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Biochemistry Flashcards

1
Q

Define glycogenesis

A

Synthesis of glycogen from glucose

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2
Q

Define glycogenolysis

A

Breakdown of glycogen to form glucose

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3
Q

Define gluconeogenesis

A

De novo synthesis of glucose from metabolic precursors (lactate, amino acids, glycerol)

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4
Q

What is glycogen?

A

Main storage form of glucose in liver and muscle cells

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5
Q

Describe liver glycogen

A

Broken down between meals and released to maintain blood glucose levels for red blood cells and brain. Glucose homeostasis

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6
Q

Describe muscle glycogen

A

Not available for maintenance of blood glucose levels. Provides energy via glycolysis and the TCA during bursts of physical activity

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7
Q

Which process fluctuates dependent upon meal times?

A

Glycogenolysis

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8
Q

What is the primary source of glucose overnight when hepatic glycogen is depleted?

A

Gluconeogenesis

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9
Q

Describe the structure of glycogen

A
  • glycogen is a polymer consisting of glucose molecules
  • joined by an alpha 1-4 glycosidic links
  • branches are introduced by alpha 1-6 glycosidic links
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10
Q

Glucose residues can only be added to where?

A

An existing glycogen chain. A glycogen ‘primer’ containing at least 4 glucose residues is required . The primer is covalently attached to a protein called glycogenin

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11
Q

Name the first enzyme in glycolysis

A

Hexokinase

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12
Q

What is the role of phosphoglucomutase?

A

Repositions phosphate 6 to position 1, this is a liver specific reaction. G1P is now primed for glycogen synthesis

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13
Q

Describe UDP-glucose

A
  • simple precursors are first converted to activated intermediates, a common feature of biosynthetic pathways
  • UDP-glucose can be though of as an ‘activated’ form of glucose
  • ATP and acetyl-CoA are activated forms of phosphate and acetate, respectively
  • the phosphate ester linkage in a nucleotide sugar releases free energy on hydrolysis
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14
Q

Describe glycogen synthase

A
  • synthesis glycogen from UDP-glucose
  • adds one glucose molecule to glycogen at a time
  • can only extend the chains of glycogen, cannot start new molecules
  • can not introduce branches
  • rate limiting enzyme of glycogenesis
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15
Q

Describe transglycosylase

A
  • branching enzyme
  • introduces an alpha 1-6 glycosidic branch into glycogen
  • approx. every 10 glycogen residues
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16
Q

What is the rate limiting enzyme of glycogenesis ?

A

Glycogen synthase

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17
Q

What catalyses glycogenolysis?

A

Glycogen phosphorylase

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18
Q

Describe the rate limiting step of glycogenolysis

A
  • catalysed by glycogen phosphorylase
  • one glucose molecule is cleaved off the ends of glycogen at a time
  • glucose 1 phosphate is then converted to glucose 6 phosphate
  • end point is release of free glucose into the bloodstream
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19
Q

Describe glycogenolysis in the liver

A

Glucose 6 phosphate can be de-phosphorylated and the resulting glucose released into the bloodstream via GLUT 2 transporter

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20
Q

Describe glycogenolysis in skeletal muscle

A

Glucose 6 phosphate cannot be de-phosphorylated but instead is used to provide energy via glycolysis and the TCA cycle

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21
Q

Describe the precursors of gluconeogenesis

A
  • lactate; synthesised by skeletal muscle under anaerobic conditions
  • amino acids; derived from muscle protein by proteolysis
  • glycerol; derived from triglycerides by lipolysis in adipose tissue
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22
Q

Describe the energy source for gluconeogenesis

A

From oxidation of fatty acids released from adipose tissue

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23
Q

Describe the location of gluconeogenesis

A

Mainly in the liver, small amounts in the kidneys

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24
Q

Why is gluconeogenesis NOT the reverse of glycolysis?

A

There are irreversible steps

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25
Q

What are the three unique liver enzymes required for gluconeogenesis?

