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BioChem > Lec21/22 - An Overview of Further Metabolism (Carbohydrate Anabolism + Storage; Glucose Metabolic Regulation; Nitrogen Fixation; Amino Acid Metabolism; Lipid Metabolism) > Flashcards

Lec21/22 - An Overview of Further Metabolism (Carbohydrate Anabolism + Storage; Glucose Metabolic Regulation; Nitrogen Fixation; Amino Acid Metabolism; Lipid Metabolism) Flashcards

1
Q

Define Gluconeogenesis

A

The synthesis of glucose from non-carbohydrate precursors, (e.g., alanine and lactate) via pyruvate

(Sort of inverse of glycolysis, but different enzymes for steps 1, 3 and 10)

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2
Q

What is the purpose of gluconeogenesis?

A

To maintain glucose levels in the blood and provide energy to the brain and muscles

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3
Q

Describe how step 1 of gluconeogenesis differs from step 10 of glycolysis

A

IN GLUCONEOGENESIS 1:
Pyruvate -> Oxaloacetate -> Phosphoenolpyruvate (catalysed by pyruvate carboxylASE, then phosphoenolpyruvate carboxylkinASE)

IN GLYCOLYSIS 10:
Phosphoenolpyruvate -> Pyruvate (Pyruvate KinASE)

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4
Q

Describe how step 8 of gluconeogenesis differs from step 3 of glycolysis

A

IN GLUCONEOGENESIS 8:
Fructose 1,6-bisphosphate -> Fructose 6-phosphate (catalysed by Fructose 1,6-bisphosphatASE)

IN GLYCOLYSIS 3:
Fructose 6-phosphate -> F1,6P (catalysed by phosphofructokinASE-1)

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5
Q

Describe how step 10 of gluconeogenesis differs from step 1 of glycolysis

A

IN GLUCONEOGENESIS 10:
Glucose 6-phosphate -> Glucose (Glucose 1,6-phosphatASE)

IN GLYCOLYSIS 1: Glucose -> Glucose 6-phosphate (HexokinASE, GlucokinASE)

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6
Q

Describe how the 4 (mentioned) non-carbohydrate precursors can enter the gluconeogenesis pathway

A
  1. Lactate <-> pyruvate
  2. Alanine <-> pyruvate
  3. Aspartate <-> oxaloacetate
  4. Glycerol/Fats <-> Dihydroxyacetone Phosphate
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7
Q

State the overall stoichiometric equation for Gluconeogenesis (and state what this shows about free energy compared to glycolysis)

A

2Pyruvate + 2NADH + 4ATP + 2GTP + 6H2O + 2H+
——>
Glucose + 2NAD+ + 4ADP + 2GDP + 6Pi

(More free energy to generate glucose from pyruvate [4+2 = 6ATP/GTP] than can be generated from glycolysis [2ATP])

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8
Q

What is pyruvate carboxylase?

A

An important enzyme at the start of gluconeogenesis, which catalyses a metabolically IRREVERSIBLE reaction that occurs in the mitochondrial matrix

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9
Q

State the equation of the reaction catalysed by Pyruvate Carboxylase

A

Pyruvate + CO2 + ATP -> Oxaloacetate + ADP + Pi

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10
Q

State the role of Biotin in Gluconeogenesis

A

It is a prosthetic group that is covalently attached to Pyruvate Carboxylase (via a Lys side chain) - the first enzyme in gluconeogenesis

It is a carrier of activated CO2 (carboxybiotin)

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11
Q

How is Pyruvate Carboxylase allosterically activated, and what is the advantage of this?

A

Allosterically activated by Acetyl CoA -> Accumulation of Acetyl CoA signals that energy is abundant, so gluconeogenesis is more useful than glycolysis

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12
Q

What is starch and where is it stored?

A

It is the glucose storage molecule in animals (equivalent to starch in plants) and is stored in cytosolic granules in the liver and muscle cells of vertebrates

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13
Q

What is Glycogenolysis, and what are the steps?

A

The degradation of glycogen:
Glycogen -> Glucose-1-phosphate -> Glucose-6-phosphate (PHOSPHOGLUCOMUTASE) -> Glucose (GLUCOSE-6-PHOSPHATASE)

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14
Q

How does glycogen compare to fatty acids as an energy store?

