Lecture 12

Textbook Reading

9.6 Fermentation

• fermentation — metabolic pathway that includes glycolysis and an additional set of reactions that oxidize stockpiles of NADH to regeneration NAD+

• lactic acid fermentation — regenerates NAD+ by reducing pyruvate to form lactate

• alcohol fermentation — yeast produces ethanol as waste

• fermentation is extremely inefficient compared with cellular respiration

  • only 2 ATP produced per glucose molecule

• facultative anaerobes — organisms that can switch between fermentation and aerobic cellular respiration

8.5 Enzymes Can Work Together in Metabolic Pathways

• metabolic pathways — linked series of biochemical reactions that sequentially change an initial substrate to form a final product

  • A -(enzyme 1)→ B -(enzyme 2)→ C -(enzyme 3)→ D

• feedback inhibition — regulate a metabolic pathway by using the final product of the reaction sequence to inactivate one of the pathway’s own enzymes

• catabolic pathways — break down molecules for sources of energy/carbon building blocks

• anabolic pathways — use energy/carbon building blocks to synthesize molecules

Lecture Slides

• we followed the path of the C and H in glucose to complete oxidation in the presence of O2:

  • glycolysis first (1 glucose → 2 pyruvate)

  • krebs cycle next (carbons released as CO2)

  • ETC and oxidative phosphorylation last (hydrogens combine with oxygen to form H2O)

• we followed the path of the energy in glucose (as electrons) to make ATP

  • some ATP generated by SLP in both glycolysis and Krebs Cycle

  • most electrons transferred to cofactors which carry them to the ETC, which uses them to create a proton gradient, which in turn powers ATP synthase to perform oxidative phosphorylation

• what happens to aerobically-respiring cells under anaerobic conditions?

  • these cells can perform glycolysis, but no Krebs cycle, ETC, or oxidative phosphorylation — all ATP comes from glycolysis

  • the pyruvate undergoes a process called fermentation:

    • serves to regenerate the NAD+ which was reduced to NADH in glycolysis

    • no additional energy is released per molecule of glucose (because glucose is not completely oxidized), but the rate of glycolysis is increased to compensate for the loss of oxidative phosphorylation

    • fermentation products accumulate

• fermentation in yeast

  • pyruvate -(pyruvate decarboxylase)→ acetaldehyde + CO2 -(alcohol dehydrogenase)→ ethanol

  • NADH gets oxidized to NAD+ in the process of making ethanol

• fermentation in muscle cells and many bacteria

  • pyruvate -(lactace dehydrogenase)→ lactic acid (lactate)

  • NADH gets oxidized to NAD+ in the process

• catabolic and biosynthetic pathways are regulated and coordinated

  • don’t use any more energy than necessary

  • coordination comes from two primary sources

    • amount of enzyme

    • activity of allosterically-regulated enzyme

• some enzymes have binding sites other than the active site to which regulatory molecules (allosteric regulation) bind

  • noncompetitive enzyme inhibition = allosteric negative regulation

  • change conformation of active site

  • can either increase or decrease activity of enzyme

    • called positive regulator if it increases activity

    • called negative regulator if it decreases activity

• sometimes, the allosteric regulator is a product of a later reaction in that pathway

  • called feedback inhibition — random collisions affect binding

  • low product (active)

    • substrate binds normally

  • high product (inactive)

    • allosteric binding results in feedback inhibition; enzyme 1 cannot bind substrate

• example: feedback inhibition of glycolysis by ATP:

  • phosphofructokinase (PFK) — phosphorylates a phosphated fructose

  • if ATP binds to regulatory site, activity of PFK is decreased (negative regulation)

  • if ADP (or AMP) binds to regulatory site, active of PFK is increase (positive regulation)

  • whichever gets to the active site first binds, so if there is more ATP, there is less ATP produced because it binds more

  • allosteric binding sites: adenine nucleotide site

• some allosteric regulators can turn “up” one reaction and turn “down” a different reaction