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glycolic processes Flashcards

1
Q

Which proteins are usually used to transport glucose into cells?

A
  • GLUT proteins
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2
Q

When are GLUT 1 and GLUT 2 proteins used for transport and how do they compare?

A
  • often GLUT 1
  • not modulated by insulin
  • has high affinity for glucose (low Km) but low capacity
  • GLUT 2 used by hepatocytes
  • low affinity for glucose (high Km), but high capacity
  • regulated by insulin – forms part of glucose-sensing system w/ glucokinase
  • phosphorylated by hexokinase to maintain cellular glucose absorption (glucose to G6P)
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3
Q

When is GLUT 4 used and why?

A
  • in muscle and adipose tissue
  • insulin-regulated with a relatively high glucose affinity (low Km)
  • a facilitative transporter
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4
Q

What is normal BGC when fasted?

A

4-7mmol/L

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5
Q

Describe the metabolism of glucose by glycolysis.

A
  • glucose breakdown (6 carbons) to 2x pyruvate (3 carbons)
  • both use and production of ATP (energy)
  • also production of reducing equivalents for energy-production in oxidative phosphorylation
  • located in cytosol
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6
Q

Describe reactions 1-5 of glycolysis with the:

  • substrate → product (enzyme)
  • the point of the reaction
A

1) glucose → glucose-6-phosphate (hexokinase)
- traps glucose in cells and destabilises structure to facilitate later reactions
2) glucose-6-phosphate → fructose-6-phosphate (phosphoglucose isomerase)
- converts 6C ring into a 5C ring in preparation for triose formation
3) fructose-6-phosphate → fructose-1,6-bisphosphate (phosphofructokinase)
- further destabilisation of structure, preparation for triose formation
4) fructose-1,6-bisphosphate → dihdroxyacetone/glyceraldehyde-3-phosphate
(aldolase)
- splits 5C ring into 2x triose sugars
5) dihdroxyacetone → glyceraldehyde-3-phosphate (triosephosphate isomerase)
- isomerisation reaction as only G-3-P can proceed for future reactions

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7
Q

Describe reactions 6-10 of glycolysis with the:

  • substrate → product (enzyme)
  • the point of the reaction
A

6) glyceraldehyde-3-phosphate → 1,3-bisphosphoglycerate (glyceraldehyde phosphate dehydrogenase)
- to provide 2x phosphate groups for ATP synthesis in subsequent reactions
7) 1,3-bisphosphoglycerate → 3-phosphoglycerate (phosphoglycerate kinase)
- substrate-level phosphorylation to produce ATP
8) 3-phosphoglycerate → 2-phosphoglycerate (phosphoglycerate mutase)
- isomerism to promote the formation of more unstable phosphoenolpyruvate
9) 2-phosphoglycerate → phosphoenolpyruvate (enolase)
- formation of unstable product for next rxn
10) phosphoenolpyruvate → pyruvate (pyruvate kinase)
- substrate-level phosphorylation to produce ATP

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8
Q

How do glycolysis phase 1 (rxns 1-5) and glycolysis phase 2 (rxns 6-10) compare?

A
  • phase 1 – uses 2x ATP

- phase 2 – uses no ATP

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9
Q

State the total energy balance of glycolysis in terms of phase 1 and phase 2.

A 
- 2x triose sugars (G-3-P) produced in phase 1
•  phase one
- ATP: -2
- NADPH: 0
•  phase two
- ATP: +4
- NADPH: +2

TOTAL = 2x ATP and 2xNADPH

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10
Q

How does glycolysis occur with galactose and fructose as a pose to glucose?

A
  • galactose converted by multi-step pathway to glucose-6-phosphate and then enters glycolysis
  • fructose phosphorylated by hexokinase (muscle + adipose tissue) to fructose-6-phosphate and then enters glycolysis
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11
Q

Describe regulation and control of glycolysis.

A
  • most bodily reactions reversible but 3 steps of glycolysis irreversible because of energy input by ATP
  • reversible reactions instead regulated via concentrations of substrates and products
  • enzymes regulated by allosteric regulation, hormonal signalling action (short-term) and induction/repression of enzyme synthesis (long-term)
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12
Q

Compare hexokinase and glucokinase activity.

