Chapter 8 Metabolism Essential Concepts --- chemical energy is necessary to life in that it allows living organisms to drive endergonic (energy requiring) reactions using energy captured from exergonic (energy releasing) reactions --- electrons can’t flow in a vacuum, oxidation reactions must always be coupled to reduction reactions --- phosphate bonds are efficient ways to transport energy, they carry a relatively large amount of energy and are stable enough to move around the cell but not too stable to be easily broken --- ion impermeable membranes can be used to establish charge separation (electrochemical potential) as a way to store energy and to convert electrochemical energy into chemical energy

Different Ways to the Same End: Strategies for producing ATP: Substrate-level Phosphorylation: Use a high energy phosphate containing molecule to transfer phosphate to ADP --- usually involves addition of free (inorganic) phosphate to a molecule and then rearrangement to increase the energy of the phosphate group Photophosphorylation: Use energy captured from light to pump protons and create a charge separation Respiration: Us energy captured from the oxidation of reduced compounds (organic or inorganic) pump protons and create a charge separation.

Charge separation across a ion impermeable membrane

A Respiratory Chain Glucose  Pyruvate + 2 NADH Electron transport Charge separation (pmf) ATP generation or other processes (flagellar rotation) transport Other oxidation reactions (produce more NADH) OR Biosynthetic reactions Produce NAD + Glycolysis Other NADH producing reactions

Chemiosmotic Theory Peter Mitchell’s idea that energy could be stored in a transmembrane ion gradient went very much against accepted theory of the time two components:  pH – pH differential across the membrane -- usually about 1.0 pH unit  -- charge potential across the membrane (-160 mV) pmf - proton motive force (-240 mV for E. coli)    - proton activity (-32 kJ/mol) Uncouplers -

An aerobic electron transport chain

ATP Synthase

Recycling is Good! --- At the heart of most respiratory chains is the concept that you must replace the oxidizing/ reducing equivalents that you use in the pathway. --- So electron transport actually has two functions: 1.) reduce NADH to NAD+ to replenish NAD+ pool 2.) produce ATP via proton pumping and charge separation

NAD + + 2H + + 2e -  NADH V

Redox Reactions: Occur in half reactions (either an oxidation or a reduction) H 2  2H + + 2e V (requires energy) (reduction) Which is great, but... electrons can’t be in solution alone So we combine the oxidation with an oxidation reaction ½ O 2 + 2H + + 2e -  H 2 O V (produces energy) (oxidation)

Total energy (  E h )= E o (oxidized) – E o (reduced) (  E h )= 0.82 V – ( V) = 1.24 V Net energy = 1.24 V Using  G = (-nF)(  E h )  G = (-2)( kJ/V)(1.24 V)  G = +239 kJ This is essentially aerobic respiration, how so?

Big Gulps: In the previous reactions O 2 is the terminal electron acceptor and 239 kJ is the maximum energy that can be extracted from this system. However, living systems cannot take H 2 and ½ O 2 directly to H 2 O in one step, too much energy is released. Living systems have a solution to this problem: 1.) Break the redox system down into multiple smaller steps, each of which release a manageable amount of energy 2.) Use mobile electron carriers to link these smaller reactions These unified systems likely evolved from simpler, less contiguous sets of reactions.

Alternate Electron Acceptors --- oxygen generates one of the largest gaps between electron donor and acceptor, and so is the most favorable terminal electron acceptor for respiratory chains. --- however, many bacteria can grow in the absence of oxygen, and oxygen was not originally present on Earth Some other electron acceptors and their energy yields: N e - + 2H +  N H V total voltage = 0.42 V – ( V) = 0.74 V  G = kJ Fe3+ + e -  Fe V  G = kJ