A

It is less reduced, so not as rich in energy - is is a quick source of glucose for a sudden demand in energy (also, released glucose can provide energy in the absence of oxygen)

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15
Q

How are glucose molecules removed from the glycogen polymer?

A

GLYCOGEN PHOSPHORLASE catalyses sequential removal of G1P (then the other enzymes convert this to glucose)

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16
Q

Describe what happens in glycogenolysis near a branch point in the molecule

A

Glycogen phosphorylase stops at 4 residues from a branch point - “limit dextrin” is then remodelled by transferase and a-1,6-glucosidase

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17
Q

Describe the role of transferase in glycogenolysis

A

At a branch point, transferase shifts three glucoses from one outer branch to the other

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18
Q

Describe the role of a-1,6-glucosidase in glycogenolysis

A

After transferase shifts 3 glucoses, a-1,6-glucosidase removes the single remaining branched glucose, leaving an elongated unbranched chain so that Glycogen Phosphorylase can continue

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19
Q

Summarise the process of glycogen synthesis

A

In the liver, cellular glucose is converted to G6P by GLUCOKINASE (using ATP)
Then one G6P molecule is incorporated into glycogen (using UTP)

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20
Q

Describe the process of UDP-Glucose (active form of glucose) formation in Glycogen Synthesis

A

The enzyme UDP-Glucose Pyrophosphorylase liberates the outer 2 Phosphates from UTP as PPi;

UMP + G1P -> UDP-Glucose (+PPi)

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21
Q

Describe the overall energy requirement of glycogen synthesis

A

Overall, 2ATP required to incorporate one glucose molecule into glycogen (1 from glycokinase reaction [G6P] and 1 to form UDP-Glucose [ATP energetically equivalent to UTP])

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22
Q

Describe the principle of Glucose Polymerisation in Glycogen Synthesis

A

Glycogen synthase catalyses addition of one glycose to glycogen via formation of a 1,4-glycosidic bond

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23
Q

Describe the role and process of Priming in Glycogen Synthesis

A

Glycogen Synthase needs at least 4 glucoses to add to (a primer)

GlycogenIN initiates glycogen synthesis by polymerising the first 10-20 glucoses

UDP-G + (glucosyl)nGlycogenin -> (glucosyl)n+1-Glycogenin + UDP

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24
Q

FACT: Transfer of glucose to growing chain via 1,4 glycosidic bonds (linear growth) is a MAJOR REGULATORY STEP IN GLYCOGEN SYNTHESIS

A

Cool beans

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25
Q

Describe how glucose enters mammalian cells

A

Down a concentration gradient, via GLUTs (e.g., GLUT1/2/3/4) - a family of passive hexose transporters that facilitate glucose transport

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26
Q

Describe the different roles of GLUT1/2/3/4

A

GLUT1 + 3 = nearly all mammalian cells, constantly transport glucose into cell under normal conditions at almost constant rate

GLUT2: Liver + Pancreatic ß Cells; transport glucose when [Glucose] in blood is high -> Pancreas detects -> Insulin production

GLUT4: Muscle and fat cells, numbers increase rapidly when insulin is present

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27
Q

How does insulin stimulate increased numbers of GLUT4?

A

It binds to an extracellular insulin receptor (A tyrosine kinase), which recruits vesicles containing GLUT4 to the cell membrane

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28
Q

How is regulation of Glycolysis and Gluconeogenesis linked?

A

Co-ordinated pathways: When one is active, the other is relatively inactive (rate of Glycolysis controlled by [Glucose]; rate of gluconeogenesis controlled by [Glucose Precursors])

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29
Q

Describe the role of Fructose 2,6-bisphosphate in regulating Glycolysis and Gluconeogenesis

A

It is a potent ACTIVATOR of PHOSPHOFRUCTOKINASE-1 (regulatory enzyme in Glycolysis);

and an INHIBITOR of FRUCTOSE 1,6-BISPHOSPHATASE (an enzyme specific to gluconeogenesis)

30
Q

Describe the synthesis / counter-conversion of Fructose 2,6-bisphosphate (and how these two enzymes are linked)

A

Synthesised by Phosphorylation of F6P by LIVER PHOSPHOFRUCTOKINASE-2 (PFK2);

Converted back to F6P by FRUCTOSE BISPHOSPHATASE-2 (FBPase2)

Both enzymes are on the same protein chain, controlled by phosphorylation of a single Ser residue (+ = FBPase2; - = PFK2)

31
Q

How is Glucagon involved in regulation of liver glycolysis?

