Energy Metabolism 1: Glycolysis and Fermentation

What is ATP used for? What is the ΔG’ of ATP hydrolysis?

Adenosine triphosphate can be used to power endergonic processes through hydrolysis

ΔG’= ΔG + RTln ([ADP][Pi] / [ATP])

ΔG’ ~= -10 kcal/mol in cells

What kind of compound can hydrolysis of ATP form?

hydrolysis of ATP can form a less energetic phosphorylated compound (like glucose-6-phosphate ΔG’= -3.3kcal/mol) but not a more energetic phosphorylated compound (like PEP ΔG’= -14kcal.mol)

What can ATP hydrolysis be coupled with?

synthetic work, concentration work, electrical work, mechanical work, bioluminescent work, heat

Other than ATP hydrolysis, what is another important energy transfer mechanism?

oxidation (the removal of electrons/hydrogen ions), also called dehydrogenation, is an important energy transfer mechanism

• electrons and hydrogens removed during oxidation usually transferred to coenzymes

oxidation and reduction reactions are always coupled

• oxidized: when an atom/molecule loses an electron

• reduced: when an atom/molecule gains an electron

Discuss an important electron carrier.

Nicotinamide adenine dinucleotide (NAD+) is the most common coenzyme in catabolism

• NAD+(oxidized) + 2H → NADH(reduced) + H+

What is an important oxidizable substrate in energy metabolism and how does glucose enter a cell?

Glucose (six carbon sugar) in animal cells is an important oxidizable substrate in energy metabolism.

glucose is imported into cells via facilitated diffusion and requires specific transporters (GLUT proteins) to pass plasma membrane

• GLUT-1: found on most cell types, has low Km

• GLUT-2: found in liver, kidney, etc, has high Km

• GLUT-4: found in skeletal muscle, Km slightly higher than glut-1

• SGLT-1 or -2: do secondary active transport of glucose in small intestine and kidneys (NOT provide glucose for metabolism in cell)

Why is glucose a good source of energy? What are some examples of glucose being used as energy?

its oxidation is highly exergonic ΔG’= -686kcal/mol with oxygen as final electron acceptor ( C6H12O6 + 6O2 → 6CO2 + 6H2O )

• complete oxidation of glucose with oxygen as terminal e- acceptor is aerobic respiration (most energy released of the three)

• many organisms (most prokaryotes) carry out anaerobic respiration using terminal acceptors like NO3-, Fe3+

• many organisms (including some that use inorganic terminal acceptors) can use organic molecules as e- acceptors, fermentation

What is glycolysis? Where is it performed? What is the net equation for glycolysis?

a ten-step catabolic pathway that converts one glucose into two molecule of pyruvate

the enzymes for glycolysis (and fermentation) are all found in cytosol

for every glucose, two net ATP and two NADH are produced

glucose + 2NAD+ + 2ADP + 2Pi → 2pyruvate + 2NADH + 2ATP + 2H+

ΔG’= -20kcal/mol

Briefly discuss glycolysis in three phases.

1. preparatory and cleavage steps (Gly-1 to -5)

2. oxidative sequence, including first ATP-generating event and yields NADH (Gly-6 and -7)

3. second ATP generating event (Gly-8 to -10)

Discuss the first most exergonic (irreversible/regulated) step in glycolysis, Gly-1.

• Gly-1 adds a phosphate to the sixth carbon of glucose to form glucose 6-phosphate

  • catalyzed by hexokinase in most cells or glucokinase in liver cells

  • this reaction results in:

    • the glucose gradient across the plasma membrane favoring glucose absorption

    • once phosphorylated the glucose can’t leave the cell (bc most cells lack glucose 6-phosphatase)

Discuss a very exergonic, regulated step in glycolysis, Gly-3.

• Gly-3 adds another phosphate to fructose 6-phosphate to form fructose 1,6-biphosphate, catalyzed by phosphofructokinase 1

  • Gly-3 is key step that feeds glucose specifically into glycolysis

fructose 6-phosphate + ATP → fructose 1,6-biphosphate + ADP

Discuss a very exergonic, regulated step in glycolysis, Gly-10.

• Gly-10: PEP goes through hydrolysis drives ATP synthesis (phosphate to ADP) catalyzed by enzyme pyruvate kinase

PEP + ADP → pyruvate + ATP

State what goes into and comes out of the first phase of glycolysis.

• Gly-1: ATP + glucose → glucose 6-phosphate + ADP

• Gly-2

• Gly-3: fructose 6-phosphate → fructose 1,6-biphosphate

• Gly-4

• Gly-5

glucose + 2ATP → 2 glyceraldehyde-3-phosphate + 2ADP

State what goes into and comes out of the second phase of glycolysis.

(some energy is captured as NADH)

• Gly-6: → NADH

  • NAD+ is e- acceptor that’s reduced to NADH

• Gly-7: → ATP

  • ATP generation from phosphorylated substrate and ADP is substrate-level phosphorylation

glyceraldehyde-3-phosphate + NAD+ + ADP + Pi → 3-phosphoglycerate + NADH + ATP + H+

State what goes into and comes out of the third phase of glycolysis.

• Gly-8

• Gly-9

• Gly-10: phosphoenolpyruvate (PEP, high energy compound) → pyruvate kinase

  • hydrolysis of PEP is one of most exergonic reactions in biological systems

3-phosphoglycerate + ADP → pyruvate + ATP

What does pyruvate do in the presence of oxygen?

Pyruvate can undergo further oxidation into acetyl-CoA which can then be completely oxidized to CO2, generating ~30 ATP per glucose

What does pyruvate do in the absence of oxygen/terminal electron acceptor?

fermentation pathways are used to regenerate NAD+ to continue glycolysis

• pyruvate is reduced by accepting e- and protons from NADH to regenerate NAD+

• no external electron accepter is consumed and no net oxidation occurs

• fermentation only give two ATP per glucose while most free energy of glucose is still present is lactate or ethanol

Discuss one type of fermentation (lactate).

under anaerobic conditions, lactate fermentation takes place

• lactate generate by e- transfer from NADH to pyruvate by lactate dehydrogenase

• entire fermentation process occurs in cytosol

glucose + 2ADP + 2Pi → 2lactate + 2ATP

Discuss one type of fermentation (ethanol).

under anaerobic conditions, plant cells, yeast, etc, ethanol fermentation takes place

glucose + 2ADP +2Pi + 2H+ → 2ethanol + 2CO2 +2ATP