Energy production: carbohydrates Flashcards
1
Q
List the two reactions in glycolysis, in which the reactants are phosphorylated compounds with high energy of hydrolysis bonds and therefore are coupled to substrate level phosphorylation
A
- Step 10:
phosphoenolpyruvates + ADP –> pyruvate + ATP - Step 7:
1,3-bisphosphoglycerate + ADP –> 3-phosphoglycerate + ATP
2
Q
What is the general formula for carbohydrates?
A
(CH2O)n
3
Q
What functional groups do carbohydrates contain?
A
- Aldehyde or keto group
2. Multiple hydroxyl groups
4
Q
Name three monosaccharides
A
Glucose
Fructose
Galactose
5
Q
What monosaccharides make up the disaccharide sucrose?
A
glucose and fructose
6
Q
What monosaccharides make up the disaccharide lactose?
A
glucose and galactose
7
Q
Name two digestible polysaccharides
A
Starch
Glycogen
8
Q
Name an indigestible polysaccharide
A
Cellulose
9
Q
Why does the body contain relatively little carbohydrate, in spite of its large intake?
A
Most is used as fuels by tissues and is oxidised to CO2 and H2O.
A small amount is stored as glycogen and as a component of cellular polymers such as nucleic acids, glycolipids and glycoproteins
10
Q
What happens to excess carbohydrate in the diet?
A
Coverted to glycogen for storage and once the glycogen stores are full to triacyglycerols for storage in adipose tissue
11
Q
Why can all monosaccharides, except for dihydroxyacetone, exist as stereoisomers?
A
Because they contain asymmetric C-atoms (four different groups attached to a C-atom)
12
Q
What is the natural form of monosaccharide steroeisomer found in the body?
A
D-enantiomer
13
Q
Describe the structure of glucose
A
It is a hexose (contains 6 carbon atoms). 5 of these carbons and 1 oxygen make a six-membered ring. Each carbon atom has one hydroxyl group.
14
Q
Glucose can exist in two forms alpha- and beta- which enzymes can distinguish between and preferentially use one or the other. What is the difference between these two forms?
A
Beta-D-glucose has all the neighbouring hydroxyl groups opposite sides of the ring to each other. Alpha-D-glucose has the hydroxyl group next to the oxygen in the ring on the same side of the ring as the next hydroxyl group
15
Q
What concentration is glucose maintained at in the blood?
A
Approximately 5mM
16
Q
When does fructose and galactose appear in the blood?
A
For short period after the consumption of fruit and dairy products
17
Q
High concentration of galactose in the blood are associated with what clinical problem?
A
Galactosaemia
18
Q
High concentration of fructose in the blood is associated with what clinical problem?
A
Fructose intolerance
19
Q
List two important physico-chemical properties of sugars
A
- Hydrophilic - water soluble, do not cross cell membranes
2. Partially oxidised - need less oxygen than fatty acids for complete oxidation
20
Q
Name the bond that links monosaccharides together to form disaccharides and polysaccharides
A
glycosidic bond
21
Q
What are most polysaccharides made from
A
One type of monosaccharide (homo-polymers)
22
Q
What two types of glycosidic bonds are found in glycogen?
A
alpha-1,4
alpha-1,6
(10:1)
23
Q
Glycogen is the major store of glucose in the body. Where is glycogen found?
A
Liver and Skeletal muscles - stored as granules
24
Q
What is glycogen made from?
A
Highly branched, glucose polymer
25
Where does starch come from?
Plants
26
What is starch made from?
Polymer of glucose. It is a mixture of amylose (alpha-,4 linkages) and amylopectin (alpha-1,6 linkages)
27
Digestive enzymes in the human GIT hydrolyse starch into...
Glucose and maltose (glucose-glucose)
28
Where does cellulose come from?
Plants - glucose polymer which has a structural role
29
Why can cellulose not be digested?
Glucose units are linked with beta-1,4 glycosidic bonds which can't be hydrolysed by human GI tract enzymes
30
Why is cellulose important in our diet?
It is a major component of dietary fibre, that is important for normal GI tract function
31
What is the name of the glycosidase enzyme secreted in the mouth?
Salivary amylase
| starch&glycogen --> glucose, maltose and smaller polysaccharides (dextrins)
32
What is the name of the glycosidase enzyme secreted into the duodenum?
