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MCBG > Energy Production: Carbphydrates > Flashcards

Energy Production: Carbphydrates Flashcards

1
Q

What are the 4 stages in catabolism?

A
  1. Breakdown of fuel molecules to building block molecules
    • Short pathways
    • Breakage of C - N and C - O bonds (no C - C)
    • No energy released
  2. Degradation of building block materials to a small number of organic precursors
    • Many pathways
    • Small fraction of energy released • C - C bonds broken
  3. Krebs cycle
    • Carbon oxidised to CO2
    • Small fraction of energy released
  4. Electron Transport and Oxidative Phosphorylation
    • Energy released
    • ATP synthesised
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2
Q

What is stage 1 catabolism?

A

• Purpose - to convert nutrients to a form that can
be taken up into cells.
• Extracellular (GI tract)
• Complex molecules -> building block molecules
• Building block molecules absorbed from GI tract into circulation
• No energy produced.

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3
Q

What is catabolism stage 2?

A

• Intracellular
(cytosolic & mitochondrial)
• Many pathways (not all in all tissues)
• Building block molecules (many) -> simpler molecules (fewer)
• Oxidative (require coenzymes which are then reduced, e.g. NAD+  NADH)
• Some energy (as ATP) produced

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4
Q

What is catabolism stage 3?

A
  • Mitochondrial
  • A single pathway – Tricarboxylic acid (TCA) cycle
  • Oxidative (requires NAD+, FAD)
  • Some energy (as ATP) produced directly
  • Acetyl (CH3CO-) converted to 2CO2
  • (Also produces precursors for biosynthesis)

Lots of reducing power removes

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5
Q

What is catabolism stage 4

A
  • Mitochondrial
  • Electron transport and ATP synthesis
  • NADH & FAD2H re-oxidised
  • O2 required (reduced to H2O)
  • Large amounts of energy substrate (ATP) produced
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6
Q

What is the general formula for carbohydrates?

A

(CH2O)n

Contain aldose or ketone groups

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7
Q

What are mono-, di-, oligo- and polysaccharides?

A

Mono: 3-9
Di: 2
Oligo: 3-12
Poly: 10-1000s

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8
Q

What are the 3 main dietary carbohydrates?

A

Glucose fructose galactose

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9
Q

Which cells have an absoute requirement of glucose

A

All tissues can metabolise glucose but some cells have an
absolute requirement.
• Red blood cells
• Neutrophils • Innermost cells of kidney medulla
• Lens of the eye (together approx. 40g/24 hours)
• Uptake depends on blood [glucose]
• CNS (brain) prefers glucose as fuel (approx. 140g/24 hours)
(can use ketone bodies for some energy requirements in times of starvation but needs time to adapt)

Need an inward gradient for absolute requires

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10
Q

What enzymes are present which digest carbohydrates

A

Saliva - Amylase - Starch, Glycogen -> dextrins
Note: lactose intolerance in some patients

Pancreas - Amylase -> monosaccharides

Small intestine - Disaccharidases attached to brush border membrane of epithelial cells
• lactase (lactose)
• sucrase (sucrose)
• Pancreatic amylase ( a1-4 bonds)
• isomaltase (a1-6 bonds)
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11
Q

Why is cellulose not digested in humans?

A

No enzymes to break down the B1-4 linkages present

Glycosidases only act on alpha

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12
Q

What is lactose intolerance

A

Primary lactase deficiency
• Absence of lactase persistence allele.
• Highest prevalence in Northwest Europe
• Only occurs in adults

Secondary lactase deficiency
• Caused by injury to small intestine:
• Gastroenteritis 
• Coeliac disease 
• Crohn's disease 
• Ulcerative colitis
• Occurs in both infants & adults 
• Generally reversible

Congenital lactase deficiency
• Extremely rare, autosomal recessive defect in lactase gene. Cannot digest breast milk.

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13
Q

How are monosaccharides transported?

