Energy Production I & II - Carbohydrate

Question 1

What is the first step in catabolism stage 1 of carbohydrates?

Answer 1

Salivary amylase breaks down starch/glycogen into dextrins (oligosaccharides).

Question 2

How do dextrins get broken down before absorption?

Answer 2

Pancreatic amylase may break them down into monosaccharides and the small intestine have disaccharidases attached to the brush border of epithelial cells (lactose, sucrose and isomaltose have alpha-1,6-glycosidic bonds.

Question 3

What are the 3 causes of lactose intolerance?

Answer 3

1. Primary lactase deficiency - absence of lactase persistence allele, only occurs in adults. Highest prevalence in northwest Europe.
2. Secondary lactase deficiency caused by injury to SI (pancreatitis, Coeliac/Crohns disease), occurs at all ages and is generally reversible.
3. Congenital lactase deficiency - extremely rare autosomal recessive defect in lactase gene - can’t digest breast milk.

Question 4

What is SGLT1?

Answer 4

Sodium dependent glucose transporter 1’ for active cotransport of glucose from the intestinal lumen into epithelial cells.

Question 5

Glucose is taken up by the tissues from the blood by facilitated diffusion using transport proteins, GLUT1-5, where may GLUT2 and GLUT4 be found?

Answer 5

GLUT2- SI, kidney, liver, PANCREATIC BETA CELLS

GLUT4- adipose and striated muscle - INSULIN REGULATED.

Question 6

Blood glucose concentration should be ~5mM because all tissues metabolise glucose, but some have an absolute requirement, name them.

Answer 6

Red blood cells, neutrophils, kidney medulla, lens of eye.
The central nervous system prefers glucose as its fuel, but in times of starvation may adapt to metabolise ketone bodies.

Question 7

What are stage 1 and stage 2 of metabolism?

Answer 7

Stage one is the breakdown of building block molecules (diet - carbohydrates - monosaccharides), whereas stage 2 is the breakdown to metabolic intermediates, release of ‘reducing power’ and energy (glucose - pyruvate/lactate - intracellular metabolic pathway of glycolysis).

Question 8

What are the functions and features of glycolysis?

Answer 8

Functions: oxidation of glucose, production of 2X NADH and 2X ATP, producing C6 and C3 intermediates.
Features: central pathway of carbohydrate metabolism, occurs in all tissues (cytosolic), exergonic and oxidative, no production of carbon dioxide and with one additional enzyme (PDH) is the only pathway to act anaerobically.

Question 9

Why are there so many stages and enzymes in glycolysis?

Answer 9

The chemistry is easier in small stages, energy efficient conservation, gives versatility (as interconnections with other pathways and intermediates may be used in reverse) and allows for fine control.

Question 10

What happens in reactions 1-3 of glycolysis? (What are the functions of hexokinase/glucokinase and phosphofructokinase?)

Answer 10

Reactions 1-3 are the investment stages as they involve the use of 2ATP.
Hexokinase (or glucokinase in liver) catalyse the phosphorylation of glucose to make G-6-P, which is negatively charged so can’t leave the cell and has an increased reactivity for subsequent steps.
Phosphofructokinase coverts fructose-6-P to F-1,6-bisphosphate, which is the committing step to glycolysis.
Both reactions have large -deltaG, so irreversible.

Question 11

What happens in phase 2 of glycolysis, the payback phase?

Answer 11

Reaction 4 cleaves C6 to 2 interchangeable C3s (reaction5). In reaction 6, reducing power is capture in NADH. ATP synthesis occurs in reactions 7&10 - substrate level phosphorylation. Reaction 10 has a large deltaG, so is irreversible.

Question 12

Gluconeogenesis involves the formation of glucose from pyruvate, what must be overcome?

Answer 12

The 3 irreversible reactions of glycolysis at stages 1, 3 and 10.

Question 13

What is the rate of glycolysis in cancer cells and how is this used clinically?

Answer 13

It is up to X200 that of normal cells, so radioactive modified hexokinase substrate (glucose analogue) may be used for imaging purposes.

Question 14

PFK is a key regulator of glycolysis as it catalysts the committing step (3), how is it allosterically regulated?

Answer 14

In muscle, high [ATP] inhibits it and high [AMP] activates it. Hormonal stimulation in the liver is also used with high [insulin] activating it and high [glucagon] inhibiting it.

