Lecture 7 - Glycolysis + Gluconeogenesis

Question 1

Metabolism is the sum of all ___ and ___ processes

Answer 1

Catabolic and anabolic

Question 2

Why is the liver so essential to the body?

Answer 2

Because it maintains blood glucose levels

Question 3

Insulin

• signals ___ blood glucose
• ___ metabolism of stored fuels
• ___ fuel storage
• secreted by ___ cells of pancreas

Answer 3

High
Decreases
Increases
Beta

Question 4

Glucagon

• signals ___ blood glucose
• ___ generation of glucose and breakdown of lipid stores
• secreted by ___ cells of pancreas

Answer 4

Low
Stimulates
Alpha

Question 5

Epinephrine

___ breakdown of glucose and lipid stores
Results in ___ fuel availability

Answer 5

Stimulates

Increased

Question 6

Catabolism is ____ (exergonic or endergonic)

Anabolism is ___ (exergonic or endergonic)

Answer 6

Exergonic

Endergonic

Question 7

What is a near equilibrium reaction?

Answer 7

One where delta G is essentially 0, they can easily go forward or reverse to restore equilibrium depending whether there is more reactants or products

Question 8

What is a far from equilibrium reaction?

Answer 8

Delta G is much less than 0 (very large and very negative thus very spontaneous)

These reactions heavily favor the forward reaction because the enzymes that catalyze these reactions don’t have the catalytic activity to come to equilibrium (enzymes are either too slow or are often saturated). This causes reactants to accumulate in excess of their equilibrium amounts and drives the reaction forward since [react]&raquo_space; [prod]. Adding more substrate will not affect these reactions since the enzyme itself is the rate limiting step.

Question 9

What are the consequences of far from equilibrium reactions on metabolism? (3)

Answer 9

1) pathways that contain these types of reactions are irreversible
2) every pathway that has these reactions has a first “committed” step
3) the catabolic (forward) and anabolic (reverse) pathways will have to differ due to the fact that the FFE rxns are irreversible

Question 10

What is allosteric control?

Answer 10

Regulation by products or coenzymes within the pathway where a substance binds to an allosteric site (not the active site) and changes the conformation of the enzyme to either increase or decrease enzyme activity.

Question 11

What is feedback inhibition?

Answer 11

A type of allosteric control. When the concentration of a product in the pathway is sufficiently high, the product can bind to an enzyme earlier in the pathway and exert allosteric inhibition on the enzyme to block more product from being made.

Question 12

What are the 4 mechanisms that can be used to control flux through a glycolytic pathway?

Which methods are short term control? Which are long term?

Answer 12

1) allosteric control (short term)
2) Covalent modification (short term)
3) Substrate cycles (short term)
4) Genetic control (long term)

Question 13

How can covalent modification alter glycolytic flux?

Answer 13

Phosphorylation or dephosphorylation of enzymes alters their activity

Question 14

How can substrate cycles alter glycolytic flux?

Answer 14

A substrate cycle occurs when far from equilibrium reactions are present in a pathway. These cycles can control flux because flux can be increased in one direction by decreasing flux in the opposite direction.

Question 15

What is a futile cycle?

How can it be avoided?

Answer 15

A futile cycle can when the FWD and REV pathways of a substrate cycle are run simultaneously such that the net reaction is the hydrolysis of ATP without any work being done or products being made.

Futile cycles are avoided by reciprocal regulation of the FWD and REV pathways (Meaning as the FWD is increased, the REV is decreased)

Question 16

How can genetic control alter glycolytic flux?

Answer 16

The rate of genetic transcription can be altered to control the rate of protein synthesis

Question 17

How can oxidation/reduction be explained in the context of H and O?

Answer 17

Reduction = increase H content, decrease O content
Oxidation = increase O content, decrease H content

Question 18

What enzymes are synonymous with oxidation?

Answer 18

Dehydrogenases (ex: NAD+ is reduced to NADH by glyceraldehyde 3 phosphate dehydrogenase while glyceraldehyde 3 phosphate is oxidized to 1,3 BPG)

Question 19

What are the 2 ways that electrons can be transferred in Red/Ox reactions?