Pancreatic amylase
| starch&glycogen --> glucose, maltose and smaller polysaccharides (dextrins)
33
What are the names of the gyocosidase enzymes which break down dietary disaccharides (maltose and sucrose), maltose and dextrins in the duodenum and jejunum, releasing the monosaccharides glucose, fructose and galactose?
lactase
glycoamylase
sucrase/isomaltase
34
Where are the glyocsidase enzymes lactase, glycoamylase and sucrase/isomaltase found in the gut?
They are large glycoprotein complexes attached to the brush border membranes of the epithelial cells lining the duodenum and jejunum
35
What happens to the activity of lactase throughout life?
It is high in infants but decreases in childhood in most populations except Northern Europeans
36
What is low activity of lactase associated with?
Reduced ability to digest the lactose present in milk products and may produce the clinical condition of lactose intolerance
37
How does lactose intolerance cause the clinical symptoms of diarrhoea, bloating and discomfort?
In individuals with a low level of lactase, if lactose is digested then it will persist into the colon where bacteria can break it down. The presence of lactose in the lumen of the colon increases the osmotic pressure of the contents and draws water into the lumen, causing diarrhoea. Colonic bacteria can produce hydrogen, carbon dioxide and methane gases from lactose, causing feelings of bloating and discomfort
38
How is glucose, fructose and galactose transported into the epithelial cells lining the gut?
Active transport (against their concentration gradient)
39
How are glucose, fructose and galactose transported from epithelial cells into the blood and from blood into tissues?
Facilitated diffusion (down a concentration gradient) using a family of glucose transport proteins (GLUT1-5)
40
How do the family of glucose transport proteins (GLUT1-5) differ?
In their affinity for glucose and relative activities. They reflect differences in the requirement of tissues for glucose
41
How does the glucose transport protein GLUT-4 function and why is this important?
GLUT-4 is found in skeletal muscle and adipose tissue. It is sensitive to insulin. High levels of insulin increase the uptake of glucose into these tissues by increasing the number of glucose transport proteins in the plasma membrane
42
Which tissues can metabolise glucose, fructose and galactose?
All tissue can remove glucose, fructose and galactose from the blood.
All tissues can metabolise glucose but the LIVER is the major site of fructose and galactose metabolism
43
Why is the concentration of the glucose in the blood kept relatively constant?
Some tissues have an absolute requirement for glucose and the rate of glucose uptake into these tissues is dependent on the concentration in the blood
44
Which tissues have an absolute requirement for glucose?
red blood cells, white blood cells, kidney medulla, lens of the eye, (brain and CNS prefer glucose)
45
What is glucose required for?
1. Tissues which have an absolute requirement
2. Tissues which prefer glucose (brain and CNS)
3. Variable amounts for specialised functions e.g. synthesis of triacylglycerol in adipose tissue requires glycerol phosphate
46
What substrate from glycolysis is used by adipose tissue to help synthesis TAGs?
DHAP is used to form glycerol phosphate which is combined with fatty acyl-CoA to form triacylglycerol
47
List some pathways that glucose can enter once in tissues?
Glycolysis
Pentose phosphate pathway (from G-6-P)
Glycogen for storage (from G-6-P)
Conversion to other sugars e.g. galactose
The importance of these pathways varies from tissue to tissue
48
Which tissues use glycolysis?
All of them!
49
What is unique about glycolysis?
It is the only pathway that can generate ATP under anaerobic conditions?
50
What does glycolysis generate?
ATP for cell function
NADH from NAD+
Building blocks for anabolism
Useful intermediated for specific cell functions
51
What is the overall equation for the 10 steps of aeorbic glycolysis?
glucose + 2Pi + 2ADP + 2NAD+ -->
| 2 pyruvate + 2ATP + 2NADH + 2H+ + 2H2O
52
```
Glycolysis can be broken down into 2 phases:
Phase 1 (steps 1-3)
Phase 2 (steps 4-10)
What is the name of each phase and the net synthesis/usage of useful carriers in each phase?
```
1 - preparative phase: Loss 2xATP
| 2 - ATP-generating phase: Gain 2x NADH and 4xATP
53
What is the first step in glycolysis and how is it catalysed?
