Carbohydrate metabolism
From IKE
For each of the major pathways of carbohydrate metabolism (glycolysis, Krebs cycle, glycogen metabolism, gluconeogenesis and pentose phosphate pathway) describe its purpose (products), strategy (general means by which the purpose is achieved), stoichiometry and regulation
Glycolysis
Purpose
- Initial metabolism of glucose and other carbohydrates
- Catabolic pathway
- produces ATP
Strategy
- substrate level phosphorylation
- oxidation-reduction (redox) reaction which produces NAD
Stoichiometry
- C6H12O6 (glucose) + 2 NAD+ (niacin derivative) --> 2 CH3-CO-COO- (pyruvate) + 2NADH2+ + 2 ATP + 2 H2O
Regulation
- allosteric inhibition of the enzyme phosphofructokinase by ATP
- this is feedback inhibition
Krebs Cycle
Purpose
- To complete the breakdown of food to produce energy-rich molecules (cellular respiration)
Strategy
Stoichiometry
- CH3-CO-COO- + 4 NAD+ + FAD + GDP + Pi + 2 H2O --> 2 CO2 + 4 NADH2+ + FADH2 + GTP
Regulation
- several enzymes of the Krebs cycle are inhibited by ATP and/or NADH2+.
Glycogen Metabolism
Purpose
- To maintain glucose levels in the blood
Strategy
- precursor of glycogen synthesis is UDP-glucose
- glycogen degradation mainly achieved by glycogen phosphorylase
Stoichiometry
Regulation
- glycogen degradation – glucagon and epinephrine (activate glycogen phosphorylase)
- glycogen biosynthesis – insulin
Gluconeogenesis
Purpose
- Produces glucose from non-carbohydrate precursors, such as glycerol and glucogenic amino acids
Strategy
- Cori cycle
- fructose -> fructose 6-P -> glucose 6-P -> glucose
- All except 2 reactions of glycolysis can be reversed. These are avoided in gluconeogenesis
Stoichiometry
- 2 pyruvate + 4 ATP + 2 GTP + 2 NADH2+ + 6H2O --> glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+
Regulation
Pentose Phosphate pathway
Purpose
- Alternative pathway for oxidation of glucose
Strategy
- First part is oxidative: produces 1 mole of CO2 and 2 moles of NADPH2+ per mole of glucose
- second part is non-oxidative: rearranges products of oxidative reaction so they can enter glycolysis or gluconeogenesis
Stoichiometry
- maximizing NADPH2+ production: glucose 6-P + 12 NADP + 6H2O --> 6CO2 + 12 NADPH2
- maximizing ribose production: 5 glucose 6-P --> 6 ribose 5-P
Regulation
- glucose 6-P dehydrogenase is the committed step.
- Regulated by availability of NADP+
Describe the biochemical basis of fetal alcohol syndrome and methanol intoxication
- FAS: Ethanol is oxidized by the fetus, producing acetaldehyde
- Methanol intoxication: Methanol is oxidized to formaldehyde, which is even more toxic than acetaldehyde.
Explain and give examples of how catabolic and anabolic pathways can both be thermodynamically favourable
- Biosynthetic pathways are inherently unfavourable, but this can be overcome by creating a high-energy precursor. One example of this is the precursor for glycogen synthesis uridine diphosphate glucose (UDP-glucose).
Describe the operations of the Cori cycle
- Lactate is produced in skeletal muscle
- transported by blood to the liver
- converted to glucose by gluconeogenesis
- transported back to muscle.
Describe the metabolic function and type of reactions involving each of the redox cofactors NAD+, FAD and NADP+
Define and give examples of integrated metabolic pathways
Integrated metabolic pathways are those that include one or more pathways working together to optimize various products. One notable example is the PPP (pentose phosphate pathway)
Describe the structure and metabolic role of glycogen
Structure
- Glycogen is a polymer of glucose residues linked mainly by α(1-4) glycosidic linkages. There are α(1-6) linkages at branch points. The chains and branches are longer than shown.
Function
- Storage method for glucose, buffer for blood-glucose concentration.
Describe how glycogen synthesis and degradation are controlled
See 1.c.
Define and give examples of high-energy biosynthetic precursors
See 3
