Chapter 8 An Introduction to Metabolism

Metabolism  Def’n: the totality of an organism’s chemical processes  Concerned with managing the material and energy resources of a cell

Catabolism vs. Anabolism  Catabolic Pathways: Release energy by breaking down complex molecules to simpler ones Example: cellular respiration  Anabolic Pathways: Consume energy to build complicated molecules from simpler ones Example: synthesis of a protein from AAs

Bioenergetics  Def’n: The study of how organisms manage their energy resources  Energy = the capacity to do work Kinetic energy: the energy of motion Potential energy: stored energy

Laws of Thermodynamics  1 st Law of Thermodynamics: Energy can be transferred and transformed, but it can be neither created nor destroyed.  2 nd Law of Thermodynamics: Every energy transfer or transformation increases the entropy (randomness) of the universe.

Energy  Spontaneous process A change that can occur without outside help  Free energy The portion of a system’s energy that can perform work when temperature is uniform throughout the system, as in a living cell Called “free” because it is available to do work

Exergonic vs. Endergonic Reactions  Exergonic Reactions Proceed with a net release of free energy “downhill” Occur spontaneously Example: cellular respiration  Endergonic Reactions Reactions that absorb free energy from their surroundings “uphill” Do NOT occur spontaneously Example: photosynthesis

Exergonic vs. Endergonic Reactions

ATP  All cellular work requires ATP!  Triphosphate tail is unstable 3 phosphate groups attached to the ribose Bonds between phosphate groups are unstable and are broken by hydrolysis ADP + P is more stable than ATP  Therefore, this is a downhill (exergonic) reaction!  ATP has to be regenerated… Cellular respiration!

ATP

Enzymes  Enzymes Catalytic proteins Catalyst = a chemical agent that changes the rate of a reaction without being consumed by the reaction

Enzymes  Activation Energy The energy required to break the bonds in the reactant molecules Heat helps… but cannot be used in cells Why not?  Enzymes can only speed up reactions that would eventually occur anyways

Selectivity of Enzymes  Enzymes are VERY selective  Substrate = the reactant an enzyme acts on Enzyme binds to the substrate(s) and converts the substrate(s) to the product(s)  Enzymes can distinguish its substrate from closely related compounds so each type of enzyme catalyzes a particular reaction

Enzyme Structure  Active site: The part of the enzyme that actually binds to the substrate  Induced Fit: As a substrate enters the active site, it induces the enzyme to change its shape slightly so that the active site fits even more snugly around the substrate

Enzymes

 How they speed up a reaction… Put 2 substrates together (blind date) Create a microenvironment conducive to a particular reaction (ie. Low pH)

Enzyme Rate of Reaction  Rate of Reaction: Partly a function of the initial concentration of a substrate  More substrate = more frequently access active sites of enzyme  There is a limit to this… Sometimes all enzymes are “busy” Enzyme is said to be “saturated” Every enzyme has an optimal temperature and pH level

Enzymes  Cofactors: Nonprotein helpers required along with enzymes for catalytic activity  zinc, iron, copper, vitamins, etc.

Enzymes  Enzyme Inhibitors: Certain chemicals selectively inhibit the action of specific enzymes Competitive inhibitors:  Mimics that compete with normal substrate molecules for admission into the active site Noncompetitive inhibitors:  Impede enzymatic reactions by binding to a part of the enzyme away from the active site, thereby making the enzyme change its shape

Enzymes  Allosteric site: A specific receptor site on some part of the enzyme molecule remote from the active site to which the molecules that naturally regulate enzyme activity bind Act as valves that control rates of key reactions in metabolic pathways On/Off switch

Enzymes  Cooperativity One substrate molecule primes an enzyme to accept additional substrate molecules Favorable conformational change in one subunit of an enzyme changes all the others too

Enzymes  Feedback Inhibition: A metabolic pathway is switched off by its end product, which acts as an inhibitor of an enzyme within the pathway Thermostat