A

When blood [Glucose] low (e.g., after night fast), glucagon rises and leads to phosphorylation of PFK-2/FBPase2, activating FBPase2, so levels of F2,6BP decrease (as converted to F6P), so gluconeogenesis dominates over glycolysis

When blood [Glucose] high (e.g., after meal), glucagon levels decrease and insulin produced, so PFK2 is activated, so F6P converted to F26BP, so PFK1 activated and glycolysis dominates

32
Q

Which 3 hormones “also” regulate glycogen metabolism?

A

Insulin, Glucagon and Epinephrine/Adrenaline

33
Q

What is the effect of insulin on regulation of glycogen metabolism?

A

Insulin (produced by pancretic ß cells) increases rate of glucose transport into muscle and adipose tissue, stimulating glycogen synthesis in the liver

34
Q

What is the effect of Glucagon on regulation of glycogen metabolism?

A

Glucagon (produced by pancreatic alpha cells in response to low [Glucose]) stimulates glycogen degradation

35
Q

What is the effect of insulin on regulation of glycogen metabolism?

A

Adrenline/Epinephrine (produced by adrenal glands in response to sudden energy requirement) stimulates the breakdown of glycogen to glucose, and the release of this glucose into blood

36
Q

Distinguish between essential and non-essential amino acids

A

Essential amino acids cannot be synthesised and must be obtained from the diet
Non-essential amino acids can be synthesised

37
Q

Where does the nitrogen in amino acids “come from”?

A

Nitrogen fixation: atmospheric N2 is converted into ammonia (NH3) by diazotrophic (nitrogen fixing) bacteria e.g., Rhizobium

38
Q

Give the equation of the reaction that takes place to fix nitrogen in Rhizobium, and which enzyme catalyses it

A

N2 + 8H+ + 8e- + 16ATP -> 2NH3 + H2 + 16ADP + 16Pi (NITROGENASE)

Note: this reaction needs ATP and reducing power

39
Q

How is ammonia incorporated into biomolecules (i.e. in what form)?

A

In the form of ammonium ions NH4+

40
Q

Describe the structure of Nitrogenase

A

A tetramer of 2alpha and 2 beta units, also contains a unique iron-molybdenum cofactor

41
Q

How is ammonium incorporated into amino acids in microorganisms (main chain)?

A

NH4+ incorporated into alpha-ketoglutarate (citric acid intermediate) to form L-GLUTAMATE

Catalysed by GLUTAMATE DEHYDROGENASE (reversible in microbes, same enzyme only catalyses reverse in humans)

Note: Needs reducing power NAD(P)H

42
Q

How is ammonium incorporated into amino acids in microorganisms (side chain)?

A

NH4+ incorporated into glutamate to form GLUTAMINE (catalysed by GLUTAMINE SYNTHETASE)

Note: Needs ATP

43
Q

How are other amino acids besides Glutamate and Glutamine synthesised in microbes?

A

Transaminase reactions: the amino group of glutamate is transferred to alpha-keto acids, generating alpha amino acids

44
Q

Where are transaminases found in humans?

A

In the liver

Note: they are a measure of liver function

45
Q

Name two examples (mentioned) of transaminase enzymes

A

Alanine aminotransferase and aspartate aminotransferase (form alanine and aspartate respectively)

46
Q

Name the 9 essential amino acids

A

Valine, Leucine, Methionine, Isoleucine, Lysine, Threonine, Histidine, Tryptophan, Phenylalanine

47
Q

Which amino acids are part of the Glycolysis -> Pyruvate biosynthetic family?

A

Alanine, VALINE, LEUCINE

48
Q

Which amino acids are part of the Glycolysis -> 3-phosphoglycerate biosynthetic family?

A

Serine (Cysteine, Glycine)

49
Q

Which amino acids are part of the Citric Acid Cycle -> Oxaloacetate biosynthetic family?

A

Aspartate (Asparagine, Methionine, Lysine, Threonine [Isoleucine])

50
Q

Which amino acids are part of the Citric Acid Cycle -> alpha ketoglutarate biosynthetic family?

A

Glutamine, Proline, Arginine

51
Q

Which amino acids are part of the Pentose Phosphate Pathway biosynthetic family?

A

Phenylalanine, (Tyrosine), Tryptophan, Histidine

52
Q

What are the three main categories of animals in terms of ammonia excretion?

