Glycolysis

Question 1

What is glycolysis ?

Answer 1

A key pathway in preparing glucose (and other CHO) for oxidative degradation.

A series of reactions.

Glycolysis - sugar splitting

Question 2

What happens in glycolysis ?

Transport of glucose

Answer 2

Blood glucose is transported into cells (e.g. Glut2 and Glut4 - enhanced by insulin), lowering blood concentrations.

Question 3

Where does glycolysis take place ?

Answer 3

In the cytosol of the cell

Question 4

Describe some fates of glucose within the cell

Answer 4

Glycolysis
Glycogen
Fat

Question 5

Glycolysis

Answer 5

Catabolism

Question 6

Glycogen

Answer 6

Anabolism

Question 7

Fat

Answer 7

Anabolism

Question 8

What is the aim of glycolysis ?

Answer 8

One molecule of glucose is converted into 2 molecules of pyruvate.

Pyruvate is then converted into acetyl co-enzyme A, which enters the Citric Acid Cycle.

Question 9

State the stages of Glycolysis

Answer 9

Investment
Cleavage
Energy Harvest

Question 10

How much energy is required for glycolysis ?

Answer 10

2 ATP are required for early reactions

Question 11

State the net yield of ATP generated by glycolysis

Answer 11

4 ATP are generated later, giving a NET YIELD of :

2 ATP per glucose

Question 12

How many carbon molecules are in pyruvate and glucose ?

Answer 12

Pyruvate - 3
Glucose - 6

Question 13

Describe the 2 fates of pyruvate produced from glycolysis

Answer 13

In AEROBIC conditions :

• pyruvate enters the citric acid cycle
• converted to acetyl-co-enzyme A

IN ANAEROBIC (low oxygen) conditions :

• Lactate is formed from pyruvate (fermentation)

Question 14

State the strategy for the glycolysis pathway

Answer 14

1. Add phosphoryl groups to glucose and intermediates
2. Chemically convert phosphorylated intermediates to compounds with high energy phosphate bond potential.
3. Chemically couple the hydrolysis of reactive compounds to generate ATP (and NADH+ and H+)

Question 15

Describe step 1 of Glycolysis

Answer 15

Phosphorylation of glucose at carbon 6

Requires ATP (INVESTMENT STAGE)

Locks glucose inside the cell
(maintains glucose gradient)

Question 16

Describe step 2 of Glycolysis

Answer 16

Conversion of glucose-6-P to fructose-6-P

G-6-P ring structure opens to enable isomerisation and subsequent ring closure. F-6-P

(G-6-P : Glucose 6 phosphate)

Question 17

Describe step 3 of Glycolysis

Answer 17

Fructose 6 phosphate is phosphorylated at carbon 1, and becomes fructose 1,6-biphosphate (FBP)

Requires ATP (INVESTMENT phase)

Phosphofructokinase (PFK) key regulatory point

Question 18

Describe step 4 of Glycolysis

Answer 18

Aldolase cleaves the FBP into 2x trioses

Glyceraldehyde-3-phosphate (GAP)
Dihydroxyacetone phosphate (DHAP)

Question 19

Describe step 5 of Glycolysis

Answer 19

DHAP is readily converted into GAP

2 GAPs for every glucose molecule.

Glyceraldehyde-3-phosphate (GAP)
Dihydroxyacetone phosphate (DHAP)

Question 20

Describe step 6 of Glycolysis

Answer 20

Oxidation and Phosphorylation of GAP by NAD+ and Pi

First ‘high energy’ intermediate-aldehyde oxidation (exergonic reaction)

This drives the synthesis of the 1-3 bisphosphoglycerate

Question 21

Describe step 7 of Glycolysis

Answer 21

First formation of ATP (energy harvest)

The newly formed high-energy phosphate bond used to synthesise ATP and 3-phosphoglycerate (3PG)

Question 22

Describe step 8 of Glycolysis

Answer 22

3PG converted to 2PG

This is essential preparation for the next energy harvest step.

Question 23

Describe step 9 of Glycolysis

Answer 23

2PG dehydration to form phosphoenolpyruvate (PEP)

Converts low energy phosphate ester bond of 2PG into high-energy intermediate phosphate bond

Question 24

Describe step 10 of Glycolysis

Answer 24

Hydrolysis of PEP high-energy bond generates ATP and pyruvate.

