Glycolysis

Question 1

What glucose transporters do humans have?

Answer 1

GLUT 1-4

Question 2

GLUT 1

Answer 2

Present in most cell types, including fetal tissues, responsible for basal glucose transport
Km is lower (1mm) than normal glucose levels (4-8mm)

Question 3

GLUT 2

Answer 3

Present in liver and pancreatic beta cells and transports only when glucose levels are high
Km is very high (15-20mm)
Bidirectional transporter; liver: glucose uptake for glycolysis and when liver levels are high, glucose is released into the circulation (e.g. during gluconeogenesis)

Question 4

GLUT 3

Answer 4

Present mostly in neurons and the placenta

Question 5

GLUT 4

Answer 5

Present in muscle and fat cells.
The number of transporters increases rapidly in the presence of insulin; endurance training increases the number in the muscles
Km is 5mM

Question 6

Glucose metabolism in anaerobic conditions

Answer 6

e.g. a sprint

pyruvate is metabolized to lactate

Question 7

Glucose metabolism in aerobic conditions

Answer 7

e.g. a long, slow run

pyruvate is more completely oxidized to CO2 with additional ATP generated

Question 8

Stage 1 of Glycolysis

Answer 8

Preparative Phase:
Glucose is turned into fructose 1,6-bisphosphate, which can be cleaved into two, 3-C units of glyceraldehyde 3-phosphate
(requires 2 ATP)

Question 9

Stage 2 of Glycolysis

Answer 9

ATP generating phase:
GAP is oxidized to generate pyruvate and ATP
4 ATP generated (net gain of 2 ATP)

Question 10

How is glucose trapped in the cell after entry?

Answer 10

Glucose is phosphorylated by Hexokinase to create Glucose-6-phosphate
G6P cannot pass through the membrane because of its negative charge. It can also NOT be transported by a glucose transporter

Question 11

Glycolysis Preparative phase

Answer 11

traps glucose into frucose 1,6-bisphosphate that can be cleaved into 3C units of glyceraldehyde-3-P
(Requires 2 ATP)

Question 12

Glycolysis Stage 2 (ATP generating phase)

Answer 12

involves oxidation of GAP to generate pyruvate and ATP

4 ATP generated

Question 13

How is glucose trapped in cells?

Answer 13

Hexokinase converts glucose to glucose-6-phosphate, which is unable to pass through the cell (is negatively charged)

Question 14

Importance of phosphofructokinase-1 in glycolysis

Answer 14

PFK-1 is a key regulatory enzyme that controls the pace of glycolysis
-is a committed, rate-limiting step

Question 15

Aldolase function in glycolysis

Answer 15

Aldolase converts fructose 1,6-bisphosphate into glyceraldehyde-3-phosphate and DHAP (which is then converted to GAP)
-GAP can generate ATP, DHAP cannot

Question 16

NAD+

Answer 16

Niacin, or vitamin B3, derivative

Can accept 2 electrons and a H+ ion

Question 17

FAD

Answer 17

Riboflavin, or vitamin B2 derivative

Can accept 2 electrons and 2 H+ ions

Question 18

fructose metabolism

Answer 18

Fructose is metabolized eventually to glyceraldehyde-3-P, which enters the glycolysis pathway.
Fructose –> fructose-1-P (by fructokinase)
Fructose-1-P –> DHAP / glyceraldehyde (by aldolase B)
DHAP & Fructose-1-P –> glyceraldehyde-3-P

Question 19

Aldolase B

Answer 19

Rate-limiting enzyme in fructose metabolism
Fructose Intolerance: Defect in aldolase B results in accumulation of fructose-1-P and phosphate & ATP depletion
Overall result is decreased glucose, lactic acidosis, hypoglycemia, nausea, convulsions, weakness, and jaundice

Question 20

Essential Fructosuria

Answer 20

Defect in fructokinase

Asymptomatic, because fructose is able to exit cells, resulting in no buildup

Question 21

Galactose metabolism

Answer 21

galactose is metabolized directly to glucose

Question 22

Classical galactosemia

Answer 22

Defect in galactose 1-phosphate uridylytransferase (step 2), results in accumulation of galactose-1-P and galactose
Presentation: intellectual disability, cataracts, liver disease
Treatment: reduce lactose consumption

Question 23

Non-classical galactosemia

Answer 23

Defect in galactokinase (step 1), results in accumulation of galactose
Presentation: cataracts, mild symptoms

Question 24

Lactate production and NAD+ Regeneration

Answer 24

NAD+ is reduced to NADH in many steps of metabolism but NAD+ supply is limited (can only get from diet - Vitamin B3).
Production of lactate during anaerobic glycolysis by lactate dehydrogenase generates NAD+

Question 25

Cori Cycle

Answer 25

RBCs can only undergo glycolysis, can generate lactate from glycolysis. The lactate can then enter the liver cells, which convert it to glucose, which is then sent back to RBCs.

Question 26

Glycerol-3-Phosphate Shuttle

Answer 26

Shuttles electrons across mitochondrial membranes to regenerate NAD+ (There are no NADH mitochondrial membrane transporters).
Cytoplasmic glycerol-3-P dehydrogenase transfers electrons from NADH to DHAP to glycerol-3-P, which diffuses into the mitochondrial membrane and donates the electrons to FAD

Question 27

Malate-Aspartate Shuttle

Answer 27

In the cytosol, oxaloacetate is converted to malate, which can be shuttled into mitochondria and once there, will contribute reducing agents to ETC.

Question 28

Key sites of glycolysis regulation

Answer 28

Hexokinase: glucose –> glucose-6-phosphