Case 6: Glycolysis and Exercise

why does glucose need transports to cross the cell membrane?

glucose is a large, polar, hydrophilic molecule, meaning it cannot easily diffuse through the hydrophobic interior of the phospholipid bilayer.

to overcome this, glucose requires specific transport proteins to cross the membrane and enter the cell.

list GLUTs with their tissue distribution

GLUT1: located in all mammalian tissues

GLUT2: located in the liver and pancreatic β cells

GLUT3: located in all mammalian tissues

GLUT4: located in muscle and fat cells

GLUT5: located in the small intestine

what molecule regulates transport of GLUT protein to the cell surface in muscle and fat cells?

insulin

list correctly the substrates and products, and enzyme, for the 10 steps of glycolysis

explain why steps 1, 3, 10 of glycolysis is energetically favorable

-the transition of molecules going from a high to low energy state makes the reactions energetically favorable with very low ΔGs

Step 1:

• ATP donates phosphate to glucose, forming glucose-6-phosphate and ADP

• ADP has lower energy than ATP, therefore energy is released in the reaction - very low ΔG

Step 3:

• ATP donates phosphate to fructose-6-phosphate, forming fructose-1,6-bisphosphate and ADP

• ADP has lower energy than ATP, therefore energy is released in the reaction - very low ΔG

Step 10:

• phosphoenolpyruvate donates phosphate to ADP, forming pyruvate and ATP

• while ATP is less stable than ADP,  phosphoenolpyruvate is much higher in energy (less stable), therefore the reaction still releases a lot of energy

what does it mean for an enzyme to be regulated?

• controls the activity level of an enzyme to make sure that reactions happen at the right speed and time

• occurs through kinetics, where different factors affect enzyme activation or inhibition

• allows the cell to conserve energy and not overwork the enzyme

for each of the regulatory enzymes, list their effector and its effect

Hexokinase:

Effector: glucose-6-phosphate

Effect: glucose-6-phosphate inhibits

Phosphofructokinase:

Effector: ATP and AMP

Effect: ATP inhibit; AMP activates

Pyruvate Kinase:

Effector: fructose-1,6-bisphosphate and ATP

Effect: fructose-1,6-bisphosphate activates; ATP inhibits

why is the oxidation of the aldehyde carbon energetically favorable?

the oxidation releases energy that is used to form a high-energy acyl-phosphate bond. the aldehyde carbon is oxidized and loses its electrons, which are given to NAD+, turning it into NADH. this process releases energy and stores it by making a high-energy bond, acyl-phosphate bond, which is later used to help make ATP. 

so this oxidation is favorable bc it helps the cell capture energy in a useful way instead of letting it go to waste.

referring to the figure, explain how glyceraldehyde-3-phosphate dehydrogenase overcomes the energetically unfavorable process of phosphorylation after oxidation

the first reaction shows oxidation occurring first, then phosphorylation takes place. this makes the activation energy very high, which can be seen in the figure.

the second reaction shows the use of the thioester intermediate, which is high in energy. it temporarily holds the energy released from oxidation.this results in a lower activation energy for the phosphorylation step, allowing the two reactions to be coupled efficiently. as a result, the process moves forward smoothly, leading to a more stable phosphorylase product.

The enzyme aldolase catalyzes the following reaction in the glycolytic pathway: (see image).

The ΔG°′ for the reaction is +23.8 kJ mol−1, whereas the ΔG in the cell is −1.3 kJ mol⁻¹. Explain how the reaction can be endergonic under standard conditions and exergonic under intracellular conditions. 

the aldolase reaction is endergonic under standard conditions because the standard conditions assume equal concentrations of reactants and products, which does not actually happen in the cell.

the reaction is exergonic in the cell bc the concentrations of substrates and products are different. in glycolysis, fructose 1,6-bisphosphate is maintained at a higher concentration than its products. this favors the forward reaction, lowering the ΔG.

ATP can be produced by 2 processes: substrate-level phosphorylation and oxidative phosphorylation.

how do the 2 processes differ? which process occurs in glycolysis?

Substrate-level phosphorylation:

• phosphate group is added to ADP to make ATP

• occurs in cytoplasm during glycolysis

• occurs in mitochondria during citric acid cycle

• phosphate comes from a high-energy molecule, and no oxygen is needed

Oxidative Phosphorylation:

• occurs in mitochondria

• electrons from nutrients (like glucose) are passed through a chain of proteins in the electron transport chain

• the flow of electrons create energy that’s used to pump protons across a membrane, creating a gradient

• this gradient powers the ATP synthase enzyme, which adds a phosphate to ADP, making ATP

• oxygen is needed at the end of the chain to accept the electrons, forming water

explain the Warburg Effect

-when cancer cells use glycolysis to make energy even when there’s enough oxygen

• normally, cells use oxygen to produce a lot of energy in the mitochondria - oxidative phosphorylation

• cancer cells switch to glycolysis, in the cytoplasm

• glycolysis is less efficient bc it only makes 2 ATP, compared to 30-32 ATP when oxygen is used

• cancer cells do this bc glycolysis is faster, and they need quick energy to support their rapid growth and division

• they also require a lot of materials to grow and divide quickly - so they use glycolysis not just for energy, but also to create materials they need to make new cells

the expression of the GLUT1 transporter usually correlates with the rate of anaerobic glycolysis. a number of cancer cells show increased synthesis of GLUT1 transporter.

