Chapter 8 Notes An Introduction to Metabolism
Describe how allosteric regulators may inhibit or stimulate the activity of an enzyme. Objectives Distinguish between the following pairs of terms: catabolic and anabolic pathways; kinetic and potential energy; open and closed systems; exergonic and endergonic reactions. Explain the laws of thermodynamics Explain how cells obtain the energy to do cellular work. Explain how ATP performs cellular work. Describe how the regulation of enzymes activity helps control metabolism. Explain why activation energy is necessary to initiate a spontaneous reaction. Describe how enzymes lower activation energy.
The Energy of Life The living cell is a chemical factory in miniature, where thousands of reactions occur within a microscopic space. The totality of an organisms chemical reactions is called metabolism. An organism s metabolism transforms matter and energy, subject to the laws of thermodynamics arises from interactions between molecules A metabolic pathway has many steps that begin with a specific molecule and end with a product each step is catalyzed by a specific enzyme
Metabolic Pathway
Metabolic Pathways Catabolic pathways break down complex molecules into simpler compounds release energy Anabolic pathways build complicated molecules from simpler ones consume energy
Forms of Energy Energy is the capacity to cause change. In everyday life, energy is important because some forms of energy can be used to do work. Two types of energy: kinetic energy-energy of motion potential energy-stored capacity to do work energy matter possesses due to location or structure
The Laws of Energy Transformation The study of the energy transformations that occur in a collection of matter is called thermodynamics. Two laws of Thermodynamics: First law of thermodynamics (energy conservation) total amount of energy in universe is constant can be transferred or transformed but cannot be created or destroyed Second law of Thermodynamics every energy transformation increases entropy energy available for doing useful work decreases with every transformation
Free Energy Organisms use free energy free energy is the energy available to do work Free energy is the portion of a system's energy that can perform work when temperature and pressure are uniform throughout the system, as in a living cell. Organisms live at the expense of free energy. During a spontaneous change, free energy decreases and the stability of a system increases. At maximum stability, the system is at equilibrium and can do no work.
Free Energy and Metabolism Based on their free-energy changes, chemical reactions can be classified as either exergonic or endergonic. endergonic reactions require input of energy energy input equals difference in potential energy between reactants and products exergonic reactions release energy energy released equals difference in potential energy between reactants and products cellular metabolism is sum total of all endergonic and exergonic reactions in cells
ATP and Cellular Work Energy coupling is a key feature in the way cells manage their energy resources to do this work A cell does three main kinds of work mechanical transport chemical
ATP (adenosine triphosphate) is the cell s energy shuttle provides energy for cellular functions most cell reactions require small amounts of energy food storage molecules contain large amounts of energy energy in food molecules is converted to energy in ATP one food molecule=many ATP (1 glucose=36 ATP)
Hydrolysis of ATP releases energy terminal covalent bonds between outer phosphate groups are energy rich and easily hydrolyzed forms ADP and phosphate group
How ATP drives Cellular Work ATP drives endergonic reactions By phosphorylation, transferring a phosphate to other molecules Phosphorylation of a protein usually results in the protein changing shape dephosphorylation (removal of the phosphate) allows protein to return to original shape phosphorylation-dephosphorylation cycle of proteins can be used to perform tasks in cells
ATP can serve as the energy currency of cells because ATP can be regenerated from ADP and Pi (inorganic phosphate) catabolic pathways drive the regeneration of ATP from ADP and phosphate endergonic reactions of cellular respiration linked to the phosphorylation of ADP reforms ATP
Enzymes Enzymes are macromolecules that acts as a catalyst a catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction an enzyme is a catalytic protein
Activation Energy Energy of activation (EA) is energy barrier, amount of energy needed to initiate a chemical reaction it is often supplied in the form of heat from the surroundings in a system Enzymes lower the activation energy barrier
Substrate Specificity of Enzymes Specific enzymes catalyze each cell reaction reactant=substrate binds to enzyme active site forms an enzyme-substrate complex substrate converted to product enzyme unchanged
Induced fit of a Substrate Brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction
Catalysis in the Enzyme s Active Site Active site is central to enzyme activity binding of substrate forms enzyme-substrate complex several mechanisms used by enzyme to lower EA orienting substrates correctly straining substrate bonds providing a favorable microenvironment covalently bonding to the substrate
Effects of Local Conditions on Enzyme Activity Factors that affect enzyme activity temperature, ph, salt concentration, and presence of cofactors and coenzymes remember that an enzyme is a protein so anything that denatures a protein affects an enzyme cofactors are non-protein enzyme helpers coenzymes are organic cofactors
Enzyme Inhibitors Inhibitors block enzyme action competitive inhibitors-bind to active site noncompetitive inhibitors-bind to second site (allostericsite) on enzyme
Enzyme Regulation Regulation of enzyme activity helps control metabolism a cell s metabolic pathways must be tightlyregulated
Allosteric Regulation Allosteric regulation is the term used to describe any case in which a protein s function at one site is affected by binding of a regulatory molecule at another site many enzymes are allosterically regulated they change shape when regulatory molecules bind to specific sites, affecting function some enzymes are activated by an allosteric activator converts inactive enzyme to active enzyme
Cooperativity is a form of allosteric regulation that can amplify enzyme activity binding of substrate can increase binding of more substrate in enzymes that have multiple active sites binding of first substrate molecule causes change in enzyme shape that favors binding of further substrate molecules.
Within the cell, many enzymes are associated with specific regions of the cell may be grouped into complexes may be incorporated into membranes Some pesticides and antibiotics function by inhibiting enzymes