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Metabolism (Semester 1) > Carbohydrate Metabolism > Flashcards

Carbohydrate Metabolism Flashcards

1
Q

What is the general formula for a Carbohydrate and are they hydrophilic or hydrophobic?

A

(CH2O)n

Hydrophilic because they contain many OH groups and interact with water.

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2
Q

How do monosaccharides enter cells?

A

They require transporters to pass across the cell membrane due to their hydrophilic nature.

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3
Q

What is a monosaccharide?

A

It is a single sugar unit with 3-6 C atoms. It is partially oxidised and so requires less O2 for complete oxidation than a fatty acid with the same length carbon chain.

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4
Q

How do monosaccharides form a disaccharide?

A

A glycosidic bond forms between the two units. -O-

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5
Q

Give three examples of polysaccharides

A

Glycogen, Cellulose, Starch

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6
Q

What is an Oligosaccharide?

A

It contains 3-12 sugar units and can also be called a Dextrin

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7
Q

Why cannot cellulose be broken down?

A

It contains B glycosidic bonds which the GI enzymes cannot digest.

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8
Q

How many stages are there in Catabolism and what happens at each stage?

A

1 - Large molecules are broken down into smaller building blocks which can be up taken by cells. This happens in the GI extracellularly.
2 - Building blocks are broken down into smaller molecules in this oxidative step (which releases some reducing power). Small amounts of ATP are produced
3 - TCA cycle. Acetyl is converted to carbon dioxide with the. Release of some energy as ATP.
4 - Mitochondrial step. H carriers are reoxidised and so this step requires oxygen. A large amount of ATP energy is released.

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9
Q

What reaction does salivary amylase support?

A

Starch/ Glycogen —> Dextrins

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10
Q

What is the enzyme which breaks down Dextrins and where does this occur?

A

Pancreatic amylase, this occurs in the small intestine and Dextrins are broken down into Monosaccharides.

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11
Q

Disaccharases are found on the brush border in the small intestine. What are the four different enzymes which can be found here?

A

Lactase, Sucrase, Maltase (a1-4 bonds), Isomaltase (a1-6 bonds)

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12
Q

How is glucose taken into cells from the blood stream?

A

Facilitated diffusion through GLUT1-5 transport proteins. This is hormonally controlled.

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13
Q

What are the four cell types which have an absolute requirement for glucose?

A

Rbc, wbc, kidney, lens of eye.

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14
Q

Where does glycolysis occur?

A

It is an intracellular pathway which occurs in the cytoplasm of all cells.

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15
Q

What is the overall reaction for glycolysis?

A

Glucose (C6) + 2NAD+ + 2Pi + 2ADP –> 2 Pyruvate (C3) + 2NADH + 2H+ + 2H2O

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16
Q

What is the function of glycolysis and what 3 useful things are produced?

A

To oxidise glucose.

ATP is synthesised from ADP, C6 and C3 intermediates are produced and NADH is produced.

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17
Q

Which enzyme catalysts the reaction Glucose –> Glucose-6-phosphate?

A

Hexokinase. In the liver it is Glucokinase.

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18
Q

What reaction does phosphofructokinase catalyse?

A

Fructose 6 phosphate –> Fructose 1,6 bis phosphate.

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19
Q

In step 10 of glycolysis, ADP –> ATP. What is the enzyme which catalysts this step and what is the end product?

A

Pyruvate kinase and then end product is pyruvate.

20
Q

In glycolysis, what is the overall ATP synthesis?

A

At the beginning of the pathway, 2 moles of ATP are invested. After cleavage, 2 moles of ATP are produced for each pyruvate and so overall 2 moles of ATP is synthesised.

21
Q

What is the effect of a slower rate on an equilibrium reaction and a mono directional reaction?

A

In an equilibrium reaction it will increase the time it takes to reach equilibrium. In a mono directional reaction it will reduce the yield.

22
Q

Why does lactose intolerance cause diarrhoea?

A

There is a lack of lactase enzyme and so lactose is not broken down. It remains in the intestine, lowering the water potential drawing water in from the cells of the intestinal walls. This means there is more water in the colon and therefore diarrhoea.
Bacteria can break down this lactose producing gases which leads to bloating and discomfort.

23
Q

What is the additional enzyme needed for glycolysis to occur anaerobically?

A

Lactate Dehydrogenase. This reaction means that the H carrier NADH is reoxidised to NAD and so glycolysis can continue.

24
Q

What are the two fates of lactate?