A
  • Active transport (low to high concentration) into intestinal epithelial cells by sodium‐dependent glucose transporter 1 (SGLT1) then,
  • passive transport (high to low concentration) via GLUT2 into blood supply
  • Transport, via blood supply, to target tissues.
  • Glucose uptake into target cells via facilitated diffusion using transport proteins (GLUT1 - GLUT5) (high to low concentration)
  • GLUTs have different tissue distribution and affinities
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14
Q

What are glucose transporters

A

• Glucose uptake into cells from blood is via facilitated
diffusion using transport proteins (GLUT1 - GLUT5).
• GLUTs can be hormonally regulated (e.g. insulin/GLUT4)

GLUT1 : Fetal tissues, adult erythrocytes, blood–brain barrier.
GLUT2: Kidney, liver, pancreatic beta cells, small intestine
GLUT3 : Neurons, placenta.
GLUT4: Adipose tissue, striated muscle *insulin-regulated
GLUT5 : Spermatazoa, intestine

  • GLUT2 and GLUT4 are particularly important and will be covered further elsewhere in your course.
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15
Q

WHAT ARE THE FUNCTIONS OF GLYCOLYSIS

A 
Functions 
• Oxidation of glucose 
• NADH production (2 per glucose) 
• Synthesis of ATP from ADP (net= 2 ATP per glucose) 
• Produces C6 and C3 intermediates
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16
Q

WHAT ARE THE FEATURES OF GLYCOLYSIS

A 
Features 
• Central pathway of CHO catabolism 
• Occurs in all tissues (cytosolic) 
• Exergonic, oxidative 
• C6 -> 2C3 (No loss of CO2) 
• With one additional enzyme (PDH), is the only pathway that can operate anaerobically 
• Irreversible pathway
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17
Q

What are 3 key enzymes of glycolysis

A
  1. Hexokinase (glucokinase in liver) - glucose -> glucose-6-phosphate
  2. Phosphofructokinase-1 - KEY CONTROL ENZYME - fructose-6-p -> fructose-1,6-bisphosphate
  3. Pyruvate kinase - phosphoenolpyruvate -> pyruvate
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18
Q

Why so many steps/enzymes?

A
  1. Chemistry easier in small stages
  2. Efficient energy conservation
  3. Gives versatility
    • allows interconnections with other pathways
    • allows production of useful intermediates
    • allows part to be used in reverse
  4. Allows for fine control
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19
Q

What is glycerol phosphate?

A

Important intermediate in glycolysis
• Important to triglyceride and phospholipid biosynthesis
• Produced from dihydroxyacetone phosphate (DHAP) in adipose tissue and liver
• Therefore, lipid synthesis in liver requires glycolysis
• (Note: Liver can phosphorylate glycerol directly)

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20
Q

What is 1,3-bisphosphoglycerate

A

Important intermediate in glycolysis
• Produced from 1,3-bisphosphoglycerate in RBC
• Important regulator of O2 affinity of haemoglobin (tense form)
• Present in red blood cells (RBC) at the same molar
concentration as haemoglobin (approx. 5 mM)

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21
Q

What is allosteric regulation of enzymes?

A

• Allostery (allo = other, steric = site) - activator/inhibitor binds at ‘another’ site

Proteins (usually enzymes) with two sites
1. Catalytic site
2. Regulatory site(s)
– Binding of specific regulatory molecule
– Affects catalytic activity
– can produce activation or inhibition

• Covalent modification (phosphorylation/dephosphorylation)

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22
Q

What are the principles of metabolic pathway regulation?

A

• Flux through pathways regulated in response
to the need
• Irreversible steps are potential sites of
regulation
• Reduced activity reduces the flux of substrates through the pathway directly
• Reducing levels of product
• Reversible steps are not regulated
• Reactions still come to equilibrium so leveksof product are unaffected

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23
Q

What is PFK?

A

Key regulator or glycolysis

Allosteric regulation (muscle)
• Inhibited by high ATP
• Stimulated by high AMP

Hormonal regulation (liver)
• Stimulated by insulin
• Inhibited by glucagon

Glucagon switches off enzyme - retain glucose in circulation - phosphorylates - inhibits
Insulin signalling high energy - use glucose - dephosphorylates - stimulates

24
Q

How else is glycolysis regulated

A 
Metabolic regulation 
Product inhibition by g6-p
High [NADH] or low [NAD+] = high energy level signal 
• Causes product inhibition of step 6 
• Inhibits glycolysis
Hormonal activation 
• PFK and Pyruvate kinase 
• increase by high insulin:glucagon ratio 
• i.e. high insulin low glucagon 
• (enzyme dephosphorylation)

Hexokinase product inhibition by g6-p

25
Q

Describe step 6

A

• 2 moles of NADH produced per mole of glucose
• Pathway therefore requires NAD+
• Total NAD+ + NADH in cell is constant, therefore, glycolysis would stop when all NAD+ is converted to NADH
• Normally, NAD+ is regenerated from NADH in stage 4 of metabolism (see Lecture 4) BUT
1. RBC have no stage 3 or 4 of metabolism
2. Stage 4 needs O2 - supply of O2 to muscles and gut often reduced
• Therefore, need to regenerate NAD+ by some
other route
Lactate Dehydrogenase (LDH)

