Carbohydrate Metabolism Flashcards
0
Q
What different forms of carbohydrates are there?
A
-Mono-, di-, polysaccharides and dextrins
1
Q
Why are carbohydrates very polar?
A
-Contain lots of aldehyde, ketone and -OH groups
2
Q
By what type of bonds are disaccharides joined?
A
-Glycosidic
3
Q
Is lactose a mono,di or polysaccharides?
A
-Disaccharides
4
Q
Is glucose a mono,di or polysaccharide?
A
-Monosaccharides
5
Q
What is the basal glucose level in the blood?
A
-5mM
6
Q
Why can’t glucose readily cross cell membranes?
A
-Hydrophillic so can not cross hydrophobic phosopholipid bilayer
7
Q
What are the main uses of carbohydrates?
A
- Main fuel source
- Energy store -> glycogen
- In anabolic nucelic acid, glycolipid and glycoprotein synthesis
- Release energy and reducing power via catabolic pathways
8
Q
What are the four stages of carbohydrate metabolism?
A
- Stage 1 -> digestion and absorption
- Stage 2 -> glycolysis
- Stage 3 -> TCA/Krebs cycle
- Stage 4 -> Oxidative phosphorylation
9
Q
What happens in stage 1 of carb metabolism?
A
- Carb-> monosaccharides in the lumen of GI tract for absorption
- Digestion -> salivary amylase (Buccal Cavity), pancreatic amylase, glycosidases and disaccharidases on brush border in duodenum/jejenum
- Absorption -> Monomers actively transported into blood via GLUT transporters to tissues
10
Q
How are GLUT transporters representative of glucose requirements?
A
-GLUT1-5 receptors have varied distribution and affinity throughout the tissues of the body which represents the glucose dependancy
11
Q
Which cells/tissues have an absolute glucose requirement?
A
-RBCs, WBCs, Kindey medulla, lens of the eye (CNS is v.preferential)
12
Q
What is the purpose of glycolysis?
A
- Breakdown into intermediate metabolites
- Release of energy and reducing power
- Provide building blocks for anabolism
13
Q
What is the end product of glycolysis?
A
-2 x Pyruvate
14
Q
Where does glycolysis occur?
A
-In all active tissues intracellularly in the cytoplasm
15
Q
Which steps in glycolysis are irreversible?
A
-1, 3,7 and 10
16
Q
Describe phase 1 of glycolysis
A
- Glucose->G6P (uses ATP,irreversible, anionic so cannot go back across PM,)
- G6P->F6P (increases reactivity)
- F6P->F16BP (irreversible, committing step, uses ATP)
17
Q
What enzyme catalyses step 1 of glycolysis (Glucose-> G6P)?
A
-Hexokinase
18
Q
What enzyme catalyses step 3 of glycolysis(F6P->F16BP)?
A
-Phosphofrucokinase-1
19
Q
What are the most important steps of phase 2 glycolysis
A
-Steps 7 and 10 as these are the irreversible steps which both produce ATP (substrate level phosphorylation)
20
Q
What is the net synthesis of ATP in glycolysis
A
-2 ATP
21
Q
Why are the intermediates DHAP and G3P important?
A
-G3P is oxidised DHAP which is used in TAG synthesis
22
Q
Why is the intermediate 1,3-BPG important?
A
-Used to synthesis 2,3-BPG which is important in regulating Hb
23
Q
What is the an overview pathway of glycolysis?
A
- C6->C6 (using 2ATP)
- C6->C3
- C3-> 2 x pyruvate (Producing 4 ATP and 2NADH)
24
What is the overall equation for glycolysis?
-Glucose + 2ADP + 2Pi +2NAD+-> 2 Pyruvate +2ATP + 2NADH +2H+ +2H2O
25
Why can humans not digest cellulose?
-Linked together by B1-4 glycosidic links and do not possess an enzyme to degrade this strong bond
26
How is glycolysis under product control?
- High ATP inhibits glycolysis
- High ATP means high NADH and low NAD+
- negative feedback inhibition of step 6 where NAD+-> to NADH
27
What are the two types of enzymatic control of glycolysis?
