energetics and metabolism biology 1

The chemistry of life is organized into metabolic pathways Organisms transform energy The energy of transformation are subject to the Laws of Thermodynamics Organisms live at the expense of free energy Cellular work is driven by ATP Enzymes act as catalysts to reactions How Enzymes work, and how they are controlled

The chemistry of life = metabolism Metabolism is the sum total of an organism’s chemical processes, requiring management of –Materials –Energy Metabolic pathways –Catabolic pathways break down complex macromolecules to simpler products, releasing energy –Anabolic pathways utilize energy to construct complex macromolecules from simpler ones –Anabolism feeds catabolism, and visa-versa Energy + H CO 2 CH 2 O + O 2 anabolic catabolic

Organisms transform energy... Energy = capacity to do work Potential energy; energy inherent to matter due to location or arrangement Kinetic energy; energy in the process of doing work through motion For example: Sun’s Energy C 6 H 12 O 6 (plants) Transformation via photosynthesis, using CO 2, H 2 0 (kinetic energy) (chemical energy)

Energy transformation conforms to the Laws of Thermodynamics 1st Law: Energy can neither be destroyed or created 2nd Law: Energy transformations occur so as to maximize entropy Organisms can be considered open systems. Although metabolic processes may be considered to decrease entropy within the system, the net change to the system and its surroundings should reflect an increase in entropy

Organisms live at the expense of free energy Not all of a systems energy is available to do work. The energy that is available is termed free energy (G), where G = H - TS H = total energy T = absolute temperature S = Entropy  TS = energy not available to do work Amount of energy harvested from a reaction is equal to change in free energy from start to end of reaction  G =  H - T  S Certain reactions occur spontaneously (don’t need energy). Thus in these cases,  G < 0

Free energy and metabolism e.g., 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2  G = kJ/mol  G = kJ/mol

How does the cell harness energy ATP (adenosine triphosphate) is the immediate source of energy that drives cellular work –Mechanical work (cilia, flagella, cytoplasmic flow, etc.) –Transport work (pumps) –Chemical work (anabolism) ATP is a nucleotide with unstable phosphate bonds. Hydrolysis of a phosphate group is an exergonic reaction that releases energy ATP + H 2 O ADP + P i  G = -55 kJ/mol

ATP Provides energy through phosphorylation (addition of temporary phosphate group to reactant forming transitional compound) Is continually being regenerated (10 7 /sec). Regeneration is endergonic, needing energy (from cellular respiration)

The role of enzymes in metabolism Enzymes act as catalysts Catalysts act to lower the activation energy of a reaction Activation energy of a reaction represents the energy needed to initially break chemical bonds in reactants Addition of activation energy (E A ) raises free energy of reaction to a transitional state As new bonds form, free energy is released.  G for the reaction is the difference in free energy between the reactants and products

Enzymes as catalysts Very selective to a specific substrate, as dictated by three-dimensional structure of protein Substrate binds to enzyme’s active site with weak hydrogen or ionic bonds (lock-and-key hypothesis), inducing a change shape of enzyme –Active site may hold two or more reactants in place so that they may react –Induced fit of enzymes active site may distort reactant’s chemical bonds, weakening them –Active site may provide localized micro-environment conducive to reaction

Enzyme activity Found freefloating in cytoplasm or bound within a membrane or organelle Activity is maximal under certain conditions of temperature (35°-40°C), pH (6-8), and substrate concentration (up to saturation) Some enzymes require a co-factor (inorganic [Zn, Fe, Cu] or organic [co- enzymes, vitamins]) that binds to enzyme to induce correct shape

Enzymes can be inhibited either reversibly or irreversibly –Competitive inhibitors block the active site form the substrate by binding with it themselves –Non-competitive inhibitors bind to sites other than the active site on the enzyme, changing its shape so that is no longer specific to original reaction (DDT, antibiotics)

Regulation of enzyme activity Allosteric enzymes use non-competitive inhibitors that bind to sites that activate or inhibit the enzyme –Binding of an activator to an allosteric site activates the enzyme –Binding of an inhibitor to an allosteric site inhibits the enzyme Another common method of control is through feedback inhibition ThreonineABCDisoleucine Enzyme 1Enzyme 2Enzyme 3Enzyme 4Enzyme 5 Feedback inhibition Initial substrate Endproduct and allosteric inhibitor of enzyme 1