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MEH > Carbohydrates II > Flashcards

Carbohydrates II Flashcards

1
Q

What is a rate-limiting Enzyme

A

Enzymes with the lowest Catalytic Activity,
Often the metabolic pathways are controlled by the rate of the “rate-limiting enzyme”

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2
Q

What is the potential site of regulation

A
  • Irreversible steps are potential sites of regulation
  • Reduced activity reduces the flux of substrates through the pathway directly
  • Reducing levels of product
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3
Q

Are reversible steps regulated

A
  • Reversible steps are not regulated
  • But products may influence reaction rates (feedback regulation)
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4
Q

How does product affect reaction rate

A

When product conc gets too high it will feedback and reduce the activity of one of the enzymes further up the pathway.

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5
Q

What does Allosteric mean and what binds to it

A

A site other than where the substrate binds to (on an enzyme) - Activators and Inhibitors bind at this site

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6
Q

What are the two sites on a protein (enzyme)

A
  1. Catalytic site. Substrate(s) —-> Product(s)
  2. Regulatory site(s)
    – Binding of specific regulatory molecule
    – Affects catalytic activity
    – can produce activation or inhibition
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7
Q

What is Phosphofructokinase

A

Phosphofructokinase is a kinase enzyme that phosphorylates fructose 6-phosphate in glycolysis.

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8
Q

What is a kinase enzyme

A

kinase, an enzyme that adds phosphate groups (PO43−) to other molecules.

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9
Q

What regulates Phosphofructokinase

A

Phosphofructokinase is regulated by allosteric regulators and by covalent modification (phosphorylation)

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10
Q

What are the Allosteric Regulations (muscle) of glycolysis
- What inhibits and Stimulates it

A

Inhibited by high ATP
Inhibited by high Citrate (a product further down the pathway)

Stimulated by high AMP
Stimulated by high F2,6,BP

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11
Q

What is the hormonal regulation (liver) of glycolysis
- What inhibits and Stimulates it

A

Stimulated by Insulin
Inhibited by Glucagon

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12
Q

What does HexoKinase do AND what

A

Hexokinase catalyzes the phosphorylation of glucose, the rate-limiting first step of glycolysis.

Hexokinase phosphorylates glucose thus it becomes charges meaning it cant travel out of cell

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13
Q

what happens when there is a high ratio of insulin to glucagon

A

There is a hormonal activation of phosphofructokinase

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14
Q

Why do some cells convert pyruvate to lactate?

A

when oxygen is limited, the body temporarily converts pyruvate into a substance called lactate, which allows glucose breakdown—and thus energy production
It also occurs when there is an inability to use NADH for
oxidative phosphorylation

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15
Q

What happens when all NAD+ is converted to NADH

A

glycolysis would stop when all NAD+ is converted to NADH

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16
Q

How is NAD+ generated

A

Normally, NAD+ is regenerated from NADH in stage 4 of metabolism

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17
Q

How many moles does NADH produce per mole of glucose

A

2 Moles of NADH

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18
Q

Why do some cells use Lactate Dehydrogenase (LDH)

A
  1. Some cells (e.g. RBC, eye lens) have no stage 3 or 4 of metabolism
  2. Stage 4 needs O2 - supply of O2 is sometimes insufficient (e.g. exercising skeletal muscles)
    * Therefore, need to regenerate NAD+ by some other route
    Lactate Dehydrogenase (LDH)
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19
Q

What is Lactate Dehydrogenase

A

LDH catalyzes the conversion of lactate to pyruvate and back, as it converts NAD⁺ to NADH and back.

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20
Q

Why is it important to recycle NAD/NADH

A

cells would run out of NAD+ and glycolysis would stop

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21
Q

Where is lactate produced and how much is produced WITHOUT MAJOR EXERCISE

A

40 - 50 g/24 hours
– RBC, skin, brain, skeletal muscle, G.I. tract

22
Q

How much lactate is produced WITH MAJOR EXERCISE

What else changes

A

30 g/5 min

– Plasma levels increase 10-fold in 2 - 5 min
– Back to normal by 90 min

23
Q

What else apart from exercise can affect lactate production

A

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

24
Q

Plasma concentration determined by relative rates of (3)

A
  1. production
  2. Utilisation
  3. Disposal
25
Q

What is Hyperlactemia

A
  • 2-5mM lactate measurement
  • Below renal threshold
  • No change in blood pH
    (buffering capacity)
26
Q

What is Lactic Acidosis

A
  • above 5mM lactate measurement
  • Above renal threshold
  • Blood pH lowered
27
Q

What is the normal lactate level

A

Concentration normally constant <1 mM

28
Q

Where is fructose mostly metabolised

A

Liver

29
Q

What is Essential fructosuria?

