Topic 4. Metabolism September 28, 2005 Biology 1001

4.1 Introduction to Metabolism Metabolism is the totality of an organism’s chemical reactions Reactions are arranged in metabolic pathways or series where each step is catalyzed by an enzyme Metabolic pathways are either catabolic or anabolic  Catabolic pathways are degradative, and release energy  Anabolic pathways consume energy to build larger molecules from smaller ones

Chemistry & Thermodynamics Bioenergetics is the study of how organisms manage their energy resources Energy is the capacity to cause change. Forms of energy include  Kinetic – the energy of motion  Thermal – the random kinetic energy of atoms or molecules  Potential – the energy of location or structure  Chemical – the potential energy of molecules during a chemical reaction Thermodynamics is the study of energy transformation  The first law of TD - the energy of the universe is constant  The second law of TD – energy transformation increases the entropy or randomness of the universe Living organisms increase entropy despite biological order

Chemistry & Thermodynamics PE & KE

4.2 - ATP Powers Cellular Work Work involves using energy to move matter against opposing forces Organisms perform three kinds of work  Mechanical, transport, & chemical To do work organisms couple endergonic reactions with exergonic ones, a process called energy coupling The molecule used to power cellular work by energy coupling is adenosine triphosphate - ATP

ATP Figure 8.8 The Structure of ATP Figure 8.9 ATP Hydrolysis An exergonic reaction A “high-energy” bond

Energy Coupling The exergonic hydrolysis of ATP is coupled to an endergonic reaction by way of a phosphorylated intermediate

The Regeneration of ATP The ATP Cycle  ADP gets phosphorylated back to ATP using energy released by catabolic reactions in the cell and inorganic phosphate  This shuttling of inorganic phosphate and energy from ATP to an intermediate and back to ATP is the ATP cycle Fig. 8.12

4.4 Enzymes and Metabolism (We will do Topic 4.3 with Topic 9) A catalyst is a chemical agent that speeds up a reaction without itself being consumed An enzyme is a protein that functions as a biological catalyst Both exergonic and endergonic reactions require enzymes to occur at appropriate rates in the cell This is because of the activation energy required to get a reaction started **Enzymes lower the activation energy barrier and allow reactions to proceed more quickly**

How Enzymes Work Figures 8.14 & 8.15

How Enzymes Work - The Catalytic Cycle Figure The active site and the catalytic cycle of an enzyme

Features of the Catalytic Cycle The reactant that an enzyme acts on is called a substrate The enzyme binds to the substrate forming an enzyme- substrate complex Only a restricted part of the enzyme usually binds to the substrate – the active site Binding to the active site may involve induced fit Enzymes use a variety of mechanisms to lower activation energy When the products are released the enzyme is free to enter another cycle

More About Enzymes Enzymes are substrate- and reaction-specific Enzymes catalyze reactions in both directions, depending on the relative concentration of reactants vs. products Enzymes do not change the overall energy of the reaction Enzymes may require cofactors, called coenyzmes if they are organic molecules Enzymes are affected by environmental factors such as temperature and pH

Temperature and pH affect enzyme activity Each enzyme has an optimal temperature at which its reaction rate is greatest, one that allows the greatest number of molecular collisions but does not denature the enzyme  o C for humans, 70 o C for prokaryotes living in hot springs Similarly, enzymes operate best at their optimal pH, usually between 6 and 8