A

• hexokinase

  • universal (most cells)
  • inhibited by glucose-6-phosphate (G6P)
  • unaffected by insulin
  • low Km (0.01 mM glucose)
  • will phosphorylate other sugars

• glucokinase

  • liver & kidney
  • no effects of G-6-P
  • regulated by insulin + glucagon
  • high Km (12mM glucose)
  • specific for glucose
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13
Q

Describe glycolysis regulation by phosphofructokinase (PFK)

A 
• inhibited by:
- ATP
- citrate
- glucagon
• stimulated by:
- AMP
- insulin
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14
Q

Describe glycolysis regulation by pyruvate kinase (PFK)

A 
• inhibited by:
- ATP
- acetyl CoA
- glucagon
• stimulated by:
- fruct-1,6-bisphos
- insulin
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15
Q

Compare where the NAD⁺ needed for pyruvate reactions comes from in aerobic and anaerobic conditions.

A

• aerobic conditions:

  • NAD⁺ regenerated in mitochondria (oxidative phosphorylation)
  • NAD⁺/NADH cannot cross mitochondrial membrane
  • glycerin-phosphate shuttle pathway
  • malate-aspartate shuttle pathway

• anaerobic conditions:

  • lack of oxidation of NADH to NAD⁺ in mitochondria
  • reduction of pyruvate to lactate used to produce NAD⁺
  • oxidation of malate to oxaloacetate (intermediate in Krebs cycle)
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16
Q

Compare the glycerin-phosphate and malate-aspartate shuttle.

A
  • dihydroxygacetone-phosphate (NADH to NAD⁺) → glycerin-3-phosphate → (FAD → FADH₂) →
    dihydroxygacetone-phosphate
  • oxaloacetate → aspartate → oxaloacetate → malate (NADH to NAD⁺) → malate (NADH to NAD⁺) → oxaloacetate

(see notes for diagrams)

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17
Q

Describe the fates of pyruvate.

A
  • (aerobic) pyruvate enters Krebs cycle, producing NADH, FADH₂ and GTP
  • (anaerobic) pyruvate can either be converted to lactate or fermented to ethanol (in microorganisms)
  • both processes use up NADH, converting it to NAD⁺
  • key in allowing regeneration of NAD⁺ to continue glycolysis
18
Q

What is gluconeogenesis and where does it occur?

A
  • formation of glucose from non-carbohydrate sources
  • mainly in liver (not all cells do this)
  • 3 irreversible steps - must be worked around
  • other reactions are close to equilibrium - concentrations of substrates and products determines direction of flow

(see notes for diagrams)

19
Q

State some gluconeogenic substrates.

A
  • AAs (not leucine or lysine)
  • lactate
  • pyruvate
  • glycerol only (from stored fats)
  • oxaloacetate

NB: 2 reactions that produce oxaloacetate from pyruvate (2 molecules of p = 1 molecule of glucose)

20
Q

Describe the reaction in gluconeogenesis which starts with triglyceride.

A

Triglyceride → glycerol → glyceraldehyde-3-phosphate → fructose 1,6-bisphosphate

triglyceride 
⬇
free FAs
⬇ 
acetyl CoA

(gylcerol can enter fluconeogenesis, animals cannot produce pyruvate from actyel-CoA and glucose cannot be synthesised from FAs)

21
Q

Describe the conversion of pyruvate to phosphoeonolpyruvate (PEP).

A
  • pyruvate (pyruvate decarboxylase) → oxaloacetate (phosphoeonolpyruvate carboxykinase) → PEP
  • requirement for ATP
  • GTP hydrolysis for substrate-level phosphorylation
  • pyruvate carboxylase is mitochondrial enzyme so must transport oxaloacetate out of mitochondria
  • mechanism depends on how cell regenerates NADH for use in G-3-P dehydrogenase catalysed reaction
22
Q

Compare conversion of pyruvate to PEP under normal conditions and under stress (during exercise).

