Chapter 6: Energy and Enzymes

1)      Cells and Energy

a)      Energy: capacity to do work.  The ultimate source of energy for all living things is the SUN.

b)      First law of thermodynamics (also called the law of conservation of energy)

i)        Energy cannot be created or destroyed, but it can be changed from one form to another.

ii)       In an ecosystem, solar energy is converted to chemical energy by the process of photosynthesis; some of the chemical energy in the plant is converted to chemical energy in an animal, which in turn can become mechanical energy or heat loss.

iii)     Neither the plant nor the animal create energy, they convert it from one form to another.

iv)     Likewise, energy is not destroyed; some becomes heat that dissipates into the environment.

c)      Second law of thermodynamics

i)        Energy cannot be changed from one form into another without a loss of usable energy.

ii)       Heat is a form of energy that dissipates into the environment; heat can never be converted back to another form of energy.

2)      Metabolic Reactions and Energy Transformation

a)      Metabolism is the sum of all the biochemical reactions in a cell.

i)        In the reaction A + B = C + D, A and B are reactants and C and D are products.

ii)       Free energy (DG) is the amount of energy that is free to do work after a chemical reaction

iii)     Change in free energy is noted as DG; a negative DG means that products have less free energy than reactants; the reaction occurs spontaneously.

(1)   Exergonic reactions have a negative DG and energy is released.

(2)   Endergonic reactions have a positive DG; products have more energy than reactants; such reactions can only occur with an input of energy.

b)      The role of ATP

i)        Adenosine triphosphate (ATP) is the energy currency of cells; when cells need energy, they “spend” ATP.

(1)   When ATP is converted into ADP + P, the energy released is sufficient for biological reactions with little wasted.

(2)   When one phosphate group is removed, about 7.3 kcal of energy is released per mole.

ii)       ATP can have any of three functions.

(1)   Chemical Work: ATP supplies energy to synthesize molecules that make up the cell.

(2)   Transport Work: ATP supplies energy to pump substances across the plasma membrane.

(3)   Mechanical Work: ATP supplies energy needed to perform mechanical processes (e.g., muscle contraction, propel cilia, etc.).

iii)     Coupled Reactions

(1)   A coupled reaction occurs when energy released by an exergonic reaction is used to drive an endergonic reaction.

(a)    ATP breakdown is often coupled to cellular reactions that require energy.

(b)   ATP supply is maintained by breakdown of glucose during cellular respiration.

(c)    Only 39% of the chemical energy of glucose is transformed into ATP; 61% is lost as heat

c)      The role of enzymes

i)        A metabolic pathway is an orderly sequence of linked reactions; each step is catalyzed by a specific enzyme.

(1)   Metabolic pathways begin with a particular reactant, end with a particular end product(s), and may have many intermediate steps.

(2)   In many instances, one pathway leads to the next; since pathways often have one or more molecules in common, one pathway can lead to several others.

(3)   Metabolic energy is captured more easily if it is released in small increments.

ii)       Enzymes are catalysts that speed chemical reactions without the enzyme being affected by the reaction.

(1)   Every enzyme is specific in its action and catalyzes only one reaction or one type of reaction.

(2)   A reactant is the substance that is converted into a product by the reaction; often many intermediate steps occur.

(3)   A substrate is a reactant for an enzymatic reaction.

(4)   Enzymes work by lowering the energy of activation for a reaction

(a)    Energy of activation is the energy that must be added to a reaction to cause it to occur.

(b)   For metabolic reactions to occur in a cell, an enzyme is needed to lower the energy of activation so that the reaction will occur.

(5) Enzymes often names for substrates and adding –ase. Ex. Lipase

iii)     Enzyme-Substrate Complex

(1)   Enzymes lower activation energy by forming enzyme-substrate complex

(2)   Enzymes bind to substrates (reactants) at the active site.

(a)    Active site is the a small region on the surface of the enzyme

(b)   This causes a change in shape of the active site that facilitates the reaction in some way

(3)   Most enzymes are not consumed in reaction so they can be reused

(4)   A particular reactant may produce more than one type of product depending on what enzymes are involved.

iv)     Factors Affecting Enzymatic Speed

(1)   Substrate concentration.

(a)    Because molecules must collide to react, enzyme activity increases as substrate concentration increases; the increase in substrate concentration increases the likelihood that the substrate and enzyme will collide; as more substrate molecules fill active sites, more product is produced per unit time. Within limits.

(2)   Temperature

(a)    As temperature rises, enzyme activity increases because there are more enzyme-substrate collisions. Within limits.

(3)   pH

(a)    Enzyme activity declines rapidly when enzyme is denatured at a certain temperature, due to a change in shape of the enzyme.

(b)   Every enzyme has optimal pH at which its rate of reaction is optimal.

(4)   Enzyme concentration

(a)    The amount of active enzyme can regulate the rate of an enzymatic reaction.  In much the same way that the substrate concentration does.

(b)   Cells can activate specific genes when certain enzymes are needed, which can increase the production and therefore presence of these enzymes.

(5)   Enzyme Cofactors

(a)    Many enzymes require an inorganic ion or non-protein cofactor to function.Inorganic cofactors are ions of metals.

(b)   A coenzyme is an organic cofactor, which assists the enzyme (i.e., it may actually contribute atoms to the reaction).

(c)    Vitamins are small organic molecules required in trace amounts for synthesis of coenzymes; they become part of a coenzyme’s molecular structure; vitamin deficiency causes a lack of a specific coenzyme and therefore a lack of its enzymatic action.

(d)   Phosphorylation of enzymes occurs when signal proteins turn on kinases, which then activate specific enzymes; some hormones use this mechanism.

(6)   Enzyme inhibition occurs when a substance (called an inhibitor) binds to an enzyme and decreases its activity; normally, enzyme inhibition is reversible.

(a)    In competitive inhibition, the substrate and the inhibitor are both able to bind to the enzyme’s active site.

(b)   In noncompetitive inhibition, the inhibitor binds to the enzyme at a location other than the active site (the allosteric site), changing the shape of the enzyme and rendering it unable to bind to its substrate.

d)      Oxidation-Reduction reactions: electrons pass from one molecule to another.

i)        Oxidation is the loss of electrons.

ii)       Reduction is the gain of electrons.

iii)     Both reactions occur at the same time because one molecule accepts electrons given up by another molecule.