A
  • glucose 6 phosphatase
  • fructose 1,6 biphosphotase
  • PEP carboxykinase
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26
Q

Which enzyme allows for the bypassing of pyruvate kinase?

A

PEP carboxykinase

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27
Q

Which enzyme allows for the bypass of fructose?

A

Fructose 1,6 biphosphate

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28
Q

Which enzyme dephosphorylates glucose 6P to form glucose?

A

Glucose 6 phosphatase

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29
Q

Describe the cori cycle

A
  • lactate as a precursor of gluconeogenesis; formed in fast-twitch muscle under conditions of heavy exercise
  • blood transports lactate to liver
  • glucose released into bloodstream
  • buys time and shifts metabolic burden from muscle to other organs
  • the process of removing lactate that is generated in the skeletal muscle, to convert it back to glucose through the gluconeogenic pathway therefore to maintain blood sugar homeostasis
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30
Q

Amino acids can act as precursors for what?

A

Gluconeogenesis

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31
Q

Describe glycogen storage disease

A
  • group of disease with increased glycogen deposits in liver or muscle or both
  • at least 10 different types, each one due to a defect in a different enzyme
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32
Q

Increased fat intake without appropriate energy expenditure leads to what?

A
  • increase in numbers of adipocytes
  • more fat in adipocytes
  • obesity
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33
Q

Control of energy balance depends on what?

A
  • genetically linked factors; eg protein messengers regulating appetite
  • environmental factors; eg, food abundance, fashionable foods
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34
Q

Uncoupling generates what?

A

Heat

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35
Q

Why is fat required for essential fatty acids?

A
  • some polyunsaturated fatty acids can not be made by the body
  • deficiencies can lead to membrane disorders, increased skin permeability, mitochondrial damage
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36
Q

Why is fat required for fat-soluble vitamins?

A
  • vitamins A,D,E and K
  • absorption of these vitamins is closely linked to that of fat
  • stored in body fat
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37
Q

What can occur if fat intake or absorption is inadequate?

A

Secondary deficiencies can occur

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38
Q

What are the three different types of lipids?

A
  • simple lipids (fatty acids, triglycerides, waxes)
  • compound lipids (phospho, glycol-lipids, lipoproteins)
  • steroids (cholesterol, steroid hormones)
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39
Q

Describe lipids

A
  • predominantly hydrocarbon
  • usually contain long chain fatty acids
  • insoluble in water
  • important for their biological function
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40
Q

Describe triglycerides

A
  • triacylglycerols
  • main energy storage for in adipose tissue
  • compact; don’t require concomitant storage of water
  • hydrophobic
  • high energy yield per gram
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41
Q

Describe the structure of triglycerides

A

Consists of glycerol and 3 fatty acids

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42
Q

Describe fatty acids

A
  • mainly straight chains
  • aliphatic (no rings)
  • usually contain an even number of C atoms
  • branched chain and odd numbers of C are rare
  • can be saturated (no double bonds)
  • unsaturated (one double bond)
  • polyunsaturated (several double bonds)
  • double bonds usually in cis configuration
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43
Q

Name the three main natural fatty acids

A
  • palmitic acid
  • stearic acid
  • oleic acid
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44
Q

Describe polyunsaturated fatty acids

A
  • occur only in small amounts
  • many can not be synthesised by the body
  • essential fatty acids
  • eg. linoleic acid
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45
Q

Which carbon is the alpha carbon in fatty acids?

A

The C adjacent to the carboxyl group

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46
Q

Which carbon is the omega carbon in fatty acids?

A

The terminal carbon

47
Q

Describe the structure of omega 3

A

There is a double bond three positions up from the terminal omega carbon

48
Q

Fatty acids with up to 8C atoms are what at room temperature?

A

Liquids

49
Q

Plant fats contain what?

A

Large proportions of unsaturated fatty acids&raquo_space; liquid

50
Q

Animal fats contain what?

