Chapter 8: Microbial Metabolism- the Chemical Crossroads of Life

The Metabolism of Microbes Metabolism: All chemical reactions and physical workings of the cell Anabolism: also called biosynthesis- any process that results in synthesis of cell molecules and structures (usually requires energy input) Catabolism: the breakdown of bonds of larger molecules into smaller molecules (often release energy) Functions of metabolism Assembles smaller molecules into larger macromolecules needed for the cell Degrades macromolecules into smaller molecules and yields energy Energy is conserved in the form of ATP or heat

Enzymes Catalyze the chemical reactions of life Enzymes: an example of catalysts, chemicals that increase the rate of a chemical reaction without becoming part of the products or being consumed in the reaction

How do Enzymes Work? Energy of activation: the amount of energy which must be overcome for a reaction to proceed. Can be achieved by: Increasing thermal energy to increase molecular velocity Increasing the concentration of reactants to increase the rate of molecular collisions Adding a catalyst An enzyme promotes a reaction by serving as a physical site upon which the reactant molecules (substrates) can be positioned for various interactions

Enzyme Structure Most- protein Can be classified as simple or conjugated Simple enzymes- consist of protein alone Conjugated enzymes- contain protein and nonprotein molecules A conjugated enzyme (haloenzyme) is a combination of a proten (now called the apoenzyme) and one or more cofactors Cofactors are either organic molecules (coenzymes) or inorganic elements (metal ions)

Figure 8.2

Apoenzymes: Specificity and the Active Site Exhibits levels of molecular complexity called the primary, secondary, tertiary, and quaternary organization The actual site where the substrate binds is a crevice or groove called the active site or catalytic site

Figure 8.3

Enzyme-Substrate Interactions For a reaction to take place, a temporary enzyme-substrate union must occur at the active site “Lock-and-key” fit The bonds are weak and easily reversible

Figure 8.4

Cofactors: Supporting the Work of Enzymes Metallic cofactors Include Fe, Cu, Mg, Mn, Zn, Co, Se Metals activate enzymes, help bring the active site and substrate close together, and participate directly in chemical reactions with the enzyme-substrate complex Coenzymes Organic compounds that work in conjunction with an apoenzyme to perform a necessary alteration of a substrate Removes a chemical group from one substrate molecule and adds it to another substrate Vitamins: one of the most important components of coenzymes

Classification of Enzyme Functions Site of action Type of action Substrate

Location and Regularity of Enzyme Action Either inside or outside of the cell Exoenzymes break down molecules outside of the cell Endoenzymes break down molecules inside of the cell

Figure 8.5

Rate of Enzyme Production Enzymes are not all produced in the cell in equal amounts or at equal rates Constitutive enzymes: always present and in relatively constant amounts Regulated enzymes: production is either induced or repressed in response to a change in concentration of the substrate

Figure 8.6

Synthesis and Hydrolysis Reactions Figure 8.7

Transfer Reactions by Enzymes Oxidation-reduction reactions A compound loses electrons (oxidized) A compound receives electrons (reduced) Common in the cell Important components- oxidoreductases Other enzymes that play a role in necessary molecular conversions by directing the transfer of functional groups: Aminotransferases Phosphotransferases Methyltranferases Decarboxylases

The Role of Microbial Enzymes in Disease Many pathogens secrete unique exoenzymes Help them avoid host defenses or promote multiplication in tissues These exoenzymes are called virulence factors or toxins

The Sensitivity of Enzymes to Their Environment Enzyme activity is highly influenced by the cell’s environment Enzymes generally operate only under the natural temperature, pH, and osmotic pressure of an organism’s habitat When enzymes subjected to changes in normal conditions, they become chemically unstable (labile) Denaturation: the weak bonds that maintain the native shape of the apoenzyme are broken

Regulation of Enzymatic Activity and Metabolic Pathways Metabolic reactions usually occur in a multiseries step or pathway Each step is catalyzed by an enzyme Every pathway has one or more enzyme pacemakers that set the rate of a pathway’s progression

Figure 8.8

Direct Controls on the Action of Enzymes Competitive inhibition: The cell supplies a molecule that resembles the enzyme’s normal substrate, which then occupies and blocks the enzyme’s active site Noncompetitive inhibition: The enzyme has two binding sites- the active site and the regulatory site; a regulator molecule binds to the regulatory site providing a negative feedback mechanism

Figure 8.9

Controls on Enzyme Synthesis Enzymes eventually must be replaced Enzyme repression: stops further synthesis of an enzyme somewhere along its pathway Enzyme induction: The inverse of enzyme repression

Figure 8.10

The Pursuit and Utilization of Energy Energy in Cells Exergonic reaction: a reaction that releases energy as it goes forward Endergonic reaction: a reaction that is driven forward with the addition of energy

Figure 8.11

A Closer Look at Biological Oxidation and Reduction Biological systems often extract energy through redox reactions Redox reactions always occur in pairs An electron donor and electron acceptor Redox pair Electron donor (reduced) + electron acceptor (oxidized)  Electron donor (oxidized) + electron acceptor (reduced) This process leaves the previously reduced compound with less energy than the now oxidized one The energy in the electron acceptor can be captured to phosphorylate to ADP or some other compound, storing the energy in a high-energy molecule like ATP

Electron Carriers: Molecular Shuttles Electron carriers repeatedly accept and release electrons and hydrogens Facilitate the transfer of redox energy Most carriers are coenzymes that transfer both electrons and hydrogens Some transfer electrns only Most common carrier- NAD

Figure 8.12

Adenosine Triphosphate: Metabolic Money ATP Can be earned, banked, saved, spent, and exchanged A temporary energy repository The Molecular Structure of ATP Three-part molecule Nitrogen base (adenine) 5-carbon sugar (ribose) Chain of three phosphate groups The high energy originates in the orientation of the phosphate groups Breaking the bonds between two successive phosphates of ATP yields ADP ADP can then be converted to AMP