Glycolysis and Fermentation

glycoloysis

a fundamental energy generating pathway present throughout biology (the point of entry of glucose into metabolism)

the purpose: the degradation of glucose to produce energy

products are used in: aerobic respiration and anaerobic fermentation

occurs in: almost all cells, all steps occur in the cytosol

what are the sources of glucose for glucolysis?

diet: mostly as sucrose which is a fructose-glucose disaccharide

storage: stored glucose comes from degradation of glycogen

can we get energy from cellulose?

no because we don’t have the enzyme that is needed to break down cellulose to glucose

glucokinase

job is to soak up glucose when there is too much glucose in the blood

trapping glucose in cells

phosphorylation of carbon-6 traps glucose in cells but requires energy of an ATP molecule

• start with glucose, add ATP and transfer the phosphate to the glucose

phases of glycolysis

• phase 1: investment of energy

  • phosphorylation of carbon-6 traps glucose in the cells (requires 1 ATP)

  • a second phosphorylation performed by phosphofructokinase also requires 1 ATP (senses if energy needs to be generated in the cell, so it allows glycolysis to continue)

• phase 2: cleavage

  • the 6-C sugar with two phosphates is cleaved into two 3-C trioses, each carrying one phosphate group

  • the rest has to be 2x to account for both groups

• phase 3: energy production

  • a third phosphorylation used Pi as a source of phosphate instead of ATP (no energy is used)

  • one molecule of NAD+ is reduced to NADH per triose (2x)

  • produces 2 ATP per triose (4 in total)

  • final product if 3-C molecule of pyruvate (2 pyruvates per glucose, so 6 carbons)

phosphofructokinase (PFK)

an enzyme that is allosterically regulated (4 subunits)

can sense if energy needs to be generated in the cell

key regulatory step of glycolysis

inhibited by ATP and activated by AMP (used to generate ATP)

• inhibited by ATP because if there is enough ATP present, then the enzyme doesn’t work but if there is little ATP then it works

  • when ATP is high, glycolysis is stopped

• responds to:

  • ATP (inhibiting)

  • energy charge (ATP/AMP ——> ATP concentrations in the cell)

substrate level phosphorylation

production of ATP during a biochemical reaction

the 2 pyruvate molecules that were produced during glycolysis does:

the TCA cycle and aerobic respiration

the NADH produced during glycolysis goes

aerobic respiration

is oxidized to NAD+ in the production of ATP

since aerobic respiration requires O2, what happens if there is not enough O2

we go through fermentation

fermentation

in the absence of oxygen, fermentation occurs to reoxidize NAD+ and allow glycolysis to continue in the cells

example: lactic fermentation

anaerobic glycolysis

the glycolysis pathway + the fermentation reaction(s)

lactic fermentation

pyruvate resulting from glycolysis is converted to lactate (lactic acid) by the enzyme lactate dehydrogenase

does not produce energy, but is necessary in order to regenerate NAD+ (the NAD+ is part of phase 3 of glycolysis)

anaerobic glycolysis in bacteria (dental caries)

• during glycolysis, PEP is produced by an enzyme called enolase

  • PEP is used as a source of phosphate instead of ATP

• during fermentation, one PEP is sent to help PTS and the other is converted into pyruvate

  • pyruvate is fermented to lactate

  • the release of lactate by the bacteria acidifies the plaque and the enamel of your teeth

  • enamel is more soluble at low pH and starts to decay to create caries

fluoride inhibits what bacterial metabolic pathway

glycolysis

• it inhibits S. mutans enolase, therefore:

  • PEP is not made and the import of sugar to make energy is blocked

    • bacteria is “starved” from getting sugar and producing energy (ATP) from anaerobic glycolysis

  • Production of lactate is blocked

    • less acid at the surface of your teeth and less tooth decay

*also helps the remineralization of your teeth

overall balance of glycolysis and fermentation

glycolysis

• input:

  • 2 ATP

  • 6 carbons

• output

  • 2 NADH

  • 4 ATP (2 net)

  • 6 carbons (2 3-C pyruvate)

fermentation

• input:

  • 2 NADH

  • 6 carbons (2 3-C pyruvate)

• output:

  • 2 NAD+

  • 6 carbons (2 3-C lactate)