Answer 19

1) H atom (1 electron, 1 proton)

2) Hydride ion (2 electrons, 1 proton)

Question 20

When NAD+ is reduced to NADH, what type of molecule does NAD+ accept?

Answer 20

Hydride (H-)

Question 21

When FAD is reduced to FADH2, what type of molecule does FAD accept?

Answer 21

FAD accepts 2 protons (H) in stepwise fashion, thus there is an intermediate between FAD and FADH2 that is possible so FAD can accept either 1 or 2 electrons.

Question 22

Which type of structure contains more energy: a reduced carbon molecule or an oxidized carbon molecule?

Answer 22

Reduced

Question 23

Oxidation is associated with ____ (catabolism or anabolism)

Reduction is associated with ___ (catabolism or anabolism)

Answer 23

Catabolism

Anabolism

Question 24

What type of bond exists between phosphate groups in ATP?

Answer 24

Phosphoanhydride

Question 25

How are the phosphates in ATP labeled (starting from the 5’ sugar to the last P group)?

Answer 25

alpha, beta, gamma

Question 26

There are 3 reasons why ATP possess a large amount of free energy potential. What are they?

Answer 26

1) Once a phosphate group is removed from ATP, the resulting ADP and Pi have greater resonance stabilization than was present in ATP
2) ATP has many negatively charged oxygens that repel one another, removal of one phosphate group reduces this repulsion and allows the charges to separate more
3) The products of ATP hydrolysis (ADP + Pi) have greater salvation (surrounded by more water molecules) than ATP alone. This helps stabilize the products in a way that is greater than the stability of the reactant (ATP)

Question 27

Why is ATP the universal energy currency?

Answer 27

There are many other kinds of phosphate compounds, but ATP has a free energy release in the middle of the field, which makes it the universal currency b/c it can be made by dephosphorylating the groups that are higher in energy and it can be used to generate the lower energy molecules

Question 28

What is the difference between substrate level phosphorylation and oxidative phosphorylation?

Answer 28

Substrate level phosphorylation = direct transfer of P to ADP, catalyzed by kinases

Oxidative phosphorylation = indirectly make ATP via chemiosmotic coupling with energy supplied by transmembrane proton gradient

Question 29

Glycolysis can be generally divided into 2 stages. Stage 1 is the ___ phase and stage 2 is the ___ phase.

Answer 29

Preparatory

Payoff

Question 30

How is NAD+ regenerated in aerobic conditions?

In active muscle under anaerobic conditions?

Why is it necessary for NAD+ to be regenerated?

Answer 30

Under aerobic conditions, NADH transfers electrons to complex I in ETC and is oxidized to NAD+

Under anaerobic conditions, NADH is oxidized to NAD+ and pyruvate is reduced to lactate by lactate dehydrogenase in the reaction pyruvate + NADH –> Lactate + NAD+

NAD+ is a coenzyme for dehydrogenases so in order to continue metabolic pathways NAD+ is needed to accept the electrons of substances that are being oxidized

Question 31

What is the role of lactate dehydrogenase?

Is it a reversible enzyme?

Answer 31

catalyzes run: pyruvate + NADH Lactate + NAD+

Yes it is reversible (near EQ rxn)

Question 32

What is the cori cycle?

Answer 32

In contracting muscle, pyruvate can either be converted to acetyl-coA by PDH or to lactate by LDH. Under anaerobic conditions (vigorous exercise), more pyruvate will be converted to lactate because the demands of cellular respiration will surpass the supply of O2. The lactate that is generated under these circumstances can enter the blood and be enter the liver, where it can be used to make glucose via gluconeogenesis. The glucose produced then travels back through blood stream to muscle to provide more glucose for muscle activity.

Question 33

Where does gluconeogenesis (GNG) occur?

What are the precursors for GNG?

Answer 33

The liver (mostly) and a the kidneys (very minimal)

Precursors are glucogenic amino acids, glycerol and lactate

Question 34

Pyruvate carboxylase is an enzyme of GNG. What reaction does it catalyze? Where is it located?

Answer 34

Pyruvate + ATP –> Oxaloacetate (OAA) + ADP + Pi

Located in mitochondrial matrix

Question 35

Phosphoenolpyruvate carboxykinase (PEPCK) is an enzyme of GNG. What reaction does it catalyze and where is it located?

