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Energy and Enzymes

ATP (Adenosine Triphosphate)

The energy in one glucose molecule is used to produce 36 ATP.  ATP has approximately the right amount of energy for most cellular reactions.

ATP is produced and used continuously. The entire amount of ATP in an organism is recycled once per minute. Most cells maintain only a few seconds supply of ATP.

ATP is a Nucleotide

Nucleotides are the building blocks of nucleic acids such as DNA and RNA.  They contain a nitrogen-containing base, a 5-carbon sugar, and a phosphate group.

ATP is a nucleotide that contains adenine (base), ribose (sugar), and three phosphate groups.

 

The phosphate bonds are high-energy bonds. Energy is required to form the bonds and energy is released when the bonds are broken.

ATP is continually produced and consumed as illustrated below.

Formation of ATP

Phosphorylation refers to the chemical reactions that make ATP by adding Pi to ADP:

ADP + Pi + energy -->  ATP + H2O

Most ATP is made in the mitochondrion and requires oxygen. When glucose is broken down, hydrogen atoms are removed from the glucose molecule and transferred to the mitochondrion. Within the mitochondrion, energy is released as hydrogen atoms from glucose and electrons from hydrogen are transferred from compound to compound and eventually to oxygen to form water. The energy that is released is used to phosphorylate ADP forming ATP. the equation for cellular respiration is below.

C6H12O6 + 6CO2 + 6H2O + 36 to 38 ATP

Smaller amounts of ATP can be produced by pathways that do not require oxygen. These pathways, called fermentation, result in the production of lactic acid in animals or alcohol and CO2 in plants.

Energy and Chemical Reactions

The energy-related reactions within cells generally involve the synthesis or the breakdown of complex organic compounds.

Some reactions consume energy while synthesizing compounds because energy is required to form chemical bonds. Energy consumed by the reaction is stored in the chemical bond.

Energy is released when chemical bonds are broken.

 

In either kind of reaction, additional energy must be supplied to start the reaction. This energy is the activation energy.

Enzymes

Catalysts are substances that speed up chemical reactions.  Organic catalysts are called enzymes.

Enzymes are specific for one particular reaction or group of related reactions.

Many reactions cannot occur without the correct enzyme present.

They are often named by adding "ase" to the name of the substrate. Example: Dehydrogenases are enzymes that remove hydrogen.

Induced-Fit Theory

An enzyme-substrate complex forms when the enzyme’s active site binds with the substrate like a key fitting a lock. 

The shape of the enzyme must match the shape of the substrate. Enzymes are therefore very specific; they will only function correctly if the shape of the substrate matches the active site.

The substrate molecule normally does not fit exactly in the active site. This induces a change in the enzymes conformation (shape) to make a closer fit.

In reactions that involve breaking bonds, the inexact fit puts stress on certain bonds of the substrate. This lowers the amount of energy needed to break them.

The enzyme does not form a chemical bond with the substrate. After the reaction, the products are released and the enzyme returns to its normal shape.

Because the enzyme does not form chemical bonds with the substrate, it remains unchanged. As a result, the enzyme molecule can be reused. Only a small amount of enzyme is needed because they can be used repeatedly.

Activation Energy and Enzymes

The amount of activation energy that is required is considerably less when enzyme is present.

Conditions that Affect Enzymatic Reactions

Rate of Reaction

Reactions with enzymes are up to 10 billion times faster than those without enzymes. Enzymes typically react with between 1 and 10,000 molecules per second. 

Fast enzymes catalyze up to 500,000 molecules per second.

Substrate concentration, enzyme concentration, Temperature, and pH  affect the rate of enzyme reactions.

Substrate Concentration

At lower concentrations, the active sites on most of the enzyme molecules are not filled because there is not much substrate.  Higher concentrations cause more collisions between the molecules.  With more molecules and collisions, enzymes are more likely to encounter molecules of reactant.

The maximum velocity of a reaction is reached when the active sites are almost continuously filled. Increased substrate concentration after this point will not increase the rate.  Reaction rate therefore increases as substrate concentration is increased but it levels off.

Enzyme Concentration

If there is insufficient enzyme present, the reaction will not proceed as fast as it otherwise would because there is not enough enzyme for all of the reactant molecules.

As the amount of enzyme is increased, the rate of reaction increases. If there are more enzyme molecules than are needed, adding additional enzyme will not increase the rate. Reaction rate therefore increases as enzyme concentration increases but then it levels off.

Temperature

Higher temperature causes more collisions and therefore increases the rate of a reaction. More collisions increase the likelihood that substrate will collide with the active site of the enzyme.

Above a certain temperature, activity begins to decline because the enzyme begins to denature.

The rate of chemical reactions therefore increases with temperature but then decreases.

pH

Each enzyme has an optimal pH.

A change in pH can alter the ionization of the R groups of the amino acids. When the charges on the amino acids change, hydrogen bonding within the protein molecule change and the molecule changes shape. The new shape may not be effective.

The diagram below shows that pepsin functions best in an acid environment. This makes sense because pepsin is an enzyme that is normally found in the stomach where the pH is low due to the presence of hydrochloric acid. Trypsin is found in the duodenum, and therefore, its optimum pH is in the neutral range to match the pH of the duodenum.

Metabolic Pathways

Metabolism refers to the chemical reactions that occur within cells. A hypothetical metabolic pathway is shown below.

Reactions occur in a sequence and a specific enzyme catalyzes each step.

Intermediates can be used as starting points for other pathways. For example, "C" in the diagram above can be used to produce "D" but can also be used to produce "F".

Photosynthesis

All organisms require energy for their chemical reactions. These reactions may be involved with reproduction, growth, or other activities. Photosynthetic organisms such as plants use light energy to produce carbohydrate (glucose). Glucose can be used at a later time to supply the energy needs of the cell. Photosynthesis is therefore a process in which the energy in sunlight is stored in the bonds of glucose for later use. 

Equation for Photosynthesis

This experiment was published by Jan Baptisa van Helmont in 1648:

"...I took an earthenware vessel, placed in it 200 pounds of soil dried in an oven, soaked this with rainwater, and planted in it a willow branch weighing 5 pounds. At the end of five years, the tree grown from it weighed 169 pounds and about 3 ounces. Now, the earthenware vessel was always moistened (when necessary) only with rainwater or distilled water, and it was large enough and embedded in the ground, and, lest dust flying be mixed with the soil, an iron plate coated with tin and pierced by many holes covered the rim of the vessel. I did not compute the weight of the fallen leaves of the four autumns. Finally, I dried the soil in the vessel again, and the same 200 pounds were found, less about 2 ounces. Therefore 169 pounds of wood, bark, and root had arisen from water only."

Most of the weight of the tree described in the above experiment came from carbon dioxide and water. The equation for photosynthesis shows that these compounds are used to produce glucose.

6CO2 + 6H2O + Energy --> C6H12O6 + 6O2

Cellular Respiration

Cellular respiration allows organisms to use (release) energy stored in the chemical bonds of glucose (C6H12O6). The energy in glucose is used to produce ATP. Cells use ATP to supply their energy needs. Cellular respiration is therefore a process in which the energy in glucose is transferred to ATP.

In respiration, glucose is oxidized and thus releases energy. Oxygen is reduced to form water.

The carbon atoms of the sugar molecule are released as carbon dioxide (CO2).

The complete breakdown of glucose to carbon dioxide and water requires two major steps: 1) glycolysis and 2) aerobic respiration.  Glycolysis produces two ATP. Thirty-four more ATP are produced by aerobic pathways if oxygen is present.

In the absence of oxygen, fermentation reactions produce alcohol or lactic acid but no additional ATP.