1 Metabolism: 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.

2 Energy: 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.

3 Bacteria 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).

4 Redox reactions release energy for use Depends on concentration, redox potential, etc. XH 2 + Y X + YH 2 shows oxidation of X, reduction of Y Note that 2 H atoms are transferred, not just electrons Familiar redox reaction that releases energy: CH 4 + 2O 2 CO 2 + 2H 2 O natural gas burning. Biological reactions release energy gradually, trap it as ATP

5 Is 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

6 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 = +1O = -2 Oxidation state of carbon in methane (CH 4 ): Not charged, so numbers add up to 0. So if all the H = 4 x 1 = +4, then C must be -4. For CO 2, 2 x -2 = -4; no net charge, then C must be = +4. Observe the origin of the term “reduced”: If carbon dioxide is ‘reduced” to methane (carbon accepts electrons), then the oxidation number of the carbon gets reduced (number gets lower) from +4 to -4.

7 Metabolic reactions require enzymes Reactions operate in pathways: A B CD Where A-D are different molecules Each step is catalyzed by a different enzyme. A Catalyst is something that speeds up a chemical reaction and is not consumed in the reaction, but can be re-used. Enzymes are biological catalysts; 99.99% of them are proteins. Enzymes are very specific; a different one is required for each type of chemical reaction. Because the 3-D shape of an enzyme is critical for its function, anything that alters that (heat, high salt, extreme pH) will affect how fast or whether it works.

8 Importance of 3D shape Every enzyme has an active site, a location where the substrate (the molecule to be acted on) fits into the enzyme. The enzyme then performs chemistry on the substrate, producing a product(s) which then diffuses away, leaving the enzyme free to act on the next substrate. Every metabolic reaction we will look at happens in this way.

9 Enzyme function depends on shape Substrates Product Enzyme brings substrates together in active site, increasing the rate at which they react.

10 More about Enzymes Sometimes an enzyme needs help –Protein alone = apoenzyme –Helper molecule: cofactor Could be inorganic like a metal ion (Fe +2 ) Could be organic coenzyme (like CoA, NAD) –Apoenzyme + cofactor = holoenzyme. –Cofactors have an effect on nutrition Bacteria have certain mineral requirements. Vitamins are cofactors that are needed in the “diet”.

11 Enzymes can be stopped Conditions that disrupt the 3D shape –Acidic, alkaline, high salt, high temperature, etc. –These conditions thus affect growth of cell also. Inhibitory molecules affect enzymes –Competitive inhibitors Fit in active site but are not changed; prevent normal substrate from binding, prevent reaction. –Non-competitive inhibitors Bind permanently to active site or other site which changes molecular shape; prevents reaction. –Allosteric inhibitor: temporary binding, regulates.

12 Competitive Inhibition Both the substrate and the inhibitor fit into the active site, but the inhibitor isn’t altered by the enzyme. As long as the inhibitor is in the active site, the substrate cannot enter the active site and react. The more inhibitor molecules that are present, the more often one of them occupies the active site. ghs.gresham.k12.or.us/.../ competitiveinhib.htm

13 Allosteric sites In allosteric site, inhibitor is not reacted, but causes a shape change in the protein. The substrate no longer fits in the active site, so it is not chemically changed either. ghs.gresham.k12.or.us/.../ noncompetitive.htm

14 Introduction 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.

15 Structure of ATP s/atp.jpg

16 The ATP cycle 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.

17 Important molecules: the electron carriers 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. The most common electron carrier is NAD: NAD + XH 2 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).

18 Other 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 FADH 2 –Used in the Krebs Cycle, other reactions

19 Oxidations for energy If this reaction is reversible, and every oxidation is coupled to a reduction, how can the oxidation of XH 2 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 YH 2 will subsequently be oxidized, driving the reaction to the right and that XH 2 has a greater tendency to give up electrons than YH 2.