Glucose + ATP -> glucose-6-phosphate + ADP
| Enzyme: hexokinase (all tissues), glucokinase (liver)
54
What is the purpose of phosphorylating glucose?
1. Makes the sugar anionic - can't cross membrane
2. Increases reactivity of sugar - so it can be metabolised by several pathways (glycolysis, pentose P, glycogen s.)
3. Allows formation of compounds with high phophoryl-group transfer potential that can transfer their phosphate group to ADP->ATP (substrate level phosphorylation)
55
What is the purpose of glycolysis reactions 2 and 3?
They isomerise glucoe-6-phosphate into a sugar phosphate that can be split into two C3 units following further phosphorylation using ATP
56
Which reactions in glycolysis are irreversible and why?
Reactions 1,3 and 10 because they have large negative delta-G values
57
Reaction 3 is the first step unique to glycolysis. Why is it known as the committing step?
It is irreversible and therefore commits glucose to metabolism via glycolysis
58
What is notable about reaction 6 of glycolysis?
It is the only reaction in glycolysis which produces NADH (x2)
59
What happens to glycolysis if NADH is not oxidised back to NAD+?
Glycolysis stop as there is a lack of NAD+ for step 6 of glycolysis
60
How is NADH oxidised back to NAD+?
1. In cells with mitochondria and an adequate oxygen supply, this occurs in the electron transport chain
2. In cells that lack mitochondria (e.g. RBCs) or in the absence of adequate oxygen (vigorously exercising muscle) it is converted back by lactate dehydrogenase reaction
61
What does lactate dehydrogenase catalyse?
pyruvate + NADH + H+ lactate + NAD+
62
Why is the lactate dehydrogenase reaction so important?
It recycles NADH --> NAD+ in anaerobic conditions and in cells without mitochondria so that glycolysis (step 6) is not inhibited and ATP (substrate level phosphorylation) can continue to be produced
63
In which reactions of glycolysis does substrate level phosphorylation occur?
Steps 7 and 10
64
What is substrate-level phosphorylation?
Substrate-level phosphorylation is a type of metabolic reaction that results in the formation of adenosine triphosphate (ATP) or guanosine triphosphate (GTP) by the direct transfer and donation of a phosphoryl (PO3) group to adenosine diphosphate (ADP) or guanosine diphosphate (GDP) from a phosphorylated reactive intermediate. Note that the phosphate group does not have to come directly from the substrate. By convention, the phosphoryl group that is transferred is referred to as a phosphate group.
65
Does loss of CO2 occur in glycolysis
NO
66
Overall how is the delta-G for glycolysis
Negative - it is overall an exergonic process
67
Why does the storage of lipid in adipose tissue depend on an adequate rate of glycolysis?
Glycerol phosphate is required for the synthesis of triacylglycerols and is synthesised from the glycolysis intermediate DHAP
68
Why is the liver less dependent on the glycerol phosphate produced by glycolysis to produce triacylglycerol than adipose tissue?
The liver can phosphorylate glycerol directly using glycerol kinase and ATP but this enzyme is not found in adipose tissue
69
What is the glycolysis intermediate 1,3-bisphosphoglycerate an important substance for?
It is converted into 2,3-bisphosphoglycerate in red blood cells. 2,3-BPG is an important regulator of oxygen affinity of haemoglobin
70
What is the role of myokinase in muscles?
It enables the high energy of hydrolysis phosphate bond in ADP to drive ATP synthesis under emergency conditions:
2ADP ATP + AMP
71
Why is not necessary to regulate the activity of all the enzymes in a metabolic pathway, only the rate limiting steps?
Most of the reactions in a metabolic pathway are close to equilibrium and the flow of material through the pathway is determined by a small number of rate-determining reactions
72
What is the most important rate limiting step in glycolysis?
Step 3 catalysed by phosphofructokinase (PFK)
73
What is the role of phosphofructokinase (PFK)?
It is the most important rate-limiting step of glycolysis and catalyses:
fructose-6-phosphate + ADP --> fructose 1,6-bisphosphate + ADP
This commits glucose to glycolysis
74
How is PFK regulated?
Muscle- allosteric regulation: inhibited by high [ATP], stimulated by high [AMP]
Liver - hormonal regulation: inhibited by insulin, stimulated by glucagon
75
By what high and low energy signals is glycolysis regulated?
