The hydrolysis of ATP liberates 7.3 kcal/mol under standard conditions, but may liberate 10-12 in the cell.
ATP powers endergonic reactions by transferring its terminal phosphate group to another molecule. Thus, the exergonic hydrolysis of ATP is coupled to an endergonic process. The transfer is controlled by enzymes and the molecule accepting the phosphate group becomes more reactive.
The conversion of glutamic acid (glutamate) to glutamine is an endergonic reaction (delta G = +3.4 kcal/mol). In a two step process, ATP is hydrolyzed (delta G = - 7.3) and the glutamate is phosphorylated. The newly acquired phosphate group on the glutamate is then replaced by ammonia yielding the related amino acid, glutamine. Overall the process becomes exergonic (3.4 - 7.3 = - 3.9) and spontaneous.
Enzymes A B C Section 1.4
Classes
A spontaneous reaction does not mean it occurs immediately, only that it will tend to go in the direction of products (proceeds to the right). Proteins, acting as biological catalysts, or enzymes, speed up and control the rate of reactions.
Energy must be added to a molecule to break its bond before new bonds may be formed. This is known as "activation energy" or the "barrier of activation". Without this barrier, important molecules would break down spontaneously at physiological temperatures. Enzymes work by introducing instability (energy) into the reactants making it easier to break the bonds. The transition state (condition where the reactants have acquired enough free energy to react) is reached sooner. In other words, enzymes act by lowering the barrier of activation.
Enzymes DO NOT change the overall delta G of a reaction. They cannot make an endergonic reaction occur spontaneously. Enzymes are, however, very selective for which reaction they will catalyze. This selectivity is due to the three dimensional structure (folding) of the enzyme.
The area on the enzyme where catalysis takes place is called the active site. The active site is usually just a few amino acids that form a groove into which the substrate (intended reactant) will fit. Only reactants that are compatible in form and properties will fit inside the active site. The active site is often changed as the substrate binds (induced fit). Hydrogen bonds temporarily hold the substrate within the active site.
In a typical reaction:
enzyme + substrate goes to enzyme-substrate complex goes to
enzyme + product
There are three factors affecting enzyme activity:
1. Environmental conditions
a. Temperature: Optimal temperature favours catalysis but not the denaturation of the enzyme. Most human enzymes work best at 35-40 degrees Celsius.
b. pH: Most enzymes work best at pH levels of 6-8. Some stomach enzymes must work at levels as low as 1-2.
c. Ionic concentrations: high salt concentrations inhibit enzyme activity.
2. Cofactors: inorganic ions (zinc, copper, iron, etc) are required for some enzymes to function. Organic cofactors are known as coenzymes; many of these are vitamins.
3. Inhibition (to be continued)