Coordination of Intermediary Metabolism

ATP Homeostasis

• Energy Consumption (adult woman/day)

– 6300-7500 kJ (>200 mol ATP)

– Vigorous exercise: 100x rate of ATP utilization

• Steady-State ATP: <0.1 mol

– 0.05% daily usage

– <1 min supply

Strict Coordinate Control

Strict Coordinate Control

• Glycogenolysis (glycogen metabolism)

• Glycolysis

• Citric Acid Cycle

• Oxidative Phosphorylation

Identification of Potential Control Sites in Electron Transport and Oxidative

Phosphorylation

Complex I and III

1/2 NADH + Cytochrome c (Fe3+) + ADP + Pi

——> 1/2 NAD+ + Cytochrome c (Fe2+) +

ATP

∆G’ = ~0

(reversible)

Complex I and III Equilibrium

Keq =

[NAD][NADH]

12 [Cytochrome c (Fe2 )]

[Cytochrome c (Fe3 )]

[ATP][ADP][Pi]

ATP Mass Action Ratio

(compare with Energy Charge)

Cytochrome c OxidaseComplex IV

Irreversible

Regulatory Site

Control by Substrate Availability

Inverse ATP Mass Action Ratio

[NADH] and [ATP] reduced Cytc c

Effectors of Electron Transport - Oxidative

Phosphorylation

• ATP mass action ratio– Availability of ADP and Pi

• Stimulation by Ca2+

• IF1: inhibitor of F1–ATPase

IF1

(Inhibitor of F1–ATPase)

Inactive(high pH)

Active(low pH)

Inactive during active respiration

Traps ATP bound to DP

Prevents ATPase activity when [O2] is low

Sources of Electrons for Mitochondrial Electron

Transport

• Glycolysis (or glycogenolysis)

• Fatty acid degradation

• Citric Acid Cycle

• Amino acid degradation

Figure 17-1

Metabolic Relationships

Figure 17-16

Regulation of the Citric Acid Cycle

Inhibition of ETC NADH

Coordinate Regulation of Citric Acid Cycle

Coordinate Regulation of Glycolysis and Pyruvate

Dehydrogenase

Citrate

Inhibition of Phosphofructokinase by

Citrate

Decline in Demand for ATP(ATP and ADP)

• Isocitrate Dehydrogenase: not activated by ADP

• α-Ketoglutarate Dehydrogenase: inhibited by ATP

• Citrate Accumulates

– Citrate transport system

– Inhibition of Phosphofructokinase

Regulation of Central Metabolic Pathways

Consequence

Enzyme or Process Negative Positive of Inhibition

Electron Transport ATP ADP + Pi Accum of NADH

-ketoglutarate DHase NADH No Oxaloacetate

Isocitrate DHase NADH Accum of Citrate ATP

Citrate Synthase NADH Accum of Acetyl-SCoA Citrate

Substrate Availability

Acetyl-SCoA Oxaloacetate

Pyruvate DHase NADH ADP Acetyl-SCoA

Pyruvate Kinase ATP PEP Citrate F-1,6-bisP

Phosphofructokinase ATP AMP No F-1,6-bisP Citrate F-2,6-bisP

Hexokinase Glc-6-P

Advantages of Aerobic Metabolism

Anaerobic glycolysis: 2 ATP

C6H12O6 + 2 ADP + 2 Pi —> 2 Lactate + 2 H+ + 2 H2O + 2 ATP

Aerobic metabolism of glucose: 32 ATP

C6H12O6 + 32 ADP + 32 Pi + 6 O2 —> 6 CO2 + 38 H2O + 32 ATP

Drawbacks or Disadvantages of Aerobic Metabolism

Sensitivity to O2 Deprivation

Production of Reactive Oxygen Species (ROS)

Oxygen Deprivation inHeart Attack and Stroke

Myocardial Infarction: interuption of the blood (O2) supply to a portion of the

heart

Stroke: interuption of the blood (O2) supply to a portion

of the brain

Consequences of O2 Limitation

• Disruption of osmotic balance (ion pumps)

• Swelling of cells and organelles — increased permeability

• Acidification (anaerobic lactic acid production) — activity of leaked lysosomal enzymes

Partial Oxygen Reduction Produces Reactive Oxygen

Species (ROS)

O2 + eĞ Ñ >

O2

[

O2 + H+ Ñ >

HO2 ] (strong oxidant)

H2O2 + Fe2+ Ñ >

OH + OHĞ + Fe3+ Or

O2 + H2O2 Ñ > O2 + H2O +

OH

Superoxide Radical

Hydroxyl Radical

Radicals Extract Electrons (Oxidize) Various

Biomolecules

• Polyunsaturated Lipids — disrupts biological membranes

• DNA — point mutations

• Proteins — enzyme inactivation

Free Radical Theory of Aging

Aging occurs, in part, from damage caused by reactive

oxygen species arising during normal oxidative metabolism

Cells are Equipped with Antioxidant Mechanims

• Superoxide Dismutase

• Catalase

• Glutathione Peroxidase

• Plant-derived Compounds

– Ascorbate (vitamin C), α-tocopherol

2

O2 + 2 H+ Ñ > H2O2 + O2

2 H2O2 —> 2 H2O + O2

2 GSH + H2O2 —> GSSG + 2 H2O

Oxidative Stress in Aging

Buffenstein, R et al; AGE 2008, 30:99-109

?