L7 - Carbohydrates and glycobiology Flashcards
Glycolysis
1) Priming of glucose
2) splitting of a phosphorylated intermediate
3) Oxidoreduction reactions and ATP synthesis
Redox imbalance MUST be adressed
- Can be done aerobically/anaerobically
- If not adressed glycolysis will stop
Inhibition of glycolysis - Sulfhydryl
Sulfhydryl reagents
- Reagens that react with Cys
- Sulfhydryl reagents
The enzyme affected is GAPDH (GAP dehydrogenase) which requires Cys
Consequently, it prevents formation of the thiohemiacetal from GAP
Inhibition of glycolysis - Fluoride
inhibits enolase
Flouride forms a complex with Pi and Mg2+ thereby interfering with binding of the normal substrate 2PG complexed with Mg2+
Inhibition of glycolysis - Arsenic
Pentavalent arsenic (arsenate) resembles Pi in structure and readily substitutes for Pi in enzyme-catalyzed reactions.
Arsenate prevents net synthesis of ATP by causing arsenolysis in the GAPDH catalyzed reaction.
Trivalent arsenic (arsenite) is more toxic, it covalently binds to pyruvate dehydrogenase and requires less dosage.
Metabolism of Fructose and Galactose
There is no galactolysis or fructolysis.
Hence they are converted into glycolytic metabolites.
Fructose metabolism (LIVER)
Fructose > (Fructokinase) > Fructose-1-phosphate > (aldolase) > Glyceraldehyde + Dihydroxyacetone (intermediate) > Glyceraldehyde > (Triose Kinase) > Glyceraldehyde-3-phosphate.
Fructokinase deficiency (essensial fructosuria): Rate of reaction is slowed down, fructose is not phosphorylated
F1P Aldolase deficiency:
Sever hypoglycemia
Accumilation of F1P as Fructokinase is still active.
Fructose metabolism (other tissues)
Fructose > (Hexokinase) > Fructose-6-phosphate
only 1 step conversion into glycolytic intermediate conversion.
Regulation of Glycolysis
Role in :
ATP generation
Carbon building blocks
Rate in glucose conversion needs to match the demand to avoid toxicity.
Enzymes that catalyze irreversible reactions are potential sites of control
For glycolysis:
Hexokinase
Phosphofructokinase
Pyruvate kinase
Enzymes become more or less active in response to allosteric effectors or covalent modification (phosphorylation)
Regulation of glycolysis (SKELETAL MUSCLE)
ATP to AMP ratio is the regulator.
Control site: PFK
High [ATP] > PFK inhibition > Glycolysis slows down
ATP binding to allosteric site lowers affinity for F6P
AMP reverses ATP glycolysis inhibition
[ATP]/[AMP] lowered > PFK more active
pH drop > PFK inhibition
Protection in anaerobic functioning muscle
(enzyme denature)
Other control sites:
Hexokinase-
controls inflow of glycolysis inhibited by its product
PFK inactive > F6P increase > G6P increase > HK inhibition
High [G6P] signals the cell no longer requires glucose for energy or glycogen synthesis
Result in glucose staying in the blood
Pyruvate kinase (PyK) Controls outflow of glycolysis Products are ATP and pyruvate [ATP] high > PyK inhibition [alanine] high > PyK inhibition (signals building blocks) [F1,6BP] high > PyK activation.
Excercise
Low ATP levels, high AMP levels.
PFK is stimulated to produce F1,6BP
Pyruvate kinase is stimulated by F1,6BP
Glycolysis increases
Regulation of glycolysis (LIVER)
Stored glucose as glycogen, which is broken down to glucose if needed.
Phosphofructokinase has more allosteric regulators
ATP regulation is same as in muscle
Low pH is not a signal as lactic acid is broken down in liver
[citrate] high > PFK inhibition
[F2,6BP] high > PFK activation
[Blood glucose] high > [F6P] high > {F2,6BP] synthesis accelerated > PFK activation.
[F2,6BP] bind to PFK > affinity for F6P higher and reduces ATP inhibition.
Hexokinase-
HK in liver is controlled in muscles but has an isoenzyme, glucokinase, not inhibited by G6P
Glucokinase has 50x lower affinity for Glc
Glucokinase main role is glycogen and FA synthesis.
Liver - Pyruvate kinase regulation in glycolysis
PK enzymes differ in affinity to undergo covalent modification.
High glucose > increase dephosphorylation of PK enzyme (more active).
Synthesis of Glc/Glc6P by Gluconeogenesis
Lactate > pyruvate > TCA > Triacyglycerol/Glycerol > G6P > product.
Only applies for certain tissues.
For plants :
CO2 fixation > 3 phosphate glycerate > product
Gluconeogenesis pathway
Non carbohydrates feed into the cycle
In animals fatty acids are not precursors for sugars
Pyruvate (cytosol) > Pyruvate (mito)
Pyruvate > oxaloacetate by biotin + pyruvate carboxylase
Role of Biotin in pyruvate carboxylation
Linked to Lysine in active site of enzyme, able to switch between 2 sites. Biotin moves CO2 from site 1 to site 2, synthesising oxaloacetate.
Once CO2 is released, the affinity is changed and reverts to site 1.