- Allosteric regulation
| - Covalent Modification
28
Describe allosteric activation
- Enzyme does not recognise substrate
- Allosteric activator binds to enzyme, at a site other than the active site
- Causes a conformational change
- Enzyme now recognises substrate
29
Describe allosteric inhibition
- Enzyme recognises substrate
- Allosteric inhibitor binds to enzyme, at a site other than the active site
- Causes a conformational change in the active site
- Enzyme no longer recognises substrate
30
Give an example of allosteric inhibition in glycolysis
- Step 1 -> G6P acts as an allosteric inhibitor of hexokinase. As G6P accumulates it binds to hexokinase and inhibits its function
- Step 3 in skeletal muscle -> High ATP:AMP ratio decreases glycolysis by allosterically binding to phosphofructokinase-1. Low ATP:AMP ratio increases glycolysis as AMP acts as an allosteric activator binding to another site on phosphofructokinase-1
31
How does insulin:glucagon have an effect on glycolysis?
In the liver
- High Insulin promotes glycolysis as the concentration of glucose is high in the blood
- High glucagon stops glycolysis as the concentration of glucose is low
32
How does covalent modification control enzymes in glycolysis?
-De/phosphorylation of enzymes will inhibit/activate the enzyme
33
Why are the irreversible steps unidirectional?
-The gibbs free energy is favourable in one direction
34
Why is lactate dehydrogenase important, particularly in RBCs?
- Regenerates NAD+ from NADH through the pathway Pyruvate +NADH + H+-> Lactate + NAD+
- Important in RBCs as they have no mitochondria which is where NADH is normally oxidised
35
Which cells produce lactate?
- RBCs
| - Skeletal muscle
36
What happens to the lactate produced?
-Transpotrted to the liver, heart and kidney where it is converted to pyruvate, oxidised to CO2 and converted to glucose
37
What is hyperlactaemia?
-An elevation in blood lactate levels but below 5mM - not significant
38
Why is an elevation of blood lactate >5mM significant?
- Exceeds renal threshold and begins to effect buffering capacity of the blood
- Leads to lactic acidosis
39
What may cause an increased production of lactate?
- Shock
- Hearty eating
- Strenuous exercise
- Congestive heart disease
40
What may cause decreased utilisation of lactate?
- Liver disease
- Thiamine deficiency
- During alcohol metabolism
- Enzyme deficiencies
41
What happens in lactose intolerance to cause diaarhoea?
- Lactose persists in colon
- Increases osmotic pressure in lumen
- Draws in water
- Causes diarrhoea
42
Describe normal galactose metabolism
- Galactose -> Galactose-1-P via GALACTOKINASE
- Galactose-1-P -> Glucose-1-P via GALACTOSE-1-P URIDYL TRANSFERASE
- Glucose-1-P enters glycolysis
43
How is Glucose made from galactose and why is this mechanism important?
- UDP-galactose gets converted to UDP-glucose by UDP-galactose EPIMERASE
- Important in lactose intolerance and lactation as it allows lactose to be made as the reaction is reversible, i.e. UDP-glucose can be converted to galactose and allow lactose to be made
44
What are the two causes of galactosaemia?
- Lack of GALACTOKINASE
| - Lack of GALACTOSE-1-P URIDYL TRANSFERASE
45
Which galactoseamia enzyme deficiency is rare?
-galactokinase deficiency
46
Which galactosemia enzyme deficiency is more severe and why?
- Galactose-1-P uridyl transferase deficience
| - Accumulation of Galactose -1-P as well as galactose as galactokinase becomes saturated
47
How can galactosaemia cause cataracts/blindness?
- Galactose accumulates in the lens of the eye
- Galactose->Galactitol by enzyme aldose reductase
- Uses NADPH->NADP+ causing a reduction in NADPH
- NADPH is an oxidative protective mechanism
- Depletion of NADPH leads to damage to lens of eye
- Inappropriate DSB formation in crystalline protein in eye due to oxidation of protein (loss of reducing power)
- Forms cataracts
- In addition non-enzymatic glycosylation of galactose increases the osmotic pressure and causes swelling of the lens -> can lead to blindness
48
Why is an accumulation of Galactose-1-P in galactosaemia a problem?
- Damage to liver, kidney and brain
- hepatomegaly due to accumulation -> galactose begins entering pathways it is not usually in
- hepatocellular damage due to depletion of Pi
- Depletion of Pi interferes with organs normal function
49
What is the substrate for the pentose phosphate pathway?
-G6P
50
How is the pentose phosphate pathway regulated?
-By G6P dehydrogenase via NADPH/NADP ratio
51
In which tissues is the pentose phosphate pathway important?