Answer 35

OAA + GTP –> PEP + GDP + Pi

Located in cytoplasm

Question 36

PC and PEPCK are 2 enzymes that work together to perform the reverse reaction of which enzyme in glycolysis?

Answer 36

Pyruvate kinase

Question 37

What is the primary reason why having a constant supply of blood glucose is essential to sustain life?

Answer 37

Because the brain uses glucose as its primary fuel and it lacks glycogen stores of its own and is unable to perform gluconeogenesis on its own. Therefore, it relies on glucose delivered to it in the bloodstream for energy.

Question 38

What are the 2 general ways that glycolysis (GCL) and GNG can be regulated?

Answer 38

1) Regulate glucose entrance into the cell

2) Regulate the enzymes of these pathways (FFE)

Question 39

Describe how glucose enters the muscle cell and how this entry changes after a meal or when [glucose] in the blood is high.

Answer 39

Muscles cells primarily rely on GLUT4 transporters to bring glucose into the cell. The affinity of GLUT4 for glucose is approximately equal to the normal [glucose] in the blood (5mM) and so when the [glucose] in the blood is high, the [blood] will exceed the [intracellular] and glucose will diffuse down its concentration gradient into the cell via the GLUT4 transporters.

Additional GLUT4 transporters are stored inside the cell in vesicles. When blood glucose is high, insulin will stimulate the transport of these transporters to the membrane and the vesicles will fuse w/ membrane to deliver more transporters. When blood glucose is low, the vesicles will sequester the transporters back into the cell via endocytosis.

Question 40

Describe how glucose enters the liver / brain cell and how this entry changes after a meal or when [glucose] in the blood is high.

Answer 40

Liver and brain cells primarily rely on other GLUT transporters (1, 2 and 3) to bring glucose into the cell. These transporters are not sensitive to insulin, so their presence on the membrane is more constant. The affinity of GLUT1 and GLUT3 for glucose is much higher than GLUT4 (lower Kd) so that even when [glucose] in the blood is low, these transporters will still uptake glucose. The affinity of GLUT2 for glucose is lower than GLUT4 (higher Kd) so that when [glucose] is low these transporters do not still uptake glucose. GLUT2 is primarily found in the liver and pancreas while GLUT1 is found in the brain and other bodily tissues.

Question 41

1) Which hexokinases are present in the muscle tissue?
2) What are their affinities for glucose?
3) How are these enzymes regulated?

Answer 41

1) Hexokinase I and III
2) These enzymes have high affinity for glucose (0.1mM) so that even when the [glucose] in the cell is low these enzymes will be able to reach saturation and phosphorylate glucose so that it can enter the glycolytic pathway.
3) These enzymes are allosterically inhibited by their product (glucose 6 phosphate) so that when energy levels are high glucose is not trapped inside the muscle cell but is available instead for other tissues

Question 42

1) Which Hexokinase is present in the liver?
2) What is its affinity for glucose?
3) How is this enzyme regulated?

Answer 42

1) Hexokinase IV
2) Low affinity (10mM) so that when [glucose] in the cell is at or below normal levels, this enzyme will not be saturated
3) Directly regulated by [glucose] in the cell (when high, glucose will be phosphorylated, when low glucose will not be phosphorylated and will be allowed to leave cell and travel to other tissues for use). Also regulated by transcription: when [glucose] in blood is high hexokinase IV transcription is upregulated , when low [glucose] in blood transcription of glucose 6 phosphatase is upregulated.

Question 43

How does energy state regulate glycolysis and gluconeogenesis?

Answer 43

1) High ATP/ADP ratio indicates that energy is being produced faster than its being used –> inhibits enzymes of glycolysis and CAC and stimulates enzymes of glycogen synthesis
2) High [citrate] indicates that energy state is high and biosynthetic intermediates are abundant –> inhibits enzymes of glycolysis and CAC, stimulates biosynthesis of FAs and AAs
3) Low ATP/ADP ratio indicates that energy is being used faster than its being produced –> increases glycolysis and slows gluconeogenesis (which requires ATP)