metabolism- includes all the chemical reactions in an organism

• these chemical reactions are ordered into metabolic pathways

  • sequence of steps, each controlled by an enzyme

    • products of one step become the reactants of the next step

• enzymes help regulate the rate (or speed) of metabolic pathways

two types of metabolic pathways

• catabolic

  • breakdown pathways

  • complex molecule to simpler compound

  • spontaneous

  • exergonic

  • release energy (kinetic)

  • downhill reactions

  • examples:

    • cellular respiration and hydrolysis of starch into glucose

• anabolic

  • synthesis pathways

  • simple compound to complex molecule

  • non-spontaneous

  • endergonic

  • consume (absorb) energy (potential)

  • uphill reaction

  • examples

    • photosynthesis and synthesis of protein from amino acids

• the energy released from catabolic pathways drives the anabolic pathways in a cell

laws of thermodynamics (energy transformations)

• an organisms metabolism transforms matter and energy

  • subject to the laws of thermodynamics

• in an open system, energy and matter may be exchanged between the system and its surroundings (organism and environment)

• 1st law of thermodynamics

  • principle of conservation of energy

    • energy can be neither created or destroyed

    • total energy of the universe is constant

• 2nd law of thermodynamics

  • entropy (disorder)

    • every energy transformation or transfer results in increasing entropy (disorder)

    • as energy is transferred or transformed more energy is wasted (unavailable for work)

      • some energy converted to heat in all chemical reactions

free energy and metabolism

• free energy (G)- the portion of a systems energy available to perform work when the systems temperature and pressure are uniform

• free energy equation- see in powerpoint notes

exergonic reactions (energy outward)

• spontaneous reaction in which there is a net release of free energy

  • see powerpoint/BILL for graphic

endergonic reactions (energy inward)

• non spontaneous reactions in which free energy is absorbed from the surroundings

  • see powerpoint/BILL for graphic

equilibrium and metabolism

• reactions in an isolated system will naturally move toward equilibrium

  • not the same as homeostasis

• cells are never at equilibrium because of the constant flow of materials in and out of the cell

  • steady supply of reactants

  • constant flow of energy

• living organisms do not work in isolation

• living organisms are open systems

• a cell at equilibrium is a dead cell

hydrolysis of ATP to ADP

• ATP is the energy source used to drive most types of cellular work

  • chemical, transport, and mechanical

• the bond between the final two phosphate groups is broken by hydrolysis

• the breaking of the final phosphate bond produces ADP, inorganic phosphate, and energy

  • because hydrolysis of ATP to ADP releases energy considered exergonic

regeneration of ATP from ADP and Pi

• cells regenerates ATP very quickly through the process of cellular respiration

  • human muscle cell @ 10 million molecules per second per cell

enzymes that are biological catalysts

• catalyst- a substance that can change the rate of a reaction without being altered in the process

• enzyme- a protein catalyst that changes the rate of a reactions without being consumed

  • speeds up rate of metabolic reactions but are unchanged by the actions

activation energy

• in chemical reactions- bonds are broken and bonds are formed

  • AB+CD to AC+BD

  • reactants must absorb energy from their surroundings for their bonds to break and energy is released when new bonds are formed to make products

• activation energy (EA)- energy needed initially to reach transition state, in which bonds can be broken and from which the reaction can proceed

  • activation energy is often supplied in the form of heat (in chemistry)

  • (in biology) high temps denature proteins and kill cells

  • heat would speed up all reactions not just those that are needed

  • instead of heat organisms use enzymes to catalyze metabolic reactions

how do enzymes speed up a reactions?

• enzymes lower the activation energy

  • allow reaction to begin sooner

  • allow reaction to proceed more quickly

• enzymes do not change the G for a reactions

• enzymes only change the rate at which a reaction occurs

• enzymes are not consumed in a reactions (enzymes are reused)

enzyme structure and specificity

• protein enzymes are macromolecules with characteristic three dimensional shapes

  • result in their being very specific- each enzymes works on one specific reaction

• substrate- the reactant that an enzyme acts on

• active site- region on an enzyme where the substrate binds

  • pocket or groove found on surface of enzyme that has a shape compatible to that of the substrate

  • forms enzyme-substrate complex

• the enzyme works by binding itself to the substrate

• biochemists have discovered and named more than 4000 different enzymes in various species

enzyme reaction rates

• one enzyme can act on 1000 substrate molecules per second

• rate at which an enzyme works depends on initial concentration of substrate

  • the more substrate molecules there are, the more frequently they will bind to the active site of available enzyme (higher rate of reaction)

• when the concentration of substrate is high enough that all of the enzymes active sites are engaged the enzyme is said to be saturated

  • when enzymes are saturated the only way to increase the reaction rate would be to add more enzyme

effects of local conditions on enzyme activity

1. temperature- each enzyme has an optimal termperature at which its reaction rate is the fastest

   • what happens to the enzyme as the temperature moves beyond the optimal temperature?

     • the heat breaks bonds in the enzyme, denaturing it and causing it to be inactive

2. pH- enzymes have a specific pH in which they are most active

   • the optimal pH for most enzymes is between 6 and 8

   • what happens to the enzyme when the pH drops below or goes above the optimal pH?

     • the enzyme is denatured

cofactors and coenzymes

• many enzymes are not able to work unless they have the help of another molecule

• cofactors- any non-protein molecule that is required for the proper functioning of an enzyme

  • example: minerals such as iron or zinc

• if a cofactor is an organic molecule, it is more specifically called a coenzyme

  • example: vitamins

regulation of enzyme activity

• the cells metabolic pathways cannot all operate simultaneously (overload)

• to regulate metabolic pathways the cell

  • switches on and off the genes that encode the specific enzymes (genetics) or

  • regulates the activity of enzymes once they are made

enzyme inhibitors

• competitive inhibitors- molecules that resemble the normal substrate and compete to bind to the active site

  • resembles/mimics normal substrate

  • competitive inhibitors bind directly to the active site

  • overcome by increasing concentration of substrate

• noncompetitive inhibitors- inhibitors that bind to the other part of the enzyme causing it to change shape so the substrate can’t bind the the active site

  • active site becomes non-functional

  • also called allosteric inhibitors

  • inhibitor binds to allosteric site

feedback inhibition

• another way in which metabolic pathways are regulated

• end product shuts down pathways by binding to allosteric site

  • prevents wasting of chemical resources

  • increases efficiency of cell

• ATP can act as a feedback inhibitor