- Liver
- Adipose tissue
- RBCs
52
Describe the first phase of the pentose phosphate pathway
- Phase 1-> oxidative decarboxylation - removes CO2 -> irreversible-> G6P + 2NADP+ -> C5 + 2NADPH + 2H+ + CO2
- Regulated by G6P DEHYDROGENASE and PHOSPHOGLUCONATE DEHYDROGENASE
53
Describe phase 2 of the pentose phophate pathway
-Converts unused C5 sugar phophates into intermediates of glycolysis -> F6P and Glyceraldehyde 6P
54
What are the functions of the pentose phosphate pathway?
- Produces C5 sugar pentose for nucleotide synthesis (more active in dividing tissues)
- In adipose tissue it provides NADPH for lipid synthesis
- In RBCs provides NADPH to protect RBCs from oxidative damage, preventing DSB formation
55
What is G6P Dehydrogenase deficiency?
-Deficiency in the enzyme Glucose 6 Phosphate Dehydrogenase
56
What kind of inheritance pattern does G6PD deficiency have?
- X linked recessive
| - affects mainly mediterranean/Black USA males
57
What type of mutation causes G6PD deficiency?
-Point mutation
58
What happens in G6PD deficiency?
- Decreased G6PD activity in RBCs
- Consequently there is a decrease in NADPH as during the pentose phosphate pathway G6PD reduces NADP+ to NADPH
- Decreased NADPH means reduced oxidative protection in RBCs resulting in DSB formation
- This causes decreased structural integrity and decreased functional activity of RBCs
- Coupled with this NADPH reduces oxidised GSSG to GSH to replenish oxidative protection
- Because there is decreased NADPH, Glutathione becomes saturated
- Oxidative protection is decreased further, more DSB formation
- Insoluble Heinz bodies form due to aggregation of Hb
59
Why can G6PD deficiency lead to haemolysis?
- Heinz bodies and decreased structural integrity can potentially cause haemolysis
- Haemolytic episodes are usually induced after exposure to an oxidative chemical, eg antimalarial drugs
60
Explain the key role of Pyruvate Dehydrogenase in glucose metabolism
- Converts Pyruvate to AcetylCoA for use in stage 3 of catabolism
- A critical step in the production of energy
- Irreversible as it involves the removal of CO2
- Reaction is sensitive to the energy status of the cell, i.e. ATP/NADH inhibit the reaction
61
How is pyruvate dehydrogenase activated?
- High pyruvate
- high NAD+
- High CoA
- insulin through phosphorylation
62
What is the overall equation for Pyruvate to AcetylCoA?
-Pyruvate + CoA + NAD+ -> AcetylCoA + NADH + H+ + CO2
63
What enzyme catalyses step 10 in glycolysis?
-Pyruvate kinase
64
Where does pyruvate->acetyl coA occur?
-Mitochondrial matrix (pyruvate is transferred from the cytoplasm)
65
How many molecules of acetyl coA does one glucose produce?
-2
66
What inhibits pyruvate dehydrogenase?
- ATP
- Acetyl CoA
- NADH
67
Why does PDH deficiency cause lactic acidosis?
- Lack of PDH
- No AcetylCoA produced
- Can not store excess pyruvate
- excess pyruvate shunted to lactate; lactate increases
- Lactate decreases pH of the blood leading to lactic acidosis
68
What are the main functions of the TCA cycle?
- To transfer the energy from glucose into chemical bond energy mainly NADH and FADH2 (reducing power)
- To synthesis intermediates to be used for the synthesis of non-essential a'a, in gluconeogenesis, haem synthesis and FA synthesis
69
Where does the TCA cycle occur?
-In the mitochondrial matrix
70
What are the 4 requirements for TCA cycle?
- Acetyl Co A
- Oxaloacetate
- NAD+
- FAD+
71
Which pathways converge at the TCA cycle?
Hint: Which pathways can be fed into the cycle?
-Sugar, FA, Ketone bodies, alcohol and a'a metabolism
72
Is the TCA cycle oxidative or reductive?
-Oxidative
73
How many unidirectional steps are there in the TCA cycle and why are they irreversible?
-2 - loss of CO2
74
What are the products of the TCA cycle per glucose?
- 6NADH
- 2FADH2
- 2GTP
75
What are the regulatory factors of the TCA cycle?
- Predominantly ATP:ADP ratio -> high ATP-> H+ cannot enter saturated ATPase-> H+ stops translocating -> stops ETC accepting e- ->inhibits glycolysis
- NADH:NAD+ ratio -> High NADH -> High H+ to drive ATP synthase -> inhibits glycolysis
76
How are the enzymes of the TCA cycle regulated?
-High energy signals such as NADH and low energy signals such as ADP allosterically bind to enzymes and inhibit/activate them, respectively
77
Why can the TCA cycle not function in the absence of O2?
- O2 is the final electron acceptor
| - Without O2 NAD+ is not regenerated and the cycle cannot function
78
What is the main function of oxidative phosphorylation?
-Primary source of energy generation
79
Where and what as is the energy stored which is released from glucose after the TCA cycle?
- Stored as chemical bond energy in NADH, FADH2
| - Also ATP from glycolysis and GTP from TCA cycle
80
`Where does oxidative phosphorylation occur?
-On the inner mitochondrial membrane
81
What has more chemical bond energy, NADH or FADH2 and why?
- NADH
| - Uses 3 ETC complexes whereas FADH2 uses 2
82
What percent of ATP is conserved from NADH and FADH2?
- approx 35% from NADH
| - Approx 31% from FADH2
83
What happens to the remaining energy which is not conserved as ATP?
-Dissipated as heat to maintain body temperature
84
Describe what happens in oxidative phosphorylation
- NADH is oxidised and H+ and e- release. e- is transferred down a series of complex multicomponent carriers which span the inner mitochondrial membrane
- This releases a large amount of free energy in a stepwise fashion
- The final electron acceptor at the end of the ETC is O2
- The free energy released is used to translocate the H+ across the inner mitochondrial membrane, which is impermeable to H+, into the intermembrane space
- This forms an electochemical potential difference across the inner membrane which is known as the proton motive force
- The PMF then drives the H+ ions through an ATPsynthase complex located on the inner membrane, back into the mitochondrial matrix
- The movement of H+ ions through the ATPsynthase complex drives ATP synthesis causing ATP synthase to catalyse ADP+Pi producing ATP
85
What is the relationship between the amount of PMF and ATP
-The greater the PMF the more ATP is produced
86
Compare and contrast oxidative phosphorylation with substrate level phosphorylation
Oxidative phosphorylation
- Requires enzyme complexes
- Energy generation occurs indirectly through the generation and utilisation of the pmf
- Cannot occur without O2
- Major process of ATP synthesis
Substrate level phosphorylation
- Requires soluble enzymes
- Energy generation occurs directly through Pi transfer
- Can occur without O2 - limited
- Minor process of ATP synthesis
87
Why are ETC and ATP synthesis controlled by the same molecules?
-Because they are not mutually exclusive, i.e. they are coupled and must occur at the same time
88
How does High ATP:Low ADP ratio regulate ETC/ATP synthesis?
- Low ADP -> ATPsynthase stops due to low substrate
- This prevents translocation of H+ back into matrix
- [H+] increases in the intermembrane space
- Prevents H+ being pumped out of matrix into intermembrane space
- Absence of H+ causes electron transport to stop
89
What are uncouplers and how do they work?
- Molecules which cause the ETC to become uncoupled from ATP synthesis
- Uncouplers increase the inner membranes permeability to H+
- H+ can now move back into the mitochondrial matrix without passing through the ATPsynthase complex and driving ATP
- ETC has become uncoupled from ATP synthesis
90
What happens to the PMF during uncoupling?
-It is no longer driving ATP synthesis as it is not passing through the ATPase complex and the energy is dissipated as heat
91
How many UCPs are endogenous in the body and where are they found?
- UCP 1-5
| - Located on the inner mitochondrial membrane, largely expressed in brown adipose tissue
92
What is non-shivering thermogenesis?
- A process which enable mammals to survive in cold environments
- UCP-1 causes H+ to be able to leak back across the membrane and be dissipated as heat
93
Where is UCP3 found and why is it important?
- Highly expressed in skeletal muscle
| - Involved in modifying FA metabolism and protection against ROS
94
How much of the BMR does proton leack account for
-20-25%
95
Give examples of exogenous uncouplers and how they work
- CO/cyanide poisoning
- Cause NADH/FADH2 not to be oxidised
- No pmf
- No ATP synthesis
- No heat
