C HAPTER 8 An Introduction to Metabolism

W HAT Y OU N EED T O K NOW : Examples of endergonic and exergonic reactions. The key role of ATP in energy coupling. That enzymes work by lowering the energy of activation. The catalytic cycle of an enzyme that results in the production of a final product. The factors that influence enzyme activity.

Metabolism is the totality of an organism’s chemical reactions Manage the materials and energy resources of a cell

Catabolic pathways release energy by breaking down complex molecules into simpler compounds Eg. digestive enzymes break down food  release energy Anabolic pathways consume energy to build complex molecules from simpler ones Eg. amino acids link to form muscle protein

E NERGY = CAPACITY TO DO WORK Kinetic energy (KE) : energy associated with motion Heat (thermal energy) is KE associated with random movement of atoms or molecules Potential energy (PE) : stored energy as a result of its position or structure Chemical energy is PE available for release in a chemical reaction converted Energy can be converted from one form to another Eg. chemical  mechanical  electrical

closed A closed system, such as liquid in a thermos, is isolated from its surroundings open In an open system, energy and matter can be transferred between the system and its surroundings Organisms are open systems T HERMODYNAMICS IS THE STUDY OF ENERGY TRANSFORMATIONS THAT OCCUR IN NATURE

T HE F IRST L AW OF T HERMODYNAMICS  The energy of the universe is constant Energy can be transferred and transformed Energy cannot be created or destroyed Also called the principle of Conservation of Energy

T HE S ECOND L AW OF T HERMODYNAMICS  Every energy transfer or transformation increases the entropy (disorder) of the universe During every energy transfer or transformation, some energy is unusable, often lost as heat

Free energy Free energy : part of a system’s energy available to perform work  G = change in free energy Exergonic reaction Exergonic reaction : energy is released Spontaneous reaction  G < 0 Endergonic reaction Endergonic reaction : energy is required Absorb free energy  G > 0

Fig. 8-5a Less free energy (lower G) More stable Less work capacity More free energy (higher G) Less stable Greater work capacity In a spontaneous change The free energy of the system decreases (∆G < 0) The system becomes more stable The released free energy can be harnessed to do work

Fig. 8-5b Spontaneous change Spontaneous change Spontaneous change (b) Diffusion(c) Chemical reaction(a) Gravitational motion

Overview: The Energy of Life Life requires a highly ordered system. Order is maintained by constant free energy input into the system. The living cell is a miniature chemical factory where thousands of reactions occur The cell extracts energy and applies energy to perform work Loss of order or free energy results in ?????? Increased disorder and entropy are offset by biological processes that maintain or increase order.

A cell does three main kinds of work: Mechanical Transport Chemical energy coupling: Cells manage energy resources to do work by energy coupling: using an exergonic process to drive an endergonic one

ATP ( adenosine triphosphate ) is the cell’s main energy source in energy coupling ATP = adenine + ribose + 3 phosphates

hydrolysis When the bonds between the phosphate groups are broken by hydrolysis  energy is released chemical change to a state of lower free energy This release of energy comes from the chemical change to a state of lower free energy, not in the phosphate bonds themselves

Overview: The Energy of Life Life requires a highly ordered system. Order is maintained by constant free energy input into the system. The living cell is a miniature chemical factory where thousands of reactions occur The cell extracts energy and applies energy to perform work Loss of order or free energy results in ?????? Increased disorder and entropy are offset by biological processes that maintain or increase order.

LIVING SYSTEMS DO NOT VIOLATE THE 2 ND LAW OF THERMODYNAMICS Order is maintained by coupling processes that increase entropy with those that decrease entropy. Energy input must exceed free energy lost to entropy to maintain order and power cellular processes. Favorable exergonic reactions such as ATP-ADP, that have negative change in free energy can be used to maintain or increase order in a system by being coupled with reactions that have a positive free energy change.

H OW ATP P ERFORMS W ORK Exergonic release of P i is used to do the endergonic work of cell When ATP is hydrolyzed, it becomes ADP (adenosine diphosphate)

NH 2 Glu P i P i P i P i NH 3 P P P ATP ADP Motor protein Mechanical work: ATP phosphorylates motor proteins Protein moved Membrane protein Solute Transport work: ATP phosphorylates transport proteins Solute transported Chemical work: ATP phosphorylates key reactants Reactants: Glutamic acid and ammonia Product (glutamine) made + + +

Catalyst Catalyst: substance that can change the rate of a reaction without being altered in the process Enzyme Enzyme = biological catalyst activation energy Speeds up metabolic reactions by lowering the activation energy (energy needed to start reaction)

S UBSTRATE S PECIFICITY OF E NZYMES substrate The reactant that an enzyme acts on is called the enzyme’s substrate enzyme-substrate complex The enzyme binds to its substrate, forming an enzyme-substrate complex active site The active site is the region on the enzyme where the substrate binds

INDUCED FIT : ENZYME FITS SNUGLY AROUND SUBSTRATE -- “CLASPING HANDSHAKE”

An enzyme’s activity can be affected by: temperature pH chemicals

C OFACTORS Cofactors Cofactors are nonprotein enzyme helpers such as minerals (eg. Zn, Fe, Cu) Coenzymes Coenzymes are organic cofactors (eg. vitamins) Enzyme Inhibitors Competitive inhibitor: active site Competitive inhibitor: binds to the active site of an enzyme, competes with substrate Noncompetitive inhibitor: another part nonfunctional Noncompetitive inhibitor: binds to another part of an enzyme  enzyme changes shape  active site is nonfunctional

I NHIBITION OF E NZYME A CTIVITY

R EGULATION OF E NZYME A CTIVITY To regulate metabolic pathways, the cell switches on/off the genes that encode specific enzymes Allosteric regulation : protein’s function at one site is affected by binding of a regulatory molecule to a separate site (allosteric site) Activator Activator – stabilizes active site Inhibitor Inhibitor – stabilizes inactive form Cooperativity Cooperativity – one substrate triggers shape change in other active sites  increase catalytic activity

F EEDBACK I NHIBITION End product of a metabolic pathway shuts down pathway by binding to the allosteric site of an enzyme Prevent wasting chemical resources, increase efficiency of cell

F EEDBACK I NHIBITION