A

Mostly palmitic and stearic acid&raquo_space; solid

51
Q

Name the main products of fat digestion

A
  • glycerol
  • fatty acids
  • monoglycerides
52
Q

What is absorbed into mucosal cells of the intestine?

A
  • short and medium length fatty acids enter portal blood

- longer chain FAs and monoglycerides are re-synthesised to triglycerides

53
Q

Describe chylomicrons

A

Coated with a layer of protein, phospholipid and cholesterol. Carrier of polar components that arises from the digestion of lipids. Enter lymph, then the blood stream

54
Q

At muscle and adipose tissue, chylomicrons are attacked and cleaved by what?

A

Lipoprotein lipases

55
Q

Once chylomicrons are cleaved by lipoprotein lipases. free fatty acids are what?

A
  • resynthesised into triaclyglycerols (in adipose tissue for storage)
  • oxidised to provide energy (in muscle)
  • depends on amount available
56
Q

What is lipolysis?

A

Breakdown of lipids

57
Q

Where is fat stored?

A

In adipose tissue

58
Q

Initial cleavage is by what molecule?

A
  • hormone sensitive lipases (eg. adrenaline sensitive)
  • releases free fatty acids and glycerol
  • occurs when energy is needed
59
Q

What do hormone sensitive lipases do?

A

Break the lipids down into constituent parts

60
Q

Describe fatty acid oxidation

A
  • before fatty acids can be oxidised to generate energy, they have to be converted to CoA derivatives
  • occurs in cytoplasm
  • requires energy (equivalent of 2 ATP)
  • further oxidation of fatty acids occurs in mitochondrial matrix so need to be transported into mitochondria by special carrier mechanism
61
Q

Describe the carnitine shuttle

A
  • in the cytoplasm, fatty acids are transferred from acyl- CoA to carnitine
  • acyl-carnitine transporter in inner membrane facilitates antiport of acyl-carnitine into the mitochondrion and carnitine out
  • net result; acyl-CoA located in mitochondrial matrix
62
Q

Name the rate limiting step of energy conversion from fatty acids to ATP

A

The carnitine shuttle

Rate limiting of lipids crossing the cytoplasm into mitochondrial matrix

63
Q

What is beta oxidation?

A
  • cycle of reactions in mitochondrial matrix

- four steps in each cycle

64
Q

Name the rate limiting molecule of beta oxidation

A

Acetyl CoA

65
Q

Name the end products for each cycle of beta oxidation

A
  • 1 acetyl-CoA
  • 1 FADH2
  • 1 NADH and H+
  • 1 fatty acyl-CoA, shortened by 2 carbon atoms
66
Q

The TCA cycle is initiated by which molecule?

A

Acetyl CoA

67
Q

What is the final yield of beta oxidation?

A
  • 8 FADH2
  • 8 NADH
  • 8 H+
  • 9 acetyl-CoA
  • 120 ATP

As the cycle is repeated 8 times

68
Q

What is the P/O ratio of FADH2

A

1.5

69
Q

What is the P/O ratio of NADH + H+

A

2.5

70
Q

Describe unsaturated fatty acids

A
  • already partially oxidised

- yield less FADH2

71
Q

Describe odd chain fatty chains

A
  • yields propionyl-CoA in last step (3 carbon atoms)
  • convert to succinyl-CoA
  • enters TCA cycle directly
72
Q

Describe branched chain fatty acids

A
  • C1 carbon oxidised to CO2

- acetyl and propionyl-CoA released in equal numbers

73
Q

Describe breakdown of glycerol

A
  • activated to glycerol-3-phosphate by glycerol kinase present in liver and kidney but absent from adipose tissue, skeletal and heart muscle
  • dehydrogenated to dihydroxyacetone phosphate, normal intermediate of carbohydrate metabolism
74
Q

Ketosis in starvation or diabetes

A
  • oxaloacetate in consumed for gluconeogenesis
  • fatty acids are oxidised
  • acetyl-CoA is converted to ketone bodies
  • high levels in blood
  • ketone bodies are moderate acids
  • accumulation leads to severe acidosis (blood cant suffer any more)
  • impairs tissue function, particularly central nervous system
  • smell of acetone can be detected in breath
75
Q

De novo synthesis of fatty acids occurs mainly where?