This is a physiological irreversible reaction

e.g. of substrate level phosphorylation

Question 25

Describe the energy yield from glycolysis

Answer 25

Glycolysis releases relatively little energy from glucose - the majority is released during the final stage, electron transfer chain and oxidative phosphorylation.

Question 26

What happens to pyruvate under anaerobic conditions ?

Answer 26

Converted to lactate

Question 27

What happens to pyruvate under aerobic conditions ?

Answer 27

Pyruvate is transported to the mitochondrial matrix and converted to acetyl-coenzyme A, which feeds into the citric acid cycle. NADH+ and H+ are oxidised to replenish NAD+ via the electron transport chain. When cellular energy levels (ATP) in excess, acetyl-coenzyme A used for synthesis of fatty acids.

Question 28

What happens to pyruvate under high cellular energy levels ?

Answer 28

Fatty acids or ketone bodies

Question 29

Describe lactate production in anaerobic conditions

Answer 29

For glycolysis to continue in anaerobic conditions, NAD+ must be replenished. When ATP demand is high and oxygen is depleted, homolactic fermentation regenerates NAD+ Reversible reaction which enables glycolysis to continue for short amounts of time. The build up of lactate causes muscle cramps and limits activity. Slow/fast twitch myofibre composition determines the rate of muscle fatigue.

Question 30

What type of reaction is the production of lactate ?

Answer 30

A redox reaction Oxidisation of NADH into NAD Reduction of pyruvate into lactate A highly exergonic reaction

Question 31

Describe the structure of lactate dehydrogenase

Answer 31

Composed of 2 different types of subunit : H or M 5 isozymes - Tissue specific expression Tetrameric - different combinations of subunits combine

Question 32

What does the presence of lactate dehydrogenase in the blood indicate ?

Answer 32

Gives an indication of where in the body damage has occurred.

Question 33

What does tetrameric mean ?

Answer 33

Different combinations of subunit combine.

Question 34

Describe lactate dehydrogenase in aerobic conditions

Answer 34

LDHB gene --> subunit H Chromosome 12 H subunits found in the heart tissue

Question 35

Describe lactate dehydrogenase in anaerobic conditions

Answer 35

LDHA gene --> subunit M Chromosome 11 M subunits found in muscle tissue

Question 36

State the use of lactate dehydrogenase in diagnosis

Answer 36

Differing properties and tissue locations Damage to specific tissues releases cell contents into circulation. Changes in serum levels - aids disease diagnosis

Question 37

Where is LDH 1 found ?

Answer 37

Predominantly / only found in cardiac tissue (also RBCs and kidneys) Elevated in myocardial infarction Fastest moving

Question 38

Where is LDH 5 found ?

Answer 38

Skeletal muscle, Liver Elevated in skeletal muscle and liver diseases Slowest moving

Question 39

State ways in which the rate of glycolysis is controlled

Answer 39

Key enzymes High ATP inhibit enzyme activity Intermediate substrates (e.g. fructose-6-P) stimulate PFK activity High citric acid inhibits Low pH inhibits Hormones

Question 40

State the key regulatory enzymes involved in glycolysis

Answer 40

Hexokinase PFK Pyruvate kinase

Question 41

Function of hexokinase

Answer 41

Locking glucose into the cell

Question 42

Describe hexokinase | What is it inhibited by ?

Answer 42

Allosterically inhibited by G-6-P

Question 43

Describe phosphofructokinase | What is it activated and inhibited by ?

Answer 43

Most IMPORTANT site of control - first step unique to glycolysis High [ATP] inhibits PFK : allosterically bind a region separate from the active site High [AMP] activates PFK (2ADP --> ATP + AMP)

Question 44

Function of phosphofructokinase

Answer 44

1st step unique to glycolysis

Question 45

Describe pyruvate kinase | What is it activated and inhibited by ?

Answer 45

Inhibited by high ATP and alanine and activated by FBP

Question 46

Function of pyruvate kinase

Answer 46

Last step from high energy compound into pyruvate