Enzymes B11
• Reference: chapter 5 of your text
• Quiz Wed March 31
• Test Thurs April 1 st
• Very short unit, builds on what you already know
• Pay close attention to the study guide you were given (with answer key)

Metabolic Reactions and
Energy Transformations
• Metabolism is the sum of all the chemical reactions that occur in a cell.
A + B C + D
(reactants) (products)

6.2 Metabolic Reactions and
Energy Transformations
• ATP: Energy for Cells
– ATP stands for adenosine triphosphate , the common energy currency for cells.
– ATP is generated from ADP (adenosine diphosphate) + an inorganic phosphate molecule ( P )

The ATP Cycle

Metabolic Reactions and
Energy Transformations
• Structure of ATP
– ATP is a nucleotide that is composed of:
• Adenine (a nitrogen-containing base)
• Ribose (a 5-carbon sugar)
• Three phosphate groups

Metabolic Reactions and
Energy Transformations
• Structure of ATP
– ATP is a “high energy” compound because a phosphate group can easily be removed.

Metabolic Reactions and
Energy Transformations
• Coupled Reactions
– The energy released by an exergonic reaction is used to drive an endergonic reaction.
– Exergonic releases energy, endergonic needs energy “put in”

Coupled Reactions

Metabolic Pathways and
Enzymes
• Metabolic pathways are a series of linked reactions.
– These begin with a specific reactant and produce an end product

Metabolic Pathways and
Enzymes
• Enzymes are usually proteins that function to speed a chemical reaction.
– Enzymes serve as catalysts

A Metabolic Pathway

Metabolic Pathways and
Enzymes
• The Energy of Activation (E a
) is the energy that must be added to cause molecules to react with one another.

Energy of Activation

Metabolic Pathways and
Enzymes
• How Enzymes Function
– Enzyme binds substrate to form a complex
– E + S  ES  E + P

Enzymatic Action

Metabolic Pathways and
Enzymes
• How Enzymes Function
– Enzyme binds substrate to form a complex
– E + S  ES  E + P
– Induced fit model
• Substrate and active site shapes don’t match exactly
• Active site is induced to undergo a slight change in shape to accommodate substrate binding

Induced Fit Model

Metabolic Pathways and
Enzymes
• Factors Affecting Enzymatic Speed
– Substrate Concentration
– Temperature and pH
– Enzyme Activation
– Enzyme Inhibition
– Enzyme Cofactors

Metabolic Pathways and
Enzymes
• Substrate Concentration
• Enzyme activity increases as substrate concentration increases because there are more collisions between substrate and enzyme
• Maximum rate is achieved when all active sites of an enzyme are filled continuously with substrate

Metabolic Pathways and
Enzymes
• Temperature
– Enzyme activity increase as temperature rises
– Higher temperatures cause more effective collisions between enzymes and substrates
– High temperatures may denature an enzyme, inhibiting its ability to bind to substrates

The Effect of Temperature on the Rate of Reaction

Metabolic Pathways and
Enzymes
• pH
• Each enzyme has an optimal pH
• Enzyme structure is pH dependent
• Extremes of pH can denature an enzyme by altering its structure

Effect of pH on the Rate of
Reaction

Metabolic Pathways and
Enzymes
• Enzyme Activation
– Cell regulates metabolism by regulating which enzymes are active
– Genes producing enzymes can be turned on or off to regulate enzyme concentration
– In some cases a signaling molecule is used to activate an enzyme

Metabolic Pathways and
Enzymes
• Enzyme Inhibition
– Occurs when enzyme cannot bind its substrate
– Activity of cell enzymes is regulated by feedback inhibition
– Ex: when product is abundant it binds to the enzyme’s active site and blocks further production
– When product is used up, it is removed from the active site
– In a more complex type of inhibition, product binds to a site other than the active site, which changes the shape of the active site
– Poisons are often enzyme inhibitors

Feedback Inhibition

Metabolic Pathways and
Enzymes
• Enzyme Cofactors
– Molecules which help enzyme function
– Copper and zinc are examples of inorganic cofactors
– Organic non-protein cofactors are called coenzymes
• Vitamins are often components of coenzymes