A

In the liver, kidney, mammary glands, adipose tissue and brain

76
Q

When does de novo synthesis of fatty acids mainly take place?

A

During excess energy intake

77
Q

What happens when excess carbohydrate is taken in?

A
  • conversion of fatty acids and triglycerides in the liver
  • free fatty acids are transported in plasma bound to albumin
  • triglycerides formed in the liver are transported to adipose tissue by VLDL for storage
78
Q

Lipogenesis is what type of process?

A

A reductive process, electrons are required

79
Q

Synthesis of fatty acids from acetyl-CoA occurs where?

A

In the cytoplasm of liver cells

80
Q

Acetyl CoA is generated where and by what?

A

Generated in mitochondria by pyruvate dehydrogenase complex

81
Q

The inner mitochondrial membrane is impermeable to what?

A

Acetyl CoA

82
Q

What transports acetyl groups into the cytoplasm?

A

Citrate

83
Q

Citrate is formed from what?

A
  • formed by condensation of acetyl-coA with oxaloacetate
  • first step in TCA cycle
  • when present in high concentrations, citrate is transported into the cytoplasm
84
Q

What is the direct precursor of lipogenesis?

A

Malonyl-CoA

85
Q

Describe the vital first step of lipogenesis

A
  • acetyl CoA carboxylase is expressed mainly in liver and adipose tissue
  • essential regulatory enzyme
  • malonyl-CoA donates carbon atoms to new lipid
86
Q

Describe fatty acid synthase

A
  • catalyses synthesis of saturated long-chain fatty acids from malonyl-CoA, acetyl CoA and NADPH
  • consists of a dimer of identical polypeptides, single polypeptide chain with seven distinct enzyme activities
  • contain an acyl-carrier protein (ACP)
87
Q

Name the three subunits of fatty acid synthase

A
  • 4 release subunit
  • 2 and 3 reduction and dehydration subunit
  • condensation subunit
88
Q

Describe lipogenesis by fatty acid synthase

A
  • fatty acids are synthesised in a cycle of reactions
  • using acetyl-CoA and malonyl-CoA as precursors, one cycle of reaction adds 2 carbon atoms to the growing acyl chain
  • derived from malonyl-CoA
  • growing acyl chain is attached to ACP
  • requires NADPH as electron donor
89
Q

When is the fatty acid released during lipogenesis by fatty acid synthase?

A

When a length of C-16 is reached

  • palmitic acid (C16) is reached, the fatty acid is released by fatty acid synthase
90
Q

Describe the control of fatty acid metabolism

A
  • stringently controlled
  • highly responsive to physiological needs
  • synthesis is maximal when carbohydrate and energy are plentiful and when fatty acids are scarce
  • essential role in regulating fatty acid synthesis and degradation; acetyl CoA carboxylase
  • AMP kinase important
91
Q

Describe regulation of Acetyl-CoA carboxylase

A
  • INSULIN; signals the fed state, stimulate storage of fuels and synthesis of proteins
  • GLUCAGON; signals the starved state
  • EPINEPHRINE; signals requirement for energy; mobilise glycogen stores
  • CITRATE; stimulates allosterically, citrate levels are high when acetyl-CoA and ATP are abundant
  • antagonised by PALMITOYL-COA; abundant when fatty acids are in excess
92
Q

Describe synthesis of triglycerides

A
  • requires glycerol-3-phosphate (G3P)
  • liver produces G-3-P from glycerol
  • adipose tissue produce G-3-P from glucose
  • adipose tissue production of triglycerides only during the fed state; insulin stimulates adipose tissue uptake of glucose
  • triglyceride formation involves esterification
  • liver also synthesises phospholipids, cholesterol and lipoproteins
93
Q

What happens to amino acids which are not used as building blocks?

