Enzymes speed up metabolic reactions by lowering energy barriers

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The laws of thermodynamics

the laws of thermodynamics tells us what will and will not happen under given conditions but say nothing about the rate of these process.


an enzyme is a macromolecule that acts as a catalyst.


a chemical agent that speeds up a reaction without being consumed by the reaction.


in the absence of regulation by enzymes, chemical traffic through the pathways of metabolism would become terribly congested because many chemical reactions would take such a long time.

The activation energy barrier

- every chemical reaction between molecules involves both bond breaking and bond forming
- The hydrolysis of sucrose involves breaking the bond between glucose and sucrose and one of the bonds of water molecule and then forming two new bonds.
- when the new bonds of the product molecules form, energy is released as heat, and the molecules return to stable shapes with lower energy than the contoured state.

Activation Energy/free energy of activation

is the initial investment of energy for starting a reaction-the energy required to contort the reactant molecules so the bonds can break.

AB + CD →AC + BD

Hypothetical exergonic reaction that swaps portions of two reactant molecules

Transition State

They are activated, and their bonds can be broken.

Activation energy....

is often supplied in the form of heat that the reactant molecules absorb from the surrounding.
* the reactants must absorb enough energy to reach the top of the activation energy barrier before the reaction can occur.

The effect of an enzyme on activation energy

Hydrocarbons and oxygen will not react because the Eᴬ barrier is too high.

How Enzymes Lower the Eᴬ Barrier

* Proteins, DNA, and other complex molecules of the cell are rich in free energy and have the potential to decompose spontaneously; that is, the laws of thermodynamics favor their breakdown.
* An enzyme catalyzes a reaction by lowering the Eᴬ barrier, enabling the reactant molecules to absorb enough energy to reach the transition state even at moderate temperatures.
* Enzymes can only hasten reactions that would occur eventually anyway, but this function makes it possible for the cell to have a dynamic metabolism, routing chemicals smoothly through the cell's metabolic pathways.

Substrate Specificity of Enzymes

the reaction catalyzed by each enzyme is very specific; an enzyme can recognize its specific substrate even among closely related compounds, such as isomers.
-The specificity of an enzyme results from its shape, which is consequence of its amino acid sequence.


the reactant an enzyme acts on is referred to as the enzyme's substrate.

Enzyme-substrate complex

the enzyme binds to its substrate(or substrates, when there are two or more reactants), forming an enzyme-substrate complex

Active site

only a restricted region of the enzyme molecule actually binds to the substrate.

Induced fit

is like a clasping handshake.

Catalysis in the Enzyme's Active site

In most enzymatic reactions, the substrate is held in the active site by so-called weak interactions, such as hydrogen bonds and ionic bonds.
- R groups of a few of the amino acids that make up the active site catalyzed the conversation of substrate to product, and the product departs from the active site.

Enzymes use a variety of mechanism that lower activation energy and speed up a reaction.

1. First, in reactions involving two or more reactants, the active site provides a template on which the substrate can come together in the proper orientation for a reaction to occur between them.
2. Second, as the active site of an enzyme clutches the bond substrates, the enzyme may stretch the substrate molecules toward their transition-state form, stressing and bending critical chemical bonds that must be broken during the reaction.
- Because Eᴬ is proportional to the difficulty of breaking the bonds, distorting the substrate helps it approach the transition state and thus reduces the amount of free energy that must be absorbed to achieve that state.

Effects of Local Conditions on Enzymes Activity

The activity of an enzyme how efficiently the enzyme functions is affected by general environmental factors, such as temperature and pH.

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