A

In the heart, it is converted to pyruvate and then oxidised to form CO2.
In the liver it undergoes gluconeogenesis and is converted back to glucose.
This can occur because these tissues both have good oxygen supplies.

25
Q

What is the consequence of increases lactate in the blood?

A

Elevations in plasma lactate can occur and this means that it exceeds the renal threshold and is excreted in the urine but it also affects the buffering capacity of the blood - lactic acidosis.

26
Q

What can lead to an increased plasma lactate?

A

Strenuous exercise, hearty eating, congestive heart disease, liver disease, thiamine deficiency, alcohol metabolism.

27
Q

Why is DHAP (dihydroxyacetone phosphate) and important intermediate in glycolysis?

A

It can be converted to glycerol phosphate by the enzyme Glycerol-3-phosphate dehydrogenase. Glycerol phosphate is very important in TAG and lipid synthesis. The liver can also phosphorylate glycerol.

28
Q

What is bisphosphoglycerate mutase?

A

It is an enzyme which acts on 1,3 bisphosphoglycerate and converts it to 2,3 bisphosphoglycerate which is an important regulator in the O2 affinity of haemoglobin.

29
Q

Why can bagged blood only be kept for a certain amount of time? (Think 2,3 BPG)

A

Blood for transfusions is fed with glucose, but as this is used up, 2,3 BPG will be converted back to 1,3 BPG and will continue down the glycolysis pathway to produce energy. This means that if the bloody was transfused it would have a high affinity for oxygen and so would not release oxygen at the tissues and so it would not be beneficial due to the lack of 2,3 BPG.

30
Q

What is an allosteric regulator?

A

It is a molecule which binds at a site which is not the active site on an enzyme to either activate or inhibit its action.

31
Q

What molecule inhibits the action of Hexokinase?

A

Glucose 6 phosphate.

32
Q

In muscle, what impact does a high ATP to AMP ratio have on the action of phosphofructokinase?

A

It inhibits its action.

33
Q

In the liver, what activates the enzyme phosphofructokinase?

A

A high insulin to glucagon ratio

34
Q

What effect does a high insulin to glucagon ratio have on pyruvate kinase?

A

It activates it by dephosphorylation of the enzyme.

35
Q

What are two common sites where lactate is produced?

A

RBC and skeletal muscle.

36
Q

List the steps for fructose to enter glycolysis.

A

Fructose –> Fructose1phosphate by fructokinase

Aldose catalysts reaction to Glyceraldehyde 3 phosphate - enters glycolysis.

37
Q

What are the clinical consequences of deficiency of Fructokinase or Aldolase?

A

A lack of fructokinase has no clinical symptoms and fructose is excreted in urine.
A lack of Aldolase means that levels of fructose1phosphate build up and this causes damage to the liver. Treatment is a fructose free diet.

38
Q

What reaction does galactokinase catalyse?

A

Galactose –> Galactose 1 phosphate

39
Q

What is the enzyme which catalysts the reaction of Glactose 1 phosphate to glucose 1 phosphate?

A

Uridyl transferase.

40
Q

What is the enzyme which is commonly deficient in galactosaemia? And what are the clinical consequences of this?

A

Uridyl transferase. This means that both galactose and galactose 1 phosphate accumulate, and they enter other pathways. When galactose is converted to galactitol it depletes the supply of NADPH which is needed to maintain sulphide ions in their reduced form. Inappropriate disulphide bridge formation occurs (in lens of eye this is cataracts).

41
Q

What is known the treatment for galactosaemia?

A

Complete removal of lactose from the diet.

42
Q

Why is the reaction in the penthouse phosphate pathway irreversible?

A

There is loss of CO2.

43
Q

What is the function of the penthouse phosphate pathway?

A

It produces NADPH in the cytoplasm which is used to maintain SH residues and also is bio synthetic reducing power used in lipid synthesis.

44
Q

What useful precursor does the pentose phosphate pathway provide?

A

It provides nucleotides for nucleic acid synthesis.

45
Q

What is the consequence of Glucose6phosphate dehydrogenase deficiency?

A

This is the enzyme involved in the pentose phosphate pathway so this means that there is reduced NADPH and so some inappropriate disulphide bridges may form which can lead to decrease protein functionality. In RBC Heinz bodies form.

46
Q

What is the name of the enzyme which catalysts the reaction Galactose –> Galactitol?

A

Aldose reductase

Metabolism (Semester 1) (20 decks)

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