26
Q

How is lactate produced

A

Produced from glucose (and alanine) via pyruvate
• without major exercise 40 - 50 g/24 hours
– RBC, skin, brain, skeletal muscle, G.I. tract

• Strenuous exercise (includes hearty eating!) 30 g/5
min
– Plasma levels x 10 in 2 - 5 min
– Back to normal by 90 min

• Pathological situations, e.g.
– SHOCK
– CONGESTIVE HEART DISEASE

27
Q

What is the lactate dehydrogenase reaction?

A

NADH + H+ + pyruvate NAD+ + lactate
CH3COCOOH -> CH3CHOHCOOH
Catalysed by LDH

• Lactate by liver and heart (via LDH)
• produced by RBC and skeletal muscle (skin, brain, GI)
• Released into blood and
• normally metabolised by liver and heart
• Liver and heart need NAD+ to be regenerated efficiently, usually
well supplied with oxygen

Keeps glycolysis running

28
Q

Where is LDH found?

A

RBC, skeletal muscle (skin, brain, GI)

In low O2 conditions pyruvate converted to lactate instead of stage 4 of metabolism which requires O2

Impaired in liver disease
Vitamin deficiency - thiamine
Alcohol NAD+ -> NADH
Enzyme deficiencies

29
Q

What is plasma lactate concentration determined by?

A

Plasma concentration determined by relative rates of

  1. production
  2. utilisation (liver, heart, muscle)
  3. disposal (kidney)
30
Q

What ate hyperlactaemia and lactate acidosis?

A

Hyperlactaemia
• 2 - 5 mM
• Below renal threshold
• No change in blood pH (buffering capacity)

Lactic acidosis
• above 5 mM
• Above renal threshold
• Blood pH lowered

31
Q

Describe fructose metabolism and its clinical importance

A

Fructose = fruit monosaccharide
Glucose-fructose disaccharide
Metabolised in liver in human

Clinical Importance 
1. Essential fructosuria - fructokinase missing
• Fructose in urine no clinical signs 
2. Fructose intolerance - aldolase missing
• Fructose-1-P accumulates in liver
- leads to liver damage 
• Treatment? 
- remove fructose from diet
32
Q

Describe galactose metabolism

A

Lactose, dissacharide in milk, galactose + glucose Metabolism in liver (major tissue)
UDP-glucose acts catalytically Clinical importance - Galactosaemia - 1 in 30,000 births

33
Q

What is galactosaemia

A

Autosomal recessive disorder which affects how the body processes galactose

Deficiency in any of the enzymes galactokinase, uridyl transferase or up-galactose epimerase can cause it
Milk rich diet - infancy
Unable to utilise galactose
Galactokinase deficiency (rare) - galactose accumulates
Transferase deficiency (common) - galactose and galactose-1-P accumulate Problem
• Galactose enters other pathways

Galactose —> galactitol
(NADPH -> NADP+)
Catalysed by aldose reductase

  • depletes lens of NADPH - structure damaged - cataracts
  • Accumulation of galactose-1-P affects liver, kidney, brain - How?
  • Treatment - no lactose in diet
  • Raised galactose concentration enters new pathways
  • Depletes NADPH levels
  • Prevents maintenance of free sulphydryl groups on proteins
  • Inappropriate disulphide bond formation
  • Loss of structural and functional integrity of some proteins that depend on free -SH groups
  • E.g. lens of eye
34
Q

What is the pentose phosphate pathway?

A

Series of non-oxidative reactions convert 5C sugars into 6C and 3C sugars which can be utilised by glycolysis
All enzymes in cytosol

2 stages
1) oxidative decarboxylation
Glucose-6-p —-> C5 sugar + CO2
(NADP+ —> NADPH)
2) rearrangement to glycolysis intermediates
3 C5 sugars ——> 2 f6P + 1 glyceralddehyde-3-p

No atp production
Loss of co2 to irreversible
Controlled by NADP+/NADPH ratio at G6P dehydrogenase

35
Q

What are the functions of the pentose phosphate pathway?

