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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
Q

Describe fructose-2,6-bisphosphate.

A
  • 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
Q

What is the importance of metabolite transport between tissues?

A
  • 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
Q

What is the energy cost of the Cori cycle?

A
  • net 2x ATP produced in anaerobic glycolysis

- however, 4x ATP and 2x GTP used in gluconeogenesis = net 4x ATP (equivalents) used

28
Q

What are the benefits of the Cori cycle?

A
  • rapid post-exercise replenishment of glycogen stores

- NAD⁺ regeneration for glycolysis + NADH regeneration for gluconeogenesis (in different tissues)

29
Q

Describe the properties of glycogen.

A
  • 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
Q

Which two main tissues store glycogen and for what functions?

A
  1. liver (glucose-6-phosphatase)
    - BGC maintenance
    - released over long periods
    - G-6-P → glucose
  2. muscle
    - energy provision
    - released when instantaneously-required
31
Q

Describe the 5 stages of glycogen catabolism, including enzymes.

A
  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
Q

What are reducing and non-reducing ends of glucose?

A
  • 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
Q

Describe branching enzyme and its activity.

A
  • 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
Q

What is glycogenolysis requires for and which 3 enzymes are involved.

A
  • 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
Q

Describe the process of glycogen breakdown.

A
  • 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
Q

Describe the regulation of glycogen metabolism.

A
  • 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
Q

Describe glycogen synthesis.

A
  • 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
Q

Summarise hormonal control of glycogen synthesis.

A
  • 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
Q

Describe glycogen storage diseases which affect the liver only.

including their:

  • deficient enzyme
  • symptoms
A

• 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
Q

Describe glycogen storage diseases which affect the liver and muscle, just muscle or all organs.

including their:

  • deficient enzyme
  • symptoms
A

• 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

N&E (6 decks)

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