Inhibited by high energy signals: high [ATP], low [AMP], {ADP]
Stimulated by low energy signals: low [ATP]. high [AMP], [ADP]
76
What is the overall equation for glycolysis under anaerobic conditions?
glucose + 2Pi + 2ADP --> 2 lactate + 2ATP + 2H2O
77
What happens to all the lactate produced by red blood cells, skin, brain, skeletal muscle and the gastrointestinal tract?
It is released into the circulation, transported to the liver and heart muscle (and kidney), where it is converted back to pyruvate and oxidised to CO2 (heart muscle) or converted to glucose (liver and kidney)
78
Under normal physiological conditions what is the rate and utilisation of lactate?
Equal, therefore the plasma concentration remains relatively stable
79
In what physiological and pathological conditions is a high level of lactate seen?
```
Strenuous exercise
Hearty eating
Shock
Congestive heart disease
Increases due to decreased utilisation occur in:
Liver disease
Thiamine deficiency
During alcohol metabolism
```
80
What causes lactic acidosis?
When plasma levels of lactate exceed the renal threshold and begin to affect the buffering capacity of plasma.
81
Galactose is used for the biosynthesis of which molecules?
Glycolipids and glycoproteins, such as blood group antigens
82
Which enzyme hydolyses dietary lactose so its products can be absorbed into the bloodstream?
Lactase
| lactose --> glucose and galactose
83
Where is galactose largely metabolise?
Liver (some in kidney and gastrointestinal tract)
84
In the liver what is the overall reaction of galactose that soluble enzymes catalyse?
galactose + ATP --> glucose-6-phosphate and ADP
85
Why is it important that the epimerase reaction in galactose metabolism is reversible?
It means that galactose can also be synthesised from glucose via UDP-galactose. This is important during lactation when breast tissue is synthesising large amounts of lactose for milk production
86
Name two clinical conditions that affect galactose metabolism
Lactose intolerance
| Galactosaemia
87
In galactosaemia individuals are unable to utilise galactose obtained from the diet because of a lack of which enzymes?
Galactokinase of Galactose-1-P transferase
88
What is the role of the galactokinase enzyme?
Converts:
| galactose + ATP --> galactose-1P + ADP
89
What is the role of the galactose-1-P uridyl transferase enzyme?
Converts:
galactose-P + UDP-glucose --> glucose-1P + UDP-galactose
Glucose-1P can then be converted to glucosose-6-P and therefore enter glycolysis or the pentose phosphate pathway
90
The lack of which enzyme in galactosaemia is more common and more serious?
Galactose-1-P uridyl transferase
91
Why is lack of Galactose-1-P uridyl transferase more serious than lack of galactose kinase?
It causes a build up of galactose and galactose-1P. In addition to the depletion in NADPH this causes, it also depletes inorganic phosphate stores leading to liver damage
92
What is the effect of a build up of galactose on NADPH stores?
The enzyme aldose reductase catalyses:
Galactose + NADPH --> Galacitol + NADP+
Depleting NADPH stores
93
Why does depletion of NADPH compromise defences against oxidative damage?
Key cellular defence mechanisms again oxidative damage require NADPH for function:
1. Glutathione - glutathione reductase uses NADPH as electron donor
2. Thioredoxin system - thioredoxin reductase used NADPH as electron donor
3. Thioltransferase system - uses gluathione as cofactor
94
How can ROS damage proteins?
ROS react with either the side chain or backbone:
1. Backbone -> fragementation -> protein degradation
2. Modified amino acid -> change in protein structure -> protein degradation/ loss of function/ gain of function
95
How does ROS damage to proteins in galactosaemia cause cataracts?
1. ROS takes electrons from cysteine residues in crystallin protein of eye
2. Inappropriate disulphide bonds form -> misfolding -> cross-linking and disruption of function
3. Aggregation of denatured crystallin protein in the eye form cataracts
96
What are the clinical symptoms of galactosaemia?
```
Hepatomegaly + cirrhosis
Renal failure
Vomiting
Seizure + brain damage
Cataracts
Hypoglycaemia
```
97
What else can cause cataracts in galactosaemia, other than inappropriate disulphide bond formation?
Non-enzymatic glycosylation of lens proteins
98
What can be the cause of glaucoma in galactosaemia?