A
  1. produce NADPH in cytoplasm
    (a) Biosynthetic reducing power
    e.g. lipid synthesis therefore, high activity in liver and adipose tissue
    (b) Maintain free -SH (cysteine) groups on certain proteins
    Prevent oxidation to - S - S - (disulphide bonds)
  2. Produce C5 sugar for nucleotides needed for nucleic acid
    synthesis
    Therefore, high activity in dividing tissues e.g. bone marrow
36
Q

Describe G6PDH deficiency

A
  • Pentose phosphate pathway has an important role in providing NADPH to maintain SH group of proteins in a reduced state
  • Structural integrity and, hence, functional activity of some proteins depends on free -SH groups
  • G6PDH deficiency is a very common inherited defect
  • e.g. in RBC, low NADPH -> disulphide bonds formed -> aggregated proteins, Heinz bodies -> haemolysis
  • Lens of eye
37
Q

What are the key points of the pentose phosphate pathway?

A 
• Starts from Glucose-6-phosphate 
• Important source of NADPH required for:
• Reducing power for biosynthesis 
• Maintenance of  GSH levels 
• Detoxification reactions
• Produces C5-sugar ribose required for synthesis of :
• Nucleotides, 
• DNA & RNA.
• No ATP synthesised. CO2 produced 
• Rate limiting enzyme is:
Glucose 6-phosphate dehydrogenase
LEARN PRINCIPLES ONLY: No need to memorise entire pathway!
38
Q

What is pyruvate dehydrogenase?

A

• Mitochondrial matrix
(pyruvate transported from cytoplasm across mitochondrial membrane)
• PDH is a large multi-enzyme complex (5 enzymes)
• The different enzyme activities require various cofactors (FAD, thiamine pyrophosphate and lipoic acid). B-vitamins provide these factors, so reaction is sensitive to Vitamin B1 deficiency.
• Reaction is irreversible, so is a key regulatory step.
• Irreversible loss of CO2
• Pyruvate cannot be formed from acetyl-CoA
• Large multi-enzyme complex
• Irreversible reaction
• Subject to multiple regulation
• PDH deficiency – lactic acidosis

39
Q

What is the TCA cycle?

A
  • Mitochondrial
  • A single pathway –
  • Acetyl (CH3CO-) converted to 2CO2
  • Oxidative (requires NAD+, FAD)
  • Some energy produced (as ATP/GTP)
  • (Also produces precursors for biosynthesis)
40
Q

Give an overview of the tca cycle

A

Citrate (c6)

  • > c6
  • > isocritate (c6)
  • > (nad+ to nadh, co2 released) c5
  • > (coA in, nad+ to nadh, co2 released) c4
  • > (gdp to gtp, coA out) c4
  • > (fad to fadh2) c4
  • > (H2O in) c4
  • > (nad+ to nadh) c4
  • > c4 + acetyl coA (c2) from link, makes citrate
41
Q

How many tca cycles per glucose?

A

2

42
Q

How is ca cycle regulated?

A

• Regulated by energy availability,
i.e. ATP/ADP ratio and NADPH/NAD+ ratio

Irreversible steps - release co2
Regulated by energy containing molecules
Adp is a low energy signal - stimulates

Enzymes: isocritate dehydrogenase, a-ketoglutarate dehydrogenase

43
Q

Give a summary of the tca cycle

A

Mitochondrial
• Central pathway in the catabolism of sugars, fatty acids, ketone bodies, amino acids, alcohol
• Strategy - to produce molecules that readily lose CO2
• Breaks C-C bond in acetate (acetyl~CoA); carbons oxidised to CO2
• Oxidative producing NADH and FADH2
• Some energy as GTP (~= ATP) produced directly
• Produces precursors for biosynthesis
• Does not function in absence of O2 (c.f. electron transport chain)
• Intermediates act catalytically - no net synthesis or degradation of Krebs cycle intermediates alone

44
Q

What happens in catabolism stage 4?

A
  • Mitochondrial
  • Electron transport and ATP synthesis
  • NADH & FADH2 re-oxidised
  • O2 required (reduced to H2O)
  • Large amounts of energy (ATP) produced
45
Q

How is reducing power used in aTp synthesis?

A

Two processes
1. Electrons on NADH and FAD2H transferred through a series of carrier molecules to oxygen (ELECTRON TRANSPORT)
Releases energy in steps

  1. Free energy released used to drive ATP synthesis
    (OXIDATIVE PHOSPHORYLATION)
46
Q

How are electrons transported

A

• Electrons are transferred through series of carrier molecules (mostly within proteins) to O2, with release of energy.
• ~30% of energy used to move H+ across membrane (a lot of the energy released as heat.)
• [H+] gradient (membrane potential) across inner mitochondrial membrane caused by energy of electrons = proton motive force
PTC = protein translocations complex

47
Q

What is a Proton translocating ATPase?

