Chemical reactions, energy and enzymes.

 

Living organisms harvest and use energy in a controlled fashion to do work.

Energy is the capacity to do work. 

 

Work is defined as moving matter against opposing forces such as gravity or fiction.  For example, when you lift a mass (e.g. a box of books) from the floor onto a table you do work because the mass that you lift is moved against the force of gravity.  It takes energy to do that work.  The energy that is used to lift the box of books comes from the breaking of chemical bonds in your body to release energy in a useable form. 

 

Living organisms carry out millions of chemical reactions that involve the making and breaking of chemical bonds to either store or release energy.  Together these controlled chemical reactions that an organism carries out in order to use energy are referred to as metabolism.  The chemical reactions in the body are arranged into orderly metabolic pathways in which molecules (specific arrangements of atoms) are built up or broken apart.  (Bear in mind that if the energy stored in your chemical bonds were to be released in an uncontrolled fashion it would mostly be released in the from of heat (i.e. you’d burst into flames).  Thank goodness for metabolism!).

 

Glucose (C6H12O6) is an example of a molecule.  It is made of 6 carbon, 12 hydrogen and 6 oxygen atoms joined to each other in a specific ring-like arrangement. 

 

Plants manufacture glucose using energy that they obtain from the sun.  This process is called photosynthesis.  Because photosynthesis involves the joining together of less complex molecules (carbon dioxide CO2 and water H2O) to make a more complex one it is an example of a general type of metabolic reaction we call biosynthetic or anabolic. 

 

Metabolic reactions that involve the breaking of a complex molecule into simpler ones we refer to as catabolic.  The process of cellular respiration in which glucose is broken down to produce CO2 and H2O with a release of useable energy (in the form of ATP Adenosine Tri-Phosphate) is an example of a catabolic metabolic pathway.

 

Photosynthesis:

 

6CO2 + 6H2O + energy from the sun à C6H12O6 + 6O2

 

Cellular metabolism:

 

            C6H12O6 + 6O2 à 6CO2 + 6H2O + useable energy in the form of ATP

 

(Note: If you are unfamiliar with atoms and molecules you should read chapter 5 of your text.  You should make sure that you understand what atoms and molecules are and familiarize yourself with the basic concepts in that chapter)

 

 

 

 

Two physical laws govern energy and its use in the universe.  These laws apply to all living systems such as cells.

 

First Law of Thermodynamics or the Law of Conservation of Energy states that the amount of energy in the Universe is constant:   Energy can be transferred or transformed, but it cannot be created or destroyed.

 

The Second Law of Thermodynamics or the Law of Entropy sates that the amount of disorder in the Universe increases:  Every energy transfer or transformation increases the entropy of the Universe.

 

Entropy is a measure of disorder or randomness. To maintain any ordered structure such as a cell a continual input of energy is required because any ordered structure naturally becomes more disordered over time. 

 

Every chemical reaction increases the amount of entropy in the universe because heat is released in the reaction. Heat is a low quality form of energy because it is the uncoordinated random movement of molecules and so cannot easily be harnessed to perform work.

 

Although cells (and other living systems) are more organized (have less entropy) than their surroundings they do not violate the 2nd Law of Thermodynamics because there is a net increase in the amount of entropy in the universe because you have to include the increase in entropy caused by the conversion of some of the sun’s energy from an ordered form (light) to a less ordered form (heat).

 

When a piece of wood is burned the energy contained in the wood is released in uncontrolled combustion as heat.  In metabolic reactions energy release is controlled and instead of all the energy being released at once it is released in a series of small steps in which individual electrons are lost or gained by atoms with the loss or gain of a small amount of energy.

 

The universal energy carried in cells is ATP.  ATP has three high-energy phosphate groups attached to it.  Each phosphate bond in ATP is a high energy bond and when it is broken the energy released can be used to power any number of useful activities in the cell (e.g. to move molecules in and out of cells or compartments or manufacture complex macromolecules).

 

Chemical reactions are controlled by simple energy laws.

 

The reaction

 

A + B à C + D

 

(where A and B are the reactants and C and D the products of the reaction) will tend to proceed in the direction shown because the products C  and D are more stable and contain less stored energy than A  and B.  In a sense the reactions “run downhill” rather as water flows down a slope.