A

They are degraded - no storage for amino acids

94
Q

Where is the major site of amino acid degradation?

A

The liver

95
Q

Describe absorption of amino acids

A
  • proteolytic enzymes in the stomach and intestine produce single amino acids and di and tri-peptides
  • absorbed into intestinal cells and released into blood for absorption by other tissues
96
Q

Describe protein turnover

A
  • tightly regulated
  • takes place at different rates; important for rapid changes
  • damaged proteins have to be removed
97
Q

Amino acids contain what?

A
  • nitrogen

- some also contain nitrogen in the side chain

98
Q

Amino acid breakdown produces what?

A
  • ammonia (NH3)

- ammonium ion (NH4+)

99
Q

Name the major nitrogen-containing excretory molecules

A
  • urea; 80%, formed in the liver
  • uric acid
  • creatinine
  • ammonium ion (NH4+)
100
Q

Name the three steps in the synthesis of urea

A
  • transamination
  • de-animation
  • urea (or ornithine) cycle
101
Q

Describe transamination

A
  • aminotransferases move the amino group from alpha amino acids to alpha keto acids
  • usually alpha-keto glutarate, a TCA intermediate
  • gives glutamate
  • occurs in all tissues
102
Q

Describe transamination for transport to the liver

A
  • amino group of glutamate is transferred to pyruvate, giving alanine
  • or glutamine synthase adds NH4+ to glutamate giving glutamine
103
Q

Name the major carriers of nitrogen in the blood to liver

A
  • alanine

- glutamine

104
Q

Where does the deamination / urea cycle occur?

A

In the liver

105
Q

Describe deamination / urea cycle

A
  • amino group of glutamate is converted to free ammonium ion
  • urea is synthesised in a complex series of reactions
  • urea / ornithine cycle
  • one nitrogen from free ammonium, the other from aspartic acid
  • carbon from CO2
106
Q

Describe the urea cycle stoichiometry

A

CO2 + NH4+ + 3 ATP + aspartate + 2 H20

urea + 2ADP + 2Pi + AMP + PPi +fumarate

Fumarate is an intermediate in the citrate cycle

107
Q

Describe degradation of carbon skeletons

A
  • after removal of alpha amino acid group, the remaining carbon skeletons are converted into major metabolic intermediates
  • can be converted to glucose or oxidised in the TCA cycle
108
Q

Describe degradation of ketogenic amino acids

A
  • degraded to acetyl-CoA or acetoacetyl-CoA

- can give rise to ketone bodies or fatty acids

109
Q

Describe degradation of glucogenic amino acids

A
  • degraded to pyruvate or TCA cycle intermediates

- can be converted into phosphoenolpyruvate and then into glucose

110
Q

Describe alkaptonuria

A

Degradation of phenylalanine and tyrosine is blocked

111
Q

Describe maple syrup urine disease

A
  • degradation of valine, isoleucine and leucine is blocked
  • urine smells like maple syrup
  • mental and physical retardation
  • prevented by appropriate diet
112
Q

Describe phenylketonuria

A
  • phenylalanine accumulates in all body fluids
  • leads to severe mental retardation if untreated
  • therapy: low phenylalanine diet
113
Q

Describe urea cycle disorders

A
  • accumulation of urea cycle intermediates
  • glutamine levels increase in the circulation
  • alpha ketoglurarate is no longer regenerated
  • alpha ketoglurarate levels become too low to fix more free ammonium ions
  • elevated levels of ammonia in the blood are toxic for the nervous system
  • treatment with low-protein diet
  • drugs which remove nitrogen
  • eg. forming complexes with amino acids which are excreted
  • gene therapy in hepatocytes

GI (11 decks)

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