A

• normal conditions:

  • pyruvate transported into mitochondria
  • pyruvate (pyruvate carboxylase) → oxaloacetate (reduced) → malate
  • malate exported to cytosol (using NADH)
  • malate (oxidised) → oxaloacetate (cytosolic malate dehydrogenase) + NADH (regenerated)
  • oxaloacetate → PEP

• unders stress/exercise:

  • lactate → pyruvate + NADH (regenerated for GAPDH reaction use)
  • in mitochondria: pyruvate → PEP (exported to cytosol)
23
Q

Describe the stages of gluconeogenesis.

A

pyruvate → oxaloacetate → PEP → 2-phosphoglycerate → 3-phosphoglycerate → 1,3-bisphosphoglycerate (up to here requires 2xATP per reaction) → glyceraldehyde 3-phosphate (requires 2x ADH)→ dihydroxyacetone phosphate → fructose 1,6-phosphate → fructose 6 phosphate → glucose-6-phosphate → glucose

(see notes for detailed diagram)

24
Q

Describe allosteric regulation of gluconeogenesis.

A

• energy status indictaor metabolites reciprocally-regulate enzymes of glycolysis + gluconeogenesis
- i.e. AMP stimulates PFK (glycolytic) while also inhibiting fructobisphosphatase-1 action (gluconeogenesis)
• also specific metabolites activating/inhibiting particular enzymes non-reciprocally
- i.e. ATP action on PFK
• fructose-2,6-bisphosphate also an important specific allosteric regulator