Accumulation of galactose and galactitol in the eye may lead to intra-ocular pressure (glaucoma) which if untreated may lead to blindness
99
What is the effect of a build-up of galactose-1-phosphate in tissues?
Causes damage to liver, kidney and brain and may be related to the sequestration of Pi making it unavailable for ATP synthesis
100
What enzyme hydrolyses dietary sucrose -> glucose and fructose?
Sucrase
101
How is fructose metabolised in the liver?
By soluble enzymes which ultimately convert it to glyceraldehyde 3-phosphate an intermediate of glycolysis
102
Glucose-1-phosphate is made from glucose-6-phoshate. What pathways is it an intermediate in?
Galactose pathway and glycogen pathway
103
Which pathways is glucose-6-phosphate an intermediate in?
Glycolysis
Pentose phosphate pathway
Galactose synthesis/conversion to glucose
Glycogen synthesis/break-down
104
What are the main functions of the pentose phosphate pathway?
1. Produce NADPH in the cytoplasm
| 2. Produce C5-sugar ribose
105
In which tissue is the pentose phosphate pathway particularly important?
Liver
Red blood cells
Adipose tissue
106
Why is NADPH production important in liver and adipose tissue?
NADPH provides reducing power for anabolic processes such as lipid synthesis
107
Why is NADPH production important in red blood cells?
It is important in the defence against oxidative damage - maintains reduced glutathione which can maintain free -SH groups on cystein residues in certain proteins
108
How is NADPH involved in the protection of cells against toxic chemicals?
It is involved in various detoxification mechanisms
109
Why is there a high activity of the pentose phosphate pathway in dividing tissue (e.g. bone marrow)?
It produces the C5 sugar ribose which is required for the synthesis of nucleotides, the building blocks of DNA and RNA
110
The pentose phosphate pathway can be considered in two phases. What happens in the first phase?
glucose-6-phosphate is oxidised and decarboxylated by the enzymes glucose 6-phosphate dehydrogenase and 6-phosphgluconate dehydrogenase in reactions that require NADP+ - producting a C5-sugar phosphate
111
The pentose phosphate pathway can be considered in two phases. What happens in the second phase?
In phase 2 a complex series of reactions convert any unused C5-sugar phosphates to intermediates of glycolysis (fructose 6-phosphate or glyceraldehyde 3-phosphate)
112
How is the pentose phosphate pathway regulated?
By controlling the activity of glucose 6-phosphate dehydrogenase, the first enzyme in the pathway.
NADP+/NADPH ratio controls the enzyme
NADP+ stimulates
NADPH inhibits
113
What is the inheritance pattern in glucose 6-phosphate dehydrogenase deficiency (G6PD) ?
X-linked
114
Which population is G6PD more commonly found in?
Mediterraneans and black USA males
115
Why are red blood cells particularly affected by oxidative damage in G6PD?
The pentose phosphate pathway is the only source of NADPH in red blood cells and their roles as oxygen carriers puts them at increased risk of oxidative damage
116
Why do Heinz bodies occur inside red blood cells of individuals with G6PD deficiency?
Due to the depletion in NADPH, oxidative damage to haemoglobin and other proteins cause them to cross-link with each other, forming insoluble aggregates called Heinz bodies
117
Why does G6PD deficiency cause an increase in oxidative damage in cells?
G6PD is an enzyme of the pentose phosphate pathway which reduces NADP+ --> NADPH. Therefore a lack of this enzyme reduces the concentration of NADPH inside cells. NADPH is used to reduce oxidised gluathione. Reduced gluathione donates electrons to radicals in cells caused by oxidative damage and therefore protects the cell against protein modification that can cause protein degradation, loss of function and gain of function
118
Why is haemolysis increased in individuals with G6PD deficiency?
Damage to the red blood cells causes them to be prematurely destroyed.
119
Why are acute haemolytic episodes of G6PD deficiency precipitated by chemicals such as: antimalarials, sulphonamides and certain glycosides found in broad beans?
These chemicals reduce the level of NAPH
120
Pyruvate is converted into acetyl-CoA by which enzyme?
