how does enzyme lower activation energy?

Question

how does enzyme lower activation energy? How does it affects rate of reation? using hexokinase and

Answer 1

Enzymes are catalysts. Most are proteins. (A few ribonucleoprotein enzymes have been discovered and, for some of these, the catalytic activity is in the RNA part rather than the protein part. Link to discussion of these ribozymes.)

Enzymes bind temporarily to one or more of the reactants of the reaction they catalyze. In doing so, they lower the amount of activation energy needed and thus speed up the reaction.

Link to a discussion of free energy (G) and "ΔG".

Examples:
Catalase. It catalyzes the decomposition of hydrogen peroxide into water and oxygen.
2H2O2 -> 2H2O + O2

One molecule of catalase can break 40 million molecules of hydrogen peroxide each second.

Carbonic anhydrase. It is found in red blood cells where it catalyzes the reaction
CO2 + H2O <-> H2CO3

It enables red blood cells to transport carbon dioxide from the tissues to the lungs. [Discussion]

One molecule of carbonic anhydrase can process one million molecules of CO2 each second.

Acetylcholinesterase. It catalyzes the breakdown of the neurotransmitter acetylcholine at several types of synapses as well as at the neuromuscular junction — the specialized synapse that triggers the contraction of skeletal muscle.

One molecule of acetylcholinesterase breaks down 25,000 molecules of acetylcholine each second. This speed makes possible the rapid "resetting" of the synapse for transmission of another nerve impulse.

Enzyme activity can be analyzed quantitatively. Some of the ways this is done are described in the page Enzyme Kinetics. Link to it.

In order to do its work, an enzyme must unite — even if ever so briefly — with at least one of the reactants. In most cases, the forces that hold the enzyme and its substrate are noncovalent, an assortment of:



The activity of enzymes is strongly affected by changes in pH and temperature. Each enzyme works best at a certain pH (left graph) and temperature (right graph), its activity decreasing at values above and below that point. This is not surprising considering the importance of
tertiary structure (i.e. shape) in enzyme function and
noncovalent forces, e.g., ionic interactions and hydrogen bonds, in determining that shape.
Examples:
the protease pepsin works best as a pH of 1–2 (found in the stomach) while
the protease trypsin is inactive at such a low pH but very active at a pH of 8 (found in the small intestine as the bicarbonate of the pancreatic fluid neutralizes the arriving stomach contents). [Discussion]
Changes in pH alter the state of ionization of charged amino acids (e.g., Asp, Lys) that may play a crucial role in substrate binding and/or the catalytic action itself. Without the unionized -COOH group of Glu-35 and the ionized -COO- of Asp-52, the catalytic action of lysozyme would cease.


Hydrogen bonds are easily disrupted by increasing temperature. This, in turn, may disrupt the shape of the enzyme so that its affinity for its substrate diminishes. The ascending portion of the temperature curve (red arrow in right-hand graph above) reflects the general effect of increasing temperature on the rate of chemical reactions (graph at left). The descending portion of the curve above (blue arrow) reflects the loss of catalytic activity as the enzyme molecules become denatured at high temperatures.

Answer 2

enzymes do bind their substrate, however, they actually bind the transition state molecule even better. this stabilization of the transition state is what lowers the activation energy and increases the rate of reaction. The affinity of enzymes for the transition state is what lead to the development of transition state analogues for use as inhibitors. Try searching for transition state theory on wikipedia or google or something.