Question 15

Aside from phosphofructokinase, which enzymes in glycolysis are allosterically regulated and how?

Answer 15

Hexokinase at step 1 is inhibited by its product - G-6-P.
Also high [NADH] or low [NAD+], inhibits step 6 of glycolysis.
Pyruvate kinase at step 10 is activated by a high insulin:glucagon.

Question 16

Triacylglycerol and phospholipid biosynthesis occurs in the liver and adipose tissue, lipid synthesis in adipose requires glycolysis, how does it join with the pathway?

Answer 16

Glycerol phosphate is an important intermediate and the enzyme glycerol-3-phosphate dehydrogenase puts it in the process.

Question 17

2,3-bisphosphoglycerate is found in RBCs and allosterically regulates Hb’s affinity with oxygen, how is it involved in glycolysis?

Answer 17

It can convert to 1,3-BPG, an intermediate, by the work of bisphoglycerate mutase.

Question 18

What enzyme must be found in RBCs to regenerate NAD+ without stage 4 metabolism which would otherwise occur in mitochondria? (As supply to gut and skeletal muscle is often reduced, it may be found there too.)

Answer 18

Lactate dehydrogenase (LDH). NADH + H+ + pyruvate NAD+ + lactate.

Question 19

What may pyruvate be used for?

Answer 19

The heart may use it for it’s own energy production (after oxidation), as may the liver, which also uses it for gluconeogenesis.

Question 20

Plasma lactate concentration, usually at 1mM is determined by what?

Answer 20

The relative production, utilisation and disposal.

Question 21

What is the difference between hyperlactaemia and lactic acidosis?

Answer 21

Hyperlactaemia is when the plasma concentration of lactic acid is 2-5mM which is below renal threshold, whereas with lactic acidosis it’s above 5mM and the buffering capacity is overcome, resulting in a change in the plasma pH of someone acutely unwell.

Question 22

Galactose metabolism can feed into glycolysis via what and a deficiency is any of which 3 enzymes may cause Galactosaemia?

Answer 22

Glucose-6-phosphate.

Galactokinase, UDP-galactase epimerise and uridyl transferase lead to Galactosaemia.

Question 23

Fructose is metabolised in the liver, how does it feed into glycolysis?

Answer 23

Via fructokinase, aldolase, triode kinase, TPI.

Question 24

What is fructosuria? How is it different to fructose intolerance?

Answer 24

Fructokinase is missing, so fructose in in urine and there are no clinical signs.
Fructose intolerance involves missing aldolase, accumulation of fructose-1-phosphate in the liver, leading to damage. Treatment is to remove fructose from diet.

Question 25

What is the point of the pentode phosphate pathway?

Answer 25

It is an important source of NADPH required for ‘reducing power’ for biosynthesis, maintenance of GSH levels and detoxification.
It produces C5 sugar ribose required for synthesis of nucleotides - DNA and RNA.

Question 26

The pentose phosphate pathway starts at glucose-6-phosphate, does it produce ATP or carbon dioxide?

Answer 26

No ATP is made, but CO2 is produced.

Question 27

What is the rate limiting enzyme of the pentose phosphate pathway?

Answer 27

Glucose-6-phosphate dehydrogenase.

Question 28

What is pyruvate dehydrogenase - what does it do, where can it be found and why is it sensitive to B1 vitamin deficiency?

Answer 28

PDH is a large multienzyme complex found in the mitochondrial matrix. Pyruvate –> acetyl CoA + CO2.
Different enzyme activities require various cofactors, which B vitamins provide, so it’s sensitive to B1 deficiency.

Question 29

The reaction catalysed by pyruvate dehydrogenase is irreversible and so a key regulatory step. It is therefore subject to multiple regulation, describe it.

Answer 29

PDH is activated by low energy products and substrates (pyruvate, NAD+, ADP, insulin), which lead it to be dephosphorylated.
PDH is inhibited by high energy metabolites (acetyl CoA - its product, NADH, ATP, citrate - from TCA cycle), which phosphorylate it.

Question 30

What clinical problem is associated with PDH deficiency?

Answer 30

Lactic acidosis, because without pyruvate dehydrogenase, pyruvate has to go down the alternative path producing lactate, for NAD+ recycling, instead of to Acetyl CoA.

