Chemical Reactions and Enzymes Chapter 8: An Introduction to Metabolism

Chemical Reactions The living cell is a miniature chemical factory where thousands of reactions occur A chemical reaction is the pathway by which atoms or groups of atoms in substances are reorganized into different substances To write a chemical reaction… Place the reactants (the substances reacting) on the left with an arrow pointing to the the products (the substances being formed) ReactantsProducts Make sure your equations are balanced

Metabolism Metabolism is the totality of an organism’s chemical reactions A metabolic pathway begins with a specific molecule and ends with a product Each step is catalyzed by a specific enzyme

Types of Pathways Catabolic pathways release energy by breaking down complex molecules into simpler compounds Example: cellular respiration, the breakdown of glucose in the presence of oxygen Anabolic pathways consume energy to build complex molecules from simpler ones Example: photosynthesis, the formation of glucose using solar energy

Energy Energy is the capacity to cause change (ability to do work) Kinetic energy is associated with motion Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules Potential energy is stored energy (because of its location or structure) Chemical energy is potential energy available for release in a chemical reaction Energy can be converted from one form to another

Thermodynamics Thermodynamics is the study of energy transformations Scientists use the word system to denote the matter being studied A closed system is isolated from its surroundings (no energy exchange) In an open system, energy is exchanged with its surroundings Organisms are open systems

Laws of thermodynamics 1st law of thermodynamics, the energy of the universe is constant Energy cannot be created nor destroyed, only transferred and transformed Also called the principle of conservation of energy

Laws of thermodynamics During every energy transfer or transformation, some energy is unusable, and is often lost as heat 2nd law of thermodynamics Every energy transfer or transformation increases the entropy (disorder) of the universe

Free Energy A living system’s free energy (∆G) is energy that can do work when temperature and pressure are uniform, as in a living cell Measure of a system’s instability Its tendency to change to a more stable state During a spontaneous change, free energy decreases and the stability of a system increases Equilibrium is a state of maximum stability A process is spontaneous and can perform work only when it is moving toward equilibrium

Free Energy & Metabolism Exergonic reactions Net release of free energy (catabolic) Spontaneous Endergonic reaction Absorbs free energy from its surroundings Nonspontaneous

Energy Coupling To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one Most energy coupling in cells is mediated by ATP (adenosine triphosphate) At the molecular level, energy from food is stored in the bonds of ATP

Releasing Energy Breaking off ATP’s third phosphate group forms ADP and releases energy ATP  ADP + P (+ energy) Breaking bonds releases energy The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis

Releasing Energy

Energy Storage Bonding a third phosphate group to ADP (DIphosphate) forms ATP (TRIphosphate) ADP + P  ATP Making bonds stores energy Called: Phosphorylation in mitochondria Photophosphorylation in chloroplast

Technical Term Review Phosphorylation adding a phosphate to ADP ADP + P > ATP Mitochondria ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant The recipient molecule is now phosphorylated Photophosphorylation e- of light energy, instead of from food are used for transport Same result: ADP + P  ATP Chloroplast

Metabolic Reactions Reactions require an input of energy to get started Called free energy of activation, or activation energy (E A ) A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction Enzymes are biological catalysts All enzymes are proteins Enzymes lower the activation energy required Example: Sucrase breaking down sucrose Example

How Enzymes Work 1) Substrate: (the “reactant”) 2) Active site: part of the enzyme where a substrate fits (specifically shaped) Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction 3) Creates an enzyme- substrate complex 4) A chemical reaction takes place (products created or broken apart) 5) Enzyme free to react with another substrate

Enzymes Enzymes tend to be very specific Only recognize certain reactant molecules (substrates) Shape determines which molecules it recognizes Example: Amylase breaks down amylose in starchAmylase breaks down amylose in starch

Enzyme Activity An enzyme’s activity can be affected by General environmental factors Each enzyme has an optimal temperature Each enzyme has an optimal pH Chemicals that specifically influence the enzyme Cofactors are nonprotein enzyme helpers Cofactors may be inorganic or organic An organic cofactor is called a coenzyme Coenzymes include vitamins

Enzyme Inhibitors Competitive inhibitors bind to the active site of an enzyme, competing with the substrate Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective May be Permanent - covalent bond Reversible - hydrogen bond Examples: Nerve gas Penicillin Cyanide Pesticides

Enzyme Inhibitors

Enzyme Regulation Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated Allosteric regulation may either inhibit or stimulate an enzyme’s activity Occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site Each enzyme has active and inactive forms Binding of an activator stabilizes the active form Binding of an inhibitor stabilizes the inactive form In feedback inhibition, the end product of a metabolic pathway shuts down the pathway Prevents a cell from wasting chemical resources by synthesizing more product than is needed

Review Questions 1.Define chemical reaction and identify and describe its major parts. 2.Define metabolism and metabolic pathway. 3.Differentiate between anabolic and catabolic pathways, naming an example of each. 4.Define energy and differentiate between the 4 types discussed in class. 5.Differentiate between open and closed systems. 6.Define thermodynamics and state the 2 laws of thermodynamics. 7.Define free energy and equilibrium. 8.Differentiate between endergonic and exergonic reactions. 9.Explain the idea of energy coupling, and explain the role ATP has. 10.Differentiate between phosphorylation and photophosphorylation. 11.Explain activation energy. 12.Define enzyme and describe how enzymes work using the following terms: active site, substrate, and enzyme-substrate complex. 13.Define various factors that may affect an enzyme’s activity. 14.Differentiate between competitive and noncompetitive inhibitors. 15.Explain enzyme regulation through allosteric regulation and feedback inhibition.