows it to combine with substrate. Enzyme carbolic anhydrase, contains zinc the cytochromes contain iron and Troponin (a muscles contraction in enzyme) contains calcium. The cofactors of an enzyme participate in the temporary bonds between the enzyme and it substrate when the enzyme substrate complex forms.

Enzymes and the Factors Affecting Enzyme Activity

Most enzymes are proteins, some are nucleic acids (RNA). Enzymes have enormous catalytic power. They greatly enhance the rate at which specific chemical reactions take place. Enzymes regulate chemical reactions. An enzyme is biological catalyst that can accelerate specific chemical reaction by lowering the required activation energy but it self remains unaltered and can be used again. The substance on which enzyme acts are called reactants or substrates. Enzyme activity depends upon the concentration of enzyme concentration of substrate, temperature and pH. Many digestive enzymes like Pepsin and trypsin, all enzyme names end with the suffix –ase and are named after their substance.
Enzyme structure:

Enzymes are complex three dimensional globular protein molecules or nucleic acids that vary in size from small, simple with a molecular weight of 10,000 to highly complex molecules with molecular weights upto one million. Many enzymes are pure proteins that are delicately folded and interlinked chains of amino acids. Binding of the substrate to the enzyme changes the enzyme’s shape, a phenomenon called induced fit. The active site’s embrace of the substrate brings chemical groups of the active site into positions that enhance their ability to work on the substrate and to catalyze the chemical reaction. When the reaction is complete, the product of catalyzed reaction is released and the enzyme resumes its previous shape and is ready to catalyze another chemical reaction. For convenience the active enzyme is termed as a holoenzyme. It may in turn comprise of major protein part of enzyme the apoenzyme. A number of other chemical substances help the enzyme to be functional. These chemicals are termed as prosthetic group and may be coenzyme, cofactors.
Enzyme function: When a substrate molecule binds to an enzyme’s active site, an enzyme substrate complex (ES) forms. This is the essential first step in enzyme catalysis and can be summarized as
Enzyme (E) + Substrate (S) ------> Enzyme Substrate Complex (ES) --------> Products (P) + Enzymes (E).
                                          <------                                                   <---------
Once the unstable high energy ES forms, amino acid side groups of the enzyme are placed against certain bonds of the substrate. These side groups stress or distort the substrate bonds lowering the activation energy needed to break the bonds. The bonds break releasing the substrate which now reacts to produce the final product and release the enzyme.
Factors affecting Enzyme activity:
Any condition that alters three dimensional shape of an enzyme also affects the enzyme’s activity. Two factors that affect enzyme activity are temperature and pH.
(1)        Temperature:
The shape of prein or nucleic acid is determined largely by hydrogen bonds present in its structure. Temperature changes easily, disrupt hydrogen bonds thus changing the structure of enzyme. In higher vertebrates such as birds and mammals, the body temperature is between 35 and 40°C as such the enzyme present in these animals function best within this narrow temperature range. Below 35°C the bonds that determine protein shapes are not flexible enough to permit the shape change necessary for substrate to fit into a reactive site. Above 40°C, the bonds are too weak to hold the protein in proper position and to maintain its shape. When proper shape is lost, the enzyme is destroyed; this loss of shape is called denaturation.
(2)        pH:
Most enzymes also have optimum pH usually between 6 and 8. With a change in pH, the shape of the enzyme can also be altered. When the pH is too low, H+ ions combine with R groups of the enzyme’s amino acids, reducing their ability to bind with substrate. Acidic environments can also denature enzymes not adapted to such conditions. Some enzymes function at low pH. For example pepsin, the enzyme found in stomach of mammals has an optimal pH of about 2. Pepsin functions at such a low pH because it has an amino acid sequence that maintains it’s ionic and hydrogen bonds, even in the presence of large number of H+ ions (low pH). On the other hand trypsin is active in more basic medium (PH9) found in small intertue of mammals. Slawary any lose acts around 7.5 pH. Generally the pH optimum of an enzyme reflects the pH of the body fluid in which the enzyme is found.

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