why would cancer cells need to increase the synthesis of the glucose transporter?

cancer cells rely on glycolysis to make energy even when there’s enough oxygen. although aerobic respiration makes a lot more ATP, glycolysis happens much faster, and cancer cells need quick energy to support their rapid growth and division

bc glycolysis consumes a lot of glucose to produce only a small amount of energy, cancer cells need to take in more glucose to keep up with high energy demand. to do this, they increase the production of the GLUT1 transporter, which allows them to absorb more glucose from the bloodstream

in addition to energy needs, cancer cells need metabolic intermediates for biosynthesis. explain how increase reliance on glycolysis would be beneficial in this respect.

glycolysis is also important for providing metabolic intermediates that are used to make up other important parts of the cancer cells.

for example, a product of glycolysis is pyruvate, which can be converted to acetyl-CoA. this can then be used to synthesize fatty acids and lipids for making cell membranes

another example, glycolysis produces intermediates like 3-phosphoglycerate, which can be used to make nucleotides

glycolysis offers a growth advantage to cancers growing under hypoxic conditions. why?

In hypoxic conditions, there’s a lack of oxygen. glycolysis does not rely heavily on oxygen, therefore cancer cells can continue to create energy without needing oxygen. this allows for more growth of the cancer without being affected by lower oxygen levels

what is the main fate of pyruvate in skeletal muscle when oxygen is limited, such in the first minute of high intensity exercise? what is the purpose of this reaction in muscle during exercise?

when oxygen is limited (such as in the first minute of high intensity exercise), pyruvate in skeletal muscle is converted to lactate through lactic acid fermentation

the purpose of this reaction is to regenerate NAD+, which is needed to keep glycolysis running and continue producing ATP for muscle contraction

What is the main fate of pyruvate in skeletal muscle when oxygen is plentiful, such as during moderate intensity exercise?  What pathway does this molecule then enter, and how does the further metabolism of this compound result in ATP synthesis? 

when oxygen is plentiful, pyruvate is converted to acetyl-CoA, which enters the citric acid cycle.

this cycle regenerates NADH and FADH2, which carry electrons to the electron transport chain.

the movement of electrons creates a proton gradient that drives ATP production through oxidative phosphorylation, providing energy for muscle activity

what tissues/organs are the two major sites of glucose storage in the body?

do these tissues/organs contribute to blood glucose levels?

• liver and skeletal muscle

• liver contributes to blood glucose levels; skeletal muscle only stores it

in what form is glucose stored in the body?

glycogen - very large molecule; has multiple branches and is a polysaccharide made up of many glucose

describe the process by which glucose is released from its stored form to be utilized by glycolysis. does this process require ATP?

glucose is released from glycogen through glycogenolysis: glycogen phosphorylase breaks it down into glucose-1-phosphate (G1P)

G1P is then converted to glucose-6-phosphate, which enters glycolysis to produce ATP

does not require ATP

regulators of muscle phosphorylase

ATP: inhibitor (excess ATP leads to decrease in glycogenolysis)

AMP: activator (excess AMP leads to increase in glycogenolysis)

glucose 6-phosphate: inhibitor

Ca2+: activator

epinephrine: activator

describe rhabdomyolysis

condition where component of muscle cells break down and is released into bloodstream

• main protein released: myoglobin

Causes:

• extreme exercise

Symptoms:

• dark, reddish urine (due to myoglobin in urine)

• decreased urine output

• weakness and muscle aches

define McArdle Disease

a genetic disorder caused by a mutation in the gene that encodes for muscle glycogen phosphorylase. this prevents muscles from breaking down glycogen into glucose 1-phosphate for energy during exercise, leading to early fatigue, muscle weakness, and rhabdomyolysis. affected individuals

which enzyme is deficient in McArdle disease?

muscle glycogen phosphorylase

discuss the effects that muscle glycogen phosphorylase deficiency has on metabolism during exercise and relate these to the observed symptoms (cramps, fatigue, dark brown urine)

the lack of the muscle glycogen phosphorylase enzyme prevents glycogen breakdown. during exercise, muscles rely on glycogen for a quick energy source. without access to energy from glycogen, they use up other slower energy sources, like fatty acids and the breakdown of protein. this leads to early fatigue and cramps due to a slower and lack of energy. intense exercise causing muscle breakdown (rhabdomyolysis), leading to myoglobin released into the bloodstream, which is filtered by the kidneys, producing a dark brown urine.

explain ischemic forearm exercise test and the results of the test for the child with McArdle disease

ischemic forearm exercise test measures lactate production under anaerobic conditions.

first, a baseline is drawn, then the blood pressure cuff is put on and the patient exercises their arm for one minute.

the blood flow is cut off to stimulate anaerobic conditions that would force the break down of glycogen for energy. it also prevents lactate from spreading throughout the body and prevents muscles from using glucose in the blood

after the blood pressure cuff is released, two more blood draws are taken one and three minutes later. the changes in lactic acid levels help detect metabolism issues in the muscle

in a healthy individual, lactate levels rise due to it being a byproduct of anaerobic glycolysis. however, for the child, lactate levels did not increase, indicating that the muscles were unable to break down glycogen, confirming McArdle disease.

what are the treatment options for an individual with McArdle disease?

• regulating diet and exercise

• drinking glucose solution before exercise can help provide an immediate energy source

• regular low-intensity exercise can improve endurance

• avoiding intense exercise and staying hydrated can help prevent muscle damage