Pyruvate dehydrogenase
121
What is the overall reaction for the conversion of pyruvate into acetyl-CoA and why is it reversible?
pyruvate + CoA + NAD+ --> acetly-CoA + NADH + H+
| The reaction is irreversible because it produces a gas CO2, which is lost through diffusion
122
Why can acetyl-CoA not produce glucose by gluconeogenesis?
Because pyruvate dehydrogenase reaction is irreversible due to the loss of CO2
123
PHD is a multi-enzyme complex that enables a complicated reaction to occur in a controlled manner. Which four B vitamins does it require as cofactors?
CoA - pantothenic acid
NAD+ - niacin
FAD - riboflavin
thiamine pyrophosphate - thiamine
124
Why is PHD very sensitive to Vitamin B deficiency?
Four Vitamin Bs are cofactors in this enzyme complex
125
Why would acetyl-CoA be synthesised from fatty acids rather than glucose, in some conditions?
To preserve glucose in times of starvation for glucose-dependent tissues such as RBCs, kidney medulla and the brain and CNS
126
Which molecules inhibit pyruvate dehydrogenase?
Allosterically - acetly-CoA, NADH, ATP
127
Which molecules activate pyruvate dehydrogenase?
Allosterically - ADP
| Promoting dephosphorylation - insulin
128
The citric acid cycle is a central pathway in the catabolism of which metabolites?
```
Sugars
Fatty acids
Ketone bodies
Alcohol
Amino acids
```
129
Where in the cell does the citric acid cycle take place?
Mitochondrial matrix
130
What does the Kreb's cycle require to function?
NAD+
FAD
oxaloacetate
(acetly-CoA!)
131
What is the main function of the citric acid pathway?
1. Break the C-C bond in acetate (acetly-CoA)
| 2. Oxidise the C-atoms in acetyl to CO2
132
What is an essential requirement to the Kreb's cycle - without which it won't function?
Oxygen
133
What useful products are made in the citric acid cycle?
Per mole of acetly-CoA:
3NADH
FADH2
GTP
134
In addition to its catabolic role, what anabolic functions does the TCA cycle have?
Creates:
1. C5 and C4 intermediates for the synthesis of non-essential amino acids
2. C4 intermediates for haem and glucose
3. C6 intermediates for synthesis of fatty acids
135
What happens to the cycle if intermediates are removed?
The cycle will stop until replacements are found. These replacements can be from:
1. Amino acid breakdown --> C5 and C4 intermediates
2. Activity of pyruvate carboxylase (major replacement):
pyruvate + CO2 + ATP + H2O --> oxaloacetate + ADP + Pi + 2H+
136
How is the TCA cycle regulated?
Signals feeding info on the rate of utilisation of ATP:
1. ATP/ADP ratio
2. NADH/NAD+ ratio
137
Explain how one of the irreversible step enzymes of the TCA cycle is regulated?
Isocitrate dehydrogenase is inhibited by NADH (high energy) and activated by ADP (low energy)
138
What has happened by the end of stage 3 catabolism (Kreb's cycle)?
1. All the C-C bonds have been broken and C-atoms oxidised to CO2
2. All the C-H bonds have been broken and the H atoms (H+ + e-) transferred to NAD+ and FAD
139
What is the net production of ATP from glycolysis?
2
140
How many ATP/ GTP are generated in the TCA cycle?
2 GTP
141
Where has all the energy gone from glucose?
1. ATP/GTP generation
| 2. Chemical bond energy of the e- in NADH and FADH2
142
How is the energy from the high energy electrons in NADH and FADH2 released?
Through a series of carrier molecules which are converted from their oxidised to reduced form in the electron transport chain
143
What is the final stage of catabolism?
Oxidative phosphorylation
144
Oxidative phosphorylation occurs where?
Inner mitochondrial membrane
145
Oxidative phosphorylation involves two processes, electron transport and ATP synthesis. What does the process of electron transport involve?
Electrons in NADH and FADH2 are transferred through a series of carrier molecules to oxygen with step-wise release of free energy. This free-energy is used by proton translocating complexes to move protons across the IMM, creating a proton concentration gradient (e.m.f.) across the impermeable membrane. This process therefore requires oxygen as the terminal electron acceptor --> H2O
146
Oxidative phosphorylation involves two processes, electron transport and ATP synthesis. What does the process of ATP synthesis involve?