 

Activation Energy

 

However, A and B may not necessarily react just because they are brought into contact.  Some energy may need to provided to jumpstart the reaction. This small amount of energy is referred to as the activation energy.  The activation energy alters the chemical configuration of the reactants enough that the reaction takes place.  Once the activation energy is provided the reaction proceeds and releases much more energy than the activation energy. 

 

As an analogy a match bursts into flame when struck.  The striking provides enough energy (in the form of friction) to cause the chemicals in the match head to react.

 

Enzymes

 

In the body the activation energy needed to make a reaction proceed can be acquired through the random collision of moving molecules.  However, that is too slow a reaction rate to sustain life processes. To make chemical reactions go faster cells use specialized proteins called enzymes. Enzymes work by binding to their reactants and lowering the activation energy required for the two to react. Similarly enzymes can stress chemical bonds and make them easier to break   As an analogy you might think of breaking a chemical bond as being like breaking a stick.  Trying to break it between your hands alone can be difficult but if you put it over your knee (enzyme) it becomes easier.

 

Enzymes are not used up in reaction, they just make it happen faster.  They are catalysts.

 

Enzyme shape determines enzyme activity.  Enzymes have specific shapes that enable them to do their job of making reactions happen faster.  Each enzyme has an active site where its reactants fit and are brought together or where a chemical bond is stressed allowing it to be more easily broken.  The active site fits only the enzymes reactants just as a key fits only a given lock. 

 

Enzymes allow reactions to happen much faster than if the reactions had to depend on the appropriate reactants just bumping into each other.  Carbonic anhydrase an enzyme that catalyses the conversion of carbon dioxide to bicarbonate ions increases  the process by a factor of nearly 10 million times.  Carbon dioxide is carried in the form of bicarbonate ions (HCO3-) in the bloodstream.

 

CO2 + H2O à HCO3- +  H+

 

Many enzymes work in sequence converting one chemical to another through a series of steps is a sequence called a chemical pathway. 

 

One advantage of arranging enzymes in sequence is that the product of one reaction can be immediately converted by the next enzyme in the sequence. 

 

     E1    E2     E3

A à  B à C à D

 

In the above example E1, E2, and E3 are enzymes that catalyze the reactions A à B, Bà C, and Cà D respectively.

 

A challenge for a metabolic pathway is for enzymes and substrates to find each other.  Cells have evolved several approaches to bring the two together efficiently.   For example, enzymes in multi-step chemical pathways are located near each other to increase the odds of the appropriate enzymes and substrates coming together.

 

Enzymes for particular pathways are often contained within a single organelle or part of an organelle.  E.g. the enzymes that are involved in cellular respiration are located in the mitochondrion.  Enzymes for the first stage of the process of respiration (the citric acid cycle) are found in the mitochondrial matrix.  Other enzymes later in the sequence are arranged along the inner mitochondrial membrane in sequence and substrates pass from one to the next. 

 

Another way in which multiple reactions can be made to go faster is to bind enzymes into multi-enzyme complexes in which several enzymes are joined to each other and so are in close proximity to each other which means that the product of one reaction is immediately available for another enzyme to work on it.

 

Building DNA.  High energy ATP and enzymes are used to assemble a DNA molecule from individual nucleotides.  First, two enzymes convert a nucleotide monophosphate into a high energy nucleotide triphosphate by each enzyme adding a phosphate to the nucleotide monophosphate.  The energy stored in the high-energy phosphate bonds on the nucleotide triphosphte is used when these bonds are broken to attach the nucleotide  (now once again a monophosphate) to one strand of the DNA and a third enzyme catalyzes this process.

 

The drug aspirin (salicylic acid) works by blocking two enzymes COX-1 and COX-2.

 

COX-2 produces pain and inflammation after an injury.  COX-1 helps to maintain the lining of the stomach.  Because aspirin affects two enzymes when it is taken to reduce pain it can also cause stomach damage.  Research is being undertaken to reduce aspirin’s effects on the stomach while allowing it to still treat pain.

 

Recent research has shown that aspirin may be effective in treating colon cancer.  Aspiring  appears to help because shutting off the COX-2 enzyme helps block the tumors blood supply.