A

Ammonotelic = excrete ammonia directly in water (marine)

Ureotelic = urea (terrestrial vertebrates)

Uricotelic = Uric Acid (birds, reptiles, insects)

53
Q

What are the two initial steps of Amino Acid Degradation?

A

Reversal of Synthesis Reactions:

  1. TRANSAMINASES transfer the amino group to alpha-ketoglutarate, forming glutarate (and alpha keto acid)
  2. GLUTAMATE DEHYDROGENASE oxidises glutamate and removes NH4+, (thus also regenerating alpha ketoglutarate)
54
Q

What happens in amino acid degradation after ammonia is formed?

A

The Urea Cycle (involving several nitrogenated metabolites) to form urea

55
Q

Why are triacylglycerols so rich in free energy (38kJ/g)?

A

Long hydrocarbon chains are the most reduced form of carbon

56
Q

How are Glycogen and Triglycerides different in terms of their stored energy?

A

Glycogen can supply ATP for muscle contraction for less than 1 hour;
Sustained work is fueled by Catabolism of TGs

57
Q

How are triglycerides stored (and broken back down into Glycerol and FAs)?

A

Stored as lipid droplets in the cytoplasm of specialised fat cells (adipocytes)
Hormone-stimulated lipases break TGs into glycerol and fatty acids

58
Q

Describe the process of glycerol metabolism in the liver

A

Converted to Dihydroxyacetone Phosphate (DHAP), which is then isomerised to glyceraldehyde-3-phosphate (an intermediate in glycolysis/gluconeogenesis)

59
Q

How are fatty acids transported in the blood and into cells?

A

In blood bound to albumin; enter cells via Fatty Acid Transport Protein (FATP) and bind to Fatty Acid Binding Protein (FABP)

(because they are water-insoluble)

60
Q

How are fatty acids activated on the Outer Mitochondrial Membrane (so they can be oxidised)?

A

By Acyl coA Synthetase - forming Fatty Acyl CoA (thioester bond):

Fatty Acid + ATP + CoA <-> Fatty Acyl CoA + (AMP+PPi)

Note: they still cannot cross the membrane, need acyl carnitine translocase

61
Q

How are Fatty Acyl CoA transported across the Inner Mitochondrial Membrane?

A

They cannot cross it, instead they are translocated by Acyl Carnitine Translocase:

Acyl CoA transfers the Acyl group to Carnitine; Acyl Carnitine then crosses the membrane (via the Translocase enzyme) then transfers the Acyl group back to a CoA

62
Q

Which enzymes catalyse the transfer of the Acyl group between Carnitine and CoA in Fatty Acyl CoA OMM transport?

A

In cytosol: Carnitine Palmitoyl Transferase I (CPTI)

In matrix: Carnitine Palmitoyl Transferase II (CPTII)

63
Q

Describe how ß-oxidation of Fatty Acyl CoA occurs

A

In the matrix, four enzymatic steps convert 2 carbons from Fatty Acyl CoA to Acetyl CoA (equivalent to 10 ATP by entering TCA)

64
Q

What would be the net ATP yield for Palmitoyl CoA (a 16 carbon Fatty Acyl CoA)?

A

8 Acetyl CoA x 10 = 80ATP
7 FADH2 x 1.5 = 10.5 ATP
7 NADH x 2.5 = 17.5
-2 for Palmitate Activation

=106 ATP

65
Q

Where does Fatty Acid synthesis occur in mammals?

A

In the cytoplasm of liver and adipocytes

66
Q

What is required for fatty acid synthesis?

A

ATP and reducing power (NADPH)

67
Q

When does fatty acid synthesis occur?

A

When glucose is plentifyl, excess acetyl coA is produced by glycolysis and is used for FA synthesis

68
Q

Which enzyme(s) catalyse Fatty Acid Synthesis?

A

Fatty Acid Synthase (consists of several enzymes in a single polypeptide chain)

69
Q

How does Fatty Acid synthesis fundamentally work?

A

Chain elongation, by sequential addition of two-carbon units derived from Acetyl CoA

70
Q

Describe the first step of Fatty Acid Synthesis (all deets)

A

Acetyl CoA + ATP + HCO3- -> Malonyl CoA + ADP + Pi + H+

Acetyl CoA carboxylase catalyses this irreversible, committed step

BioChem (13 decks)

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