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Catabolic and Anabolic Reactions
The energy-producing reactions within cells generally involve the breakdown of complex organic compounds to simpler compounds. These reactions release energy and are called catabolic reactions.
Anabolic reactions are those that consume energy while synthesizing compounds.
ATP produced by catabolic reactions provides the energy for anabolic reactions. Anabolic and catabolic reactions are therefore coupled (they work together) through the use of ATP.
Diagram: next slide
Enzymes
Catalysts are substances that speed up chemical reactions. Organic catalysts (contain carbon) 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 – Most current
An enzyme-substrate complex forms when the enzyme’s active site binds with the substrate like a key fitting a lock.
The substrate molecule does not fit exactly in the active site. This induces a change in the enzymes conformation (shape) to make a closer fit.
After the reaction, the products are released and the enzyme returns to its normal shape.
Only a small amount of enzyme is needed because they can be used repeatedly.
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.
Effect of Temperature on Enzyme Activity
Temperature
Higher temperature causes more collisions between the atoms, ions, molecules, etc. It therefore increases the rate of a reaction – “Turnover Rate”. 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 (unfold).
The rate of chemical reactions therefore increases with temperature but then decreases.
Denaturation
If the hydrogen bonds within an enzyme are broken, the enzyme may unfold or take on a different shape. The enzyme is denatured.
A denatured enzyme will not function properly because the shape of the active site has changed.
If the denaturation is not severe, the enzyme may regain its original shape and become functional.
The following will cause denaturation:
Heat
Changes in pH
Heavy-metal ions (lead, arsenic, mercury)
Alcohol
UV radiation
Effect of pH on Enzyme Activity
pH
Each enzyme has an optimal pH. Pepsin, an enzyme found in the stomach, functions best at a low pH. Trypsin, found in the intestine, functions best at a neutral 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 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 (small intestine), 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.
Reactions occur in a sequence and a specific enzyme catalyzes each step.
Metabolic Pathways
A Cyclic Metabolic Pathway
Regulation of Enzymes
The next several slides illustrate how cells regulate enzymes. For example, it may be necessary to decrease the activity of certain enzymes if the cell no longer needs the product produced by the enzymes.