An Introduction to Metabolism Chapter 8

Metabolism Chemical reactions are occurring continuously in our bodies The totality of these reactions is called metabolism There are two broad categories of metabolic reactions: Anabolic pathways Catabolic pathways

Metabolic Reactions Anabolic Catabolic Link simple molecules to form more complex molecules Require the input of energy and capture it in the chemical bonds that are formed Ex) photosynthesis making glucose or the synthesis of a protein from amino acids The breakdown of complex molecules into simpler ones Energy is released from the chemical bonds Ex) cellular respiration breaking down glucose

Catabolic and anabolic reactions are often linked Catabolic and anabolic reactions are often linked. The energy released in catabolic reactions is often used to drive anabolic reactions. Ex) the energy released by the breakdown of glucose (catabolism) is used to drive anabolic reactions such as the synthesis of nucleic acids and proteins.

Energy Energy comes in many forms: chemical, heat, electrical, light, and mechanical All forms of energy can be categorized into to basic types: Potential Kinetic

Types of Energy Potential Kinetic The energy of state or position; stored energy Can be stored in many forms – in chemical bonds, as a concentration gradient, or even as an electric charge imbalance The energy of movement Energy that does work and makes things change

Example of an Energy Conversion A cat leaping requires the potential energy stored in the covalent bonds of its muscles (chemical energy) to be converted into the kinetic energy of muscle contractions (mechanical energy)

Laws of Energy Conversions 1st Law of Thermodynamics 2nd Law of Thermodynamics Energy can be transferred and transformed, but it cannot be created or destroyed In any conversion of energy, the total energy before and after the conversion is the same. Every energy transfer or transformation increases the entropy of the universe Entropy is a measure of disorder Disorder (entropy) tends to increase

Ex) when we eat (chemical energy) some is converted into kinetic energy to make us move but most is converted into heat, which is energy associated with the random motion of atoms or molecules. In other words, no physical process or chemical reaction is 100 percent efficient; some of the released energy is lost to a form associated with disorder Think of disorder as a kind of randomness due to the thermal motion of particles; this energy is of such a low value and so dispersed that it is unusable

∆G = G final state – G initial state Free Energy Free energy (G) refers to energy that can perform work Scientists study free energy to determine if chemical reactions will occur spontaneously This is done by comparing the amount of free energy before the reaction occurs to the amount of free energy after the reaction has occurred Change in free energy = ∆G ∆G = G final state – G initial state

Unstable systems have high G (able to do a lot of work) Stable systems have low G (not able to do a lot of work) This means systems tend to change from unstable conditions to stable conditions OR Systems change from more organized to less organized Remember: 2nd law of thermodynamics – the universe follows an unstoppable trend toward disorder (entropy)

Exergonic and Endergonic Reactions Releases free energy Spontaneous (-∆G) Absorbs free energy Non-spontaneous (+∆G)

Systems at Equilibrium Reactions in a closed system eventually reach equilibrium and then do no work Cells are not in equilibrium; they are open systems experiencing a constant flow of materials A defining feature of life is that metabolism is never at equilibrium

Energy Coupling A cell does three main kinds of work: Chemical Transport Mechanical 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

ATP Structure ATP (adenosine triphosphate) is the cell’s energy shuttle ATP is composed of ribose (a pentose sugar), adenine (a nitrogenous base), and three phosphate groups The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis Energy is released from ATP when the terminal phosphate bond is broken

How ATP Performs Work In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction Overall, the coupled reactions are exergonic ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant

ATP Fun Facts The total quantity of ATP in the human body is about 0.1 mole (6.02x1023 molecules). The energy used by human cells requires the hydrolysis of 200 to 300 moles of ATP daily. This means that each ATP molecule is recycled 2000 to 3000 times during a single day. ATP cannot be stored, so its consumption must closely follow its synthesis

Enzymes An enzyme is a catalytic protein A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction Enzymes catalyze reactions by lowering the activation energy barrier The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA) Activation energy is often supplied in the form of heat from the surroundings Enzymes do not affect the change in free energy (∆G); instead, they speed up reactions that would occur eventually

How Enzymes Work The reactant an enzyme acts on is called a substrate. The substrate will bind to a spot on the enzyme called the active site forming an enzyme-substrate complex. An enzyme is very specific; it will recognize its specific substrate (reactant) even among closely related compounds, such as isomers. As the substrate binds to the active site, interactions between the chemical groups of the substrate and the R-groups of the active site cause the enzyme to change shape slightly so the active site fits more snugly around the substrate; this is called an induced fit.

Conditions that Effect Enzyme Activity Temperature – enzymes function best in specific temperature ranges. When temperatures get too hot, the enzyme denatures The active site may change shape which prevents the substrate from entering Other conditions include pH Salt concentration Inhibitor molecules

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

Allosteric Regulation Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site Most allosterically regulated enzymes are composed of more than one polypeptide, each with its own active site The entire complex is usually activated at all times or inactivated All subunits are effected by the attachment of a single activator or inhibitor

Feedback Inhibition In feedback inhibition, the end product of a metabolic pathway shuts down the pathway When enough product is produced, the product actually stops the production of more…genius!