25
Describe fructose-2,6-bisphosphate.
- synthesised from fructose-6-phosphate - allosteric binding to enzymes results in increased/decreased affinity for substrates - provides mechanism modification of glycolysis/gluconeogenesis via insulin + glucagon signalling which results in dephosphorylation/phosphorylation of dual-function enzyme called PFK2 - high BGC results in removal of Ser-32 residue allowing hormonal regulation - PFK2 catalyses both synthesis or degradation: fructose-2,6-bisphosphate ⇌ fructose-6-phosphate
26
What is the importance of metabolite transport between tissues?
- glycolysis found in almost all cell types in body - gluconeogenesis occurs primarily in liver - thus, organs and tissues which produce lactate (e.g. hypoxic tissue) cannot convert it back to glucose - transporting these metabolic products like to liver, can be recycled back to glucose and stored in the body by Cori cycle
27
What is the energy cost of the Cori cycle?
- net 2x ATP produced in anaerobic glycolysis | - however, 4x ATP and 2x GTP used in gluconeogenesis = net 4x ATP (equivalents) used
28
What are the benefits of the Cori cycle?
- rapid post-exercise replenishment of glycogen stores | - NAD⁺ regeneration for glycolysis + NADH regeneration for gluconeogenesis (in different tissues)
29
Describe the properties of glycogen.
- insoluble glucose polymer → carbohydrate storage - monomers linked by α-1,4 bonds (straight chains) and α-1,6 bonds for branch points - heavily-hydrated - glycogenin core - highly-branched - allows improved solubility and sites for synthesis + degradation interactions so they can rapidly occur
30
Which two main tissues store glycogen and for what functions?
1. liver (glucose-6-phosphatase) - BGC maintenance - released over long periods - G-6-P → glucose 2. muscle - energy provision - released when instantaneously-required
31
Describe the 5 stages of glycogen catabolism, including enzymes.
1. conversion: glucose → glucose-1-phosphate (glycogen phosphorylase) 2. addition: UDP-Glucose + glycogen (via α-1,4 bond) (glycogen synthase) - addition of glucose, via α-1,4 linkage, to non-reducing end of growing glycogen chains - released UDP regenerated to UTP for use in glucose activation 3. reversal of above for catabolism (glycogen branching + debranching enzymes) 4. conversion: glucose-1-phosphate → glucose-1-phosphate (phosphoglucomutase) - ATP generates UTP: UDP → UTP + Pi - UTP + G-1-P → UDP-glucose + PPi (UDP-glucose pyrophosphorylase) - phosphorylation: ATP + glucose → G-1-P (traps glucose in cell) - for every 1x mole of glucose = 2x moles ATP consumed in this step 5. activation of glucose: UDP + G-1-P → glucose (UDP-glucose pyrophosphorylase)
32
What are reducing and non-reducing ends of glucose?
- in solution, glucose ring structure is dynamic - at C1, reducing end, there is formation of aldehyde group (oxidation possible) when open - at C4, non-reducing end on other side of this structure (see notes for diagrams)
33
Describe branching enzyme and its activity.
- or amylo-(1-4→1-6) transglycosylase - adds branches to glycogen (via glucose units) - after 10 glucose units added to glycogen, an α-1,6 branch point formed (branching enzyme) - enzyme breaks one of the α-1,4 glycosidic bonds and transfers block of residues (about 7) to more interior site in glycogen - these are then re-attached by an α-1,6 glycosidic bond (see notes for diagram)
34
What is glycogenolysis requires for and which 3 enzymes are involved.
- catabolic process resulting in the formation of free glucose or glucose-6-phosphate - 3 enzymes required: 1. glycogen phosphorylase 2. transferase enzyme 3. glycogen-debranching enzyme
35
Describe the process of glycogen breakdown.
- glycogen phosphorylase activated by: energy demand (AMP), muscle contraction (Ca2⁺ ions) and stress (adrenaline + glucagon) - glycogen (non-reducing end) + pi (phosphate) G-1-P (breaks α-1,4 glycosidic bonds) - processive enzyme - attaches to glycogen and removes glucose residues until >5 away from branchpoint - remaining residues added to existing chain with α-1,4 glycosidic bond (transferase enzyme) - residue at the α-1,6 branching point removed (glycogen-debranching enzyme) - debranching enzyme has dual activity (transferase and debranching)
36
Describe the regulation of glycogen metabolism.
- G-1-P cannot directly enter glycolysis so phosphoglucomutase converts it to G-6-P first - glucose-6-phosphate stays inside cell (ionised and saves energy) and can go straight into glycolysis - glycogen phosphorylase degrades glycogen by breaking α-1,4 glycosidic bonds to release glucose units 1 at a time from non-reducing end of a glycogen chain (end with free 4’-OH group) - glucose released as glucose-1-phosphate: (glycogen) n + Pi ⇌ (glycogen)n-1 + glucose-1-phosphate - differences in muscle and liver glucagon acts on liver only - in liver cells glycogen phosphorylase is inactivated by high BGC
37
Describe glycogen synthesis.
- enzyme glycogen synthase inactivated by phosphorylation - achieved via kinase cascade activation by hormones epinephrine and glucagon - insulin signalling results in phosphatase activation - this de-phosphorylates glycogen synthase and activates enzyme
38
Summarise hormonal control of glycogen synthesis.
- glycogen degradation or synthesis occurs depending on low or high BGC - to prevent futile cycles and produce appropriate responses (see notes for diagram summary)
39
Describe glycogen storage diseases which affect the liver only. including their: - deficient enzyme - symptoms
• Von gierke: - glucose-6-phosphatase - increased glycogen stores, enlarged liver, kidney failure, hypoglycaemia • Hers: - liver glycogen phosphorylase - increased glycogen stores, hypoglycaemia • Type 0: - glycogen synthase - hypoglycaemia, ketosis, failure to thrive • Type VIII/IX: - phosphorylase kinase - enlarged liver, lack of response to glucagon and epinephrine
40
Describe glycogen storage diseases which affect the liver and muscle, just muscle or all organs. including their: - deficient enzyme - symptoms
• McArdle: (muscle only) - liver glycogen phosphorylase - moderate increase in muscle glycogen, exercise-induced cramps • Cori: (liver + muscle) - debranching enzyme (Cori cycle affected) - enlarged liver, mild hypoglycemia • Andersen: (liver + muscle) - branching enzyme - enlarged liver, failure to thrive (physically develop), not fatal • Pompe: (all organs) - lysosomal α-1,4-gucosidase - herat failure in infantile form, muscle defects in juvenile form

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