Question 31

What is stage 3 of carbohydrate metabolism? Where does it occur and what is the point?

Answer 31

TCA cycle (tricarboxylic acid cycle - Krebs). It is mitochondrial and involves a single pathway where acetyl (CH3CO) is converted to 2CO2. It's oxidative so needs NAD+ and FADH. Some energy providing molecules are produced in the form of ATP and GTP, as are some precursors for biosynthesis.
Pathway acts catalytically - no change in level of intermediates. 2 cycles per glucose molecule entering glycolysis.

Question 32

What are the products of a single TCA cycle?

Answer 32

1 GTP, 3 NADH, 1 FADH2 and 2 CO2.

Question 33

What is the TCA cycle regulated by?

Answer 33

Energy availability (ATP:ADP / NADH:NAD+).

Question 34

In the TCA cycle, steps from C6 isocitrate to C5 alpha-ketoglutarate then to C4 succinyl-CoA involve the removal of oxygen and so are irreversible. It will not function in the absence of oxygen. It is a central pathway in many biosynthetic processes, as intermediates can go on to produce what?

Answer 34

Fatty acids, amino acids, harm or glucose.

Question 35

Describe stage 4 in the catabolism of glucose.

Answer 35

Mitochondrial.
Electron transport and ATP synthesis.
NADH and FADH2 get reoxidised, oxygen is required, large amounts of ATP are produced.

Question 36

In electron transport of stage 4 of glucose metabolism, 30% of the energy is used to pump protons across the inner membrane of the mitochondria (lots is lost as heat), where does it come from and what complexes are involved?

Answer 36

Electrons from reducing power carriers NADH and FADH2 are transferred through a series of carrier molecules to oxygen, releasing energy in steps. NADH starts at proton translocating complex 1 (PTC1), so overall results in the pumping of 6H+ across the membrane, whereas FADH2 feeds in at PTC2, so only moves 4H+ over.

Question 37

What is pmf?

Answer 37

Proton motive force.

H+ gradient across the inner mitochondrial membrane (membrane potential).

Question 38

What is the role of proton translocating ATPase?

Answer 38

It could use dephosphorylation of ATP to drive 2H+ across the membrane, but instead works in reverse and is known as ATP synthase.

Return of H+ is favoured energetically by the electrochemical potential, but the membrane is largely impermeable, so they must only return via ATP synthase, to drive ATP synthesis.

Question 39

What is the consequence, with regards to the electron transport chain of a high ATP/low ADP concentration?

Answer 39

There is no substrate for synthesis, so the concentration of H+ in the intermembrane space increases, which prevents further proton pumping, stopping electron transport.

Question 40

What do inhibitors do in terms of the electron transport chain?

Answer 40

Block electron transport, for instance CN-/cyanide, which prevents the acceptance of electrons by oxygen.

Question 41

Normally oxidative phosphorylation and electron transport are tightly coupled, as they are both regulated by mitochondrial ATP concentration. What do uncouplers do?

Answer 41

Increase the permeability of the mitochondrial inner membrane to protons, so the proton gradient dissipates, reducing the proton motive force, so there’s no drive for ATP synthesis.
E.g. Fatty acids.

Question 42

What are oxidative phosphorylation diseases?

Answer 42

Genetic defects in proteins encoded by mitochondrial DNA, which decrease electron transport and ATP synthesis.

Question 43

What does the efficiency of stage 4 glucose catabolism depend on?

Answer 43

Tightness of the coupling.

Question 44

How does brown adipose tissue react to the cold?

Answer 44

The response to stress is to release noradrenaline.
This noradrenaline activates lipase to release fatty acids from triacyglycerols. It also activates fatty acid oxidation, to produce NADH and FADH2, which drive the electron transport chain.
Fatty acids also activate UCP1 thermogenin protein (a natural uncoupler), so the energy of the proton motive force is released as extra heat.

Question 45

What’s the difference between oxidative/substrate level phosphorylation?

Answer 45

Requires membrane associated complexes / requires soluble enzymes.
Energy coupling occurs indirectly through generation and subsequent utilisation of proton gradient / energy coupling occurs directly through formation of high energy hydrolysis bond.
Can’t occur in oxygen absence / can occur to a limited extent.
Major process for ATP synthesis in cells requiring large amounts of energy / minor.