1Metabolism: the chemical reactions of a cell

• All organisms need two things with which to grow:– Raw materials (especially carbon atoms)– Energy.

• Types of metabolic reactions:– Anabolism: biosynthesis; reactions that create

large/complex molecules from smaller, simpler ones. Use raw materials and energy.

– Catabolism: degradation; reactions that break down large/complex molecules, used to generate energy for use and to produce smaller, building block molecules.

2Energy: where from? What for?

• Chemotrophs vs. phototrophs– Chemotrophs get energy from molecules

• Chemolithotrophs get energy from oxidation of inorganic substances.

• Chemoorganotrophs get energy from oxidation of organic compounds (like we do).

– Phototrophs get energy from sunlight

• Energy is needed to power the cell– Biosynthesis to respond to environment, to grow– Active transport, motility, etc.

3Bacteria obtain energy through oxidation/reduction reactions

• Oxidation: molecule gives up electrons• Reduction: molecule accepts electrons• Oxidation/reduction (redox) reactions always occur

in pairs; if electrons are removed, they must go somewhere!

• Biological redox reactions usually involve PAIRS of electrons.

• Biological redox reactions often involve entire hydrogen atoms, not just the electrons (so called dehydrogenation reactions).

4Redox reactions release energy for use

• Depends on concentration, redox potential, etc.

• XH2 + Y X + YH2 shows oxidation of X, reduction of Y

• Note that 2 H atoms are transferred, not just electrons

• Familiar redox reaction that releases energy:

• CH4 + 2O2 CO2 + 2H2O natural gas burning.

• Biological reactions release energy gradually, trap it as ATP

5Is it good to eat?

• Reduced molecules have lots of energy.

• Have lots of H, few O

• Oxidized molecules have little energy;

• lots of O or few H.

glucose

Carbon dioxide

Redox Calculations• One can assign an oxidation number to the carbon

atoms in a molecule to determine how much energy an organic molecule has.

• Oxidation numbers: H = +1 O = -2– For an uncharged organic molecule, all the redox

numbers must add up to 0

– in methane (CH4): oxidation number C is -4.

– For CO2, 2 x -2 = -4; no net charge, then C must be = +4

– In going from CO2 to CH4 although a carbon has gained electrons, the oxidation number for the carbon is lower, thus “reduced”. http://www.chem.vt.edu/RVGS/ACT/notes/oxidation_numbers.html

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7Introduction to important molecules in metabolism

• Biological reactions release energy from redox reactions gradually, trap it as ATP

• ATP is the energy molecule that cells use to power most of their activities. “energy currency”

• ATP is a molecule under stress:– too many negative charges in one place. Release of 1

phosphate: ATP → ADP + Pi relieves that stress, releases energy which can be used for:

– cellular activities such as transport, motility, biosynthesis, etc.

8Structure of ATP

http://www.ustboniface.mb.ca/cusb/abernier/Biologie/Module1/Images/atp.jpg

9The ATP cycle

www.cat.cc.md.us/.../ metabolism/energy/fg1.html

ATP is hydrolyzed to ADP to release energy.

Energy is used to reattach the phosphate to ADP to regenerate ATP.

Other molecules used for energy include GTP and PEP.

10Important molecules: the electron carriers -1

• The energy released in redox reactions is often thought of as the energy in the bonds between the H and the C; when a molecule is reduced by transfer of the H, the energy is conserved in that reduced molecule.

11Important molecules: the electron carriers -2

• The most common electron carrier in biological redox reactions is NAD:

• NAD + XH2 X + NADH + H+ – where NAD carries 2 e-, 1 H+

– Reduced NAD (NADH) is like poker chips, energy that can’t be spent, but can be “cashed in” later to make ATP (which can be “spent”, i.e. used as an energy source for cell activities).

Structure of NAD 12

http://www.bact.wisc.edu/themicrobialworld/NAD.jpg

13Other electron carriers

• NADP – Reduced during some reactions such as those in the

Pentose Phosphate pathway, Photosynthesis– NADPH used to donate H for biosynthesis reactions such

as the Calvin Cycle, amino acid biosynthesis.

• FAD– Reduced to FADH2

– Used in the Krebs Cycle, other reactions

14Oxidations for energy

If this reaction is reversible, and every oxidation is coupled to a reduction, how can the oxidation of XH2 yield energy?

Describes the tendency of a reaction to occur spontaneously, to release energy. Two major factors are the tendencies of X and Y to give up or accept electrons and their concentrations.

In this example, we expect that YH2 will subsequently be oxidized, driving the reaction to the right and that XH2 has a greater tendency to give up electrons than YH2 .

Go = - R T ln K