INTRODUCTORY BIOCHEMISTRY MODULE

CHEMICAL REACTIONS AND ENERGETICS
Use these links to navigate to selected parts of this section of the Introductory Biochemistry module.

Chemical reactions
Chemical elements combine with each other via the formation of chemical bonds.
The formation or breaking of chemical bonds is termed a chemical reaction.
In general terms chemical reactions are of two types :
  • synthesis reactions
  • decomposition reactions
Chemical equations use formulae to describe reactions. Examples of chemical equations appear below in the discussion of reaction types.

Synthesis reactions
As an example of a synthesis reaction :

Ca + S -------------------> CaS

1 calcium atom reacts with 1 sulphur atom to give 1 molecule of calcium sulphide

The + sign means "reacts with" or "combines with" and the arrow indicates "to form".
A second more complex example of a synthesis reaction :

2Al + 3S -------------------> Al2S3

2 aluminium atoms react with 3 sulphur atoms to give 1 molecule of aluminium sulphide

The numbers in front of the reactants are called coefficients and specify the proportions of the units involved in the reaction.
(Note that whenever the coefficient is 1 it is not written.)

Decomposition reactions
An example of a decomposition reaction is :

The products of this decomposition reaction have quite different chemical properties to the initial compound.

Energetics
An understanding of the basic principles controlling the energetics of chemical reactions will assist you to understand what is occurring in many chemical processes.
The following statements outline these principles.
  • An exergonic reaction is one in which there is a nett energy yield i.e. energy is released (sometimes as chemical bond energy or sometimes as heat).
  • An endergonic reaction is one in which there is a nett energy input i.e. energy is needed to drive the reaction (again, sometimes as chemical bond energy and sometimes as heat).
  • Both types of reaction, however, have a energy barrier which must be overcome to initiate the reaction - this is termed the activation energy of the reaction (abbreviated to Ea).
  • It is the Ea (and not the overall energy change) which determines the rate of reaction.
  • To initiate a reaction between two molecules, they must collide and "stick" together. This "sticking" forms an activated intermediate which goes on to form products. This minimum collision energy which causes them to stick is the activation energy.
If the rate of molecules colliding and sticking is increased, the rate of reaction is increased.

The rate of collision can be increased generally in three ways :

  • increase the temperature so the molecules are moving at higher speed
  • increase the concentration of molecules so the chance of a collision is increased
  • use a catalyst

What is a catalyst?
A catalyst, when added to a reaction, dramatically increases the rate of reaction.
This is achieved by lowering the activation energy.
Thus proportionally more molecules will overcome the activation energy barrier at any one time and more molecules will react to form product in a given time.

Enzymes
Enzymes are a group of specialised protein molecules which act as biological catalysts. They make it possible for chemical reactions to occur inside cells in the absence of large activation energy input.
They achieve this by lowering the activation energy required for a particular reaction. An essential part of this mechanism is the binding of the enzyme with the reacting molecule or molecules - these are referred to as the substrate or substrates for that enzyme. After binding to the enzyme, the substrate is converted to product. The site on the enzyme at which the substrate binds is referred to as the active site or catalytic centre.
Each enzyme is highly specific for both the substrate bound and the reaction catalysed i.e. there is a specific enzyme for virtually every biological reaction.
A full discussion of the factors affecting enzyme reactions (the study of enzyme kinetics) is beyond the scope of this module. Only a few brief points to aid an understanding of the role of enzymes in cellular metabolism are included.
The rate of an enzyme catalysed reaction is affected by a number of variables.
  • Substrate concentration

    An increase in substrate concentration increases the rate of reaction up to a point at which the rate becomes constant, no matter how much more substrate is added. At this point the enzyme is described as saturated with substrate.

  • Enzyme concentration

    An increase in enzyme concentration increases the rate of reaction in a proportional manner.

  • Time

    The longer time that an enzyme has to catalyse the reaction, the more product will be formed.

  • Temperature of the reaction solution

    An increase in temperature increases the rate of any chemical reaction. In the case of enzymes, which are proteins, there is a limit to this as the enzyme molecules are subject to denaturation at high temperatures i.e. a change in their 3-dimensional shape (tertiary structure) resulting in loss of biological activity. Thus all enzymes have an optimum temperature - a temperature at which the rate of the catalysed reaction is maximal. Most enzymes are no longer active at temperatures above about 65oC.

  • pH of the reaction solution

    The binding of the enzyme and substrate is via weak secondary bonds such as hydrogen bonds and ionic bonds. These interactions are affected by changes in pH. Enzymes all have an optimum pH at which the rate of the catalysed reaction is maximal.

  • Inhibitors and activators

    Various molecules will affect the rate of an enzyme catalysed reaction by binding to the enzyme. Some bind at the same site as the substrate (the active site) and prevent the substrate from binding. Others bind at sites on the enzyme remote from the active site and affect activity by modifying the shape of the enzyme. Many of these molecules reduce the activity of the enzyme and are referred to as inhibitors. Others aid the course of the reaction and are referred to as activators. Many hormones affect the activity of enzymes either by inhibition or activation. In some cases this activation involves covalent modification of an enzyme, often via the addition or removal of phosphate groups from selected amino acids in the enzyme polypeptide chain.

  • Cofactors and coenzymes

    Many enzymes, once folded into their correct tertiary structure, exhibit their full biological activity. Others require an additional component for activity. These additional components are called coenzymes or cofactors. (The two terms are used more or less interchangeably.) The additional requirement may be a metal ion or a small organic molecule. Often they are involved in maintenance of the enzyme's shape, binding of the substrate to the enzyme or participating in the actual reaction which converts substrate to product.

    Vitamins, which are organic molecules required in the diet, are the source of many of the coenzymes required by various enzymes in the body. After the vitamins are absorbed from the food in the gut they are transported to the cells which require them prior to conversion to their active coenzyme form.

That completes the section on chemical reactions and energetics.

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