A

Essential fructosuria
- fructokinase missing
* Fructose in urine no clinical signs

30
Q

What is Fructose intolerance

A

Fructose intolerance
- aldolase missing
* Fructose-1-P accumulates in the liver
- leads to liver damage and possible death
* Managed by removing fructose and sucrose from the diet

31
Q

Where is lactose found

A

Lactose is a disaccharide found in milk

32
Q

What is lactose metabolised into

A

galactose + glucose

33
Q

Where does galactose metabolism occur

A

Metabolism in the liver (major tissue)

34
Q

What is Galactosaemia

A

Galactosemia is a disorder that affects how the body processes a simple sugar called galactose.

35
Q

What are the three enzymes involved in Galactosaemia
what is wrong with the enzyme

A
  1. Galactokinase
  2. UDP-galactose epimerase
  3. Uridyl transferase

Deficiency in any of these 3 enzymes can cause Galactosaemia

36
Q

How rare Galactosaemia

A

1 in 30,000 births

37
Q

What does Galactokinase do

A

The enzyme is responsible for the conversion of Galactose into Galactose-1P.

38
Q

what does UDP - galactose epimerase do?

A

The enzyme is responsible for catalyzing the reversible conversion of galactose - 1P to UDP-galactose.

39
Q

What does Uridyl transferase do?

A

facilitate change from galactose-1-P to glucose-1-P

40
Q

What happens when you are unable to utilise galactose (2)

A

-Galactokinase deficiency (rare) - galactose accumulates
-Transferase deficiency (common) - galactose and galactose-1-P accumulate

41
Q

What does the accumulation of galactose in tissues lead to

A

-Accumulation of galactose in tissues leads to its reduction to galactitol (aldehyde group reduced to alcohol group) by the activity of the enzyme aldose reductase:
-This reaction depletes some tissues of NADPH. In the eye the lens structure is damaged, (cross-linking of lens proteins by –S-S- bond formation) causing cataracts. In addition, there may be non-enzymatic glycosylation of the lens proteins because of the high concentration of galactose and this may contribute to cataract formation. The accumulation of galactose and galactitol in the eye may lead to raised intra-ocular pressure (glaucoma) which if untreated may cause blindness.

42
Q

What does the accumulation of galactose 1-phosphate in tissues cause

A

Accumulation of galactose 1-phosphate in tissues causes damage to the liver, kidney and brain and may be related to the sequestration of Pi making it unavailable for ATP synthesis.

43
Q

What is the treatment to Galactosemia

A

No lactose in diet

44
Q

What is the pentose phosphate pathway

A

The pentose phosphate pathway is a metabolic pathway parallel to glycolysis.
It generates NADPH and pentoses as well as ribose 5-phosphate, a precursor for the synthesis of nucleotides. While the pentose phosphate pathway does involve the oxidation of glucose,
its primary role is anabolic rather than catabolic

45
Q

Where does Pentose Phosphate Pathway start from

A

Starts from Glucose-6-phosphate

46
Q

Why is the production of NADPH important

A
  • Reducing power for biosynthesis
  • Maintenance of GSH levels
  • Detoxification reactions
47
Q

What is the C5-sugar ribose used to sythenthise (2)

A
  • Nucleotides,
  • DNA & RNA.
48
Q

what is the rate-limiting enzyme in the pentose sugar pathway

A

Glucose 6-phosphate dehydrogenase

49
Q

Is ATP or CO2 produced in Pentose phosphate pathway

A

No ATP is synthesised. CO2 produced

50
Q

What happens in Glucose-6-phosphate dehydrogenase (G6PDH) deficiency

A

-G6PDH is the rate limiting enzyme of the pentose phosphate pathway
-It supplies reducing energy by maintaining NADPH levels
- NADPH is required to protect against oxidative stress by maintaining the level of reduced glutathione.
- Since the pentose phosphate pathway is the only source of reduced glutathione in red blood cells, these are particularly affected by defects in the glucose-6-phosphate dehydrogenase enzyme.
-Patients with G6PDH deficiency are therefore at risk of haemolytic anaemia in states of oxidative stress such as infection or exposure to certain chemicals or medications. Damaged red cells are phagocytosed in the spleen and metabolism of the excessive haemoglobin to bilirubin can lead to jaundice.

51
Q

What does pyruvate dehydrogenase in glucose metabolism do ?

A

Pyruvate does not enter stage 3 of catabolism directly but is first converted to acetyl~CoA by the enzyme pyruvate dehydrogenase

PDH is a multi-enzyme complex that catalyses the overall reaction: CH3COCOOH + CoA + NAD+ → CH3CO~CoA + CO2 + NADH + H+

MEH (13 decks)

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