A

ATP synthase
Uses hydrogen ion gradient to drive the synthesis of atp
Giving up energy of proton gradient
• Return of protons (H+) is favoured energetically by the
electrochemical potential (electrical and chemical gradient)
• Protons (H+) can only return across membrane via the ATP synthase and this drives ATP synthesis

48
Q

What is oxidative phosphorylation?

A

• Electrons are transferred from NADH and FAD2H to
molecular oxygen
• Energy released is used to generate a proton gradient, proton motive force (pmf)
• Energy from the dissipation of the proton motive force is coupled to the synthesis of ATP from ADP

  • Electrons in NADH have more energy than in FAD2H,
  • so NADH uses 3 PTCs, FADH2 uses only 2.
  • The greater the p.m.f. -> more ATP synthesized
  • Oxidation of 2 moles of NADH -> synthesis of 5 moles of ATP (P/O = 2.5)
  • Oxidation of 2 moles of FADH2 -> synthesis of 3 moles of ATP (P/O = 1.5)
49
Q

How is oxyphos regulated?

A

• When [ATP] is high, i.e. [ADP] is low, so no substrate for ATP synthase (synthetase)
• so inward flow of H+ stops
• Concentration of H+ in the intermitochondrial space increases
• Prevents further H+ pumping - stops electron transport
• Normally oxidative phosphorylation and electron transport
are tightly coupled
• Both regulated by mitochondrial [ATP]
• High ATP = Low ADP
• When [ADP] is low, so no substrate for ATP synthase (synthetase)
• so inward flow of H+ stops
• Concentration of H+ in the intermitochondrial space increases
• Prevents further H+ pumping - stops electron transport
• Reverses with low [ATP]

50
Q

How is oxidative phosphorylation inhibited

A

• Inhibitors block electron transport, e.g. cyanide (CN-) prevents acceptance of electrons by O2
Bind with higher affinity than oxygen - reduced h+ translocation reduces pmf - synthesis of ATP reduced - leads to cell death
No electron transport - Block the whole of oxyphos and all of catabolic

  • Uncouplers increase the permeability of the mitochondrial inner membrane to protons - h+ enter mitochondria without driving ATP synthase
  • dissipate the proton gradient, thereby reducing the proton motive force
  • No drive for ATP synthesis
51
Q

What are oxyphos diseases

A

Genetic defects in proteins encoded by mtDNA (some subunits of the PTCs and ATP synthase) so decrease in electron transport and ATP synthesis

52
Q

=what is coupling of oxyphos

A
  • What happens to the rest of the energy ?
  • Lost as HEAT
  • Efficiency depends on tightness of coupling
  • Can be varied in some tissues
  • Brown adipose tissue - degree of coupling controlled by fatty acids (uncouplers) - allows extra heat generation
53
Q

Describe e- transport in brown adipose tissue

A

Contains thermogenin (UCP1) - naturally-occurring uncoupling
In response to cold, noradrenaline (norepinephrine) activates :
1. Lipase which releases fatty acids from Triacylglycerol
2. Fatty acid oxidation -> NADH/FADH2 -> electron transport
3. Fatty acids activate UCP1
4. UCP1 transports H+ back into mitochondria

So, Electron Transport uncoupled from ATP Synthesis. Energy of p.m.f. is then released as extra heat.
Family of UCPs – role in heat generation by uncoupling but may have other functions.

54
Q

Where is brown adipose found

A

Newborn infants to mainatain heat around vital organs

Hibernating animals to generate heat to maintain body temp

55
Q

Compare oxyphos and sub level phos

A

Oxy
Requires membrane-associated complexes (inner mitochondrial membrane)
Energy coupling occurs indirectly through generation & subsequent utilisation of a proton gradient (pmf)
Cannot occur in the absence of O2
Major process for ATP synthesis in cells requiring large amounts of energy

Sub
Requires soluble enzymes (cytoplasmic & mitochondrial matrix)
Energy coupling occurs directly through formation of high energy of hydrolysis bond (phosphoryl-group transfer)
Can occur to a limited extent in the absence of O2
Minor process for ATP synthesis in cells requiring large amouts of energy

MCBG (18 decks)

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