The free energy released in electron transport is used to create a proton motive force. The diffusion of protons down this gradient through ATP synthase complex drives ATP synthesis from ADP + Pi. Thus the greater the proton motive force the more ATP is synthesised
147
Which complexes in the electron transport chain also act as PROTON TRANSLOCATING COMPLEXES?
Complexes I,III & IV
148
What do proton translocating complexes do?
They use the free energy derived from electron transport to move protons from the inside to the outside of the inner mitochondrial membrane (which is impermeable to protons)
They therefore convert chemical bond energy of the electrons into electo-chemical potential difference of protons.
149
What is the proton motive force (pmf)?
It is the force generated by proton translocating complexes pumping protons against their concentration gradient across a membrane impermeable to protons.
It's a measure of the potential energy stored as a combination of proton and voltage gradients across a membrane (differences in proton concentration and electrical potential).
150
Why does NADH result in more translocated protons than FADH2?
NADH has electrons with a greater chemical bond energy than FADH2, therefore NADH electrons have the energy to use all three proton translocating complexes whereas FADH2 only uses two
151
What happens to the energy not conserved in ATP?
Lost as heat and help to maintain body temperature at 37oC
152
List some differences between oxidative phosphorylation and substrate level phosphorylation
1. OP requires membrane associated complexes, whereas SLP requires soluble (cytoplasmic and Mt) enzymes
2. OP has indirect energy coupling through pmf whereas SLP has direct coupling through generation of high energy of hydrolysis bond
3. OP cannot occur in the absence of O2
4. OP is major process for ATP synthesis in cells that require large amounts of energy whereas SLP is not
153
How is the coupling of electron transport and ATP synthesis controlled so that one cannot happen without the other?
Mitchondrial concentration of ATP plays an important role in regulating both processes. When [ATP] is high:
1. [ADP] is low and ATP synthase stops (lack of substrate), preventing transport of protons back into mitochondria matrix
2. The [H+] outside increases to a level that prevents more protons being pumped
3. In the absence of proton pumping electron transport stops
The reverse occurs when [ATP] is low
154
What is an electron transport uncoupler?
A substance, like dinitrophenol, which increases the permeability of the inner mitochondrial membane to protons. This enables protons that have been pumped out of the mitochondrial matrix to reenter without driving ATP synthesis, hence electron transport and ATP synthesis are uncoupled and the p.m.f. is lost as heat.
155
Uncouplers effectively stop ATP synthesis, but what do they do to the electron transport chain?
The electron transport chain continues, so excessive heat generation continues
156
How much does proton leak account for the basal metabolic rate?
20-25%
157
Where can the natural uncoupling proteins (UCP1-5) be found and what is their role?
They are found in the inner mitochondrial membrane of brown adipose tissues and are involved in non-shivering thermogenesis, which enables mammals to survive cold environments
158
How does the sympathetic nervous system drive non-shivering thermogenesis in response to cold stimuli in brown adipose tissue?
1. Releases noradrenaline which stimulates lipolysis which releases fatty acids to provide fuel for oxidation
2. As a result of beta-oxidation NADH and FADH2 are formed, driving ET and increasing the pmf
3. Noradrenaline also activates UCP-1, allowing protons back into the mitochondrial matrix without driving ATP synthesis, which dissipate the p.m.f. as heat
159
What substance can inhibit the electron transport chain?
1. Anaerobic conditions
| 2. CO and various posions (cyanide...)
160
Why do antimalarials such as primaquine precipitate haemolysis in an individual with glucose-6-phosphate deficiency?
NADPH levels are limited in these individuals due to the pentose phosphate pathway not being active and therefore they have less protection against oxidative stress due to a lower capacity to recycle oxidised gluathione. Anti-malarials like primquine further deplete the levels of reduced glutathione (they oxidise it) leaving the cells very susceptible to oxidative damage
161
A newborn girl is vomiting after her feeds and has diarrhoea. She becomes jaundiced over the next few days. Her urine is tested and found to contain galactose. She is diagnosed as having galactossaemia. Which enzyme is most likely to be deficient in classical (Type 1) galactossaemia where both galactose and galactose-1-phosphate accumulate in tissues?
galactose-1-phosphate uridyl transferase
162
Which rare enzyme deficiency causes a less severe form of galctossaemia characterised by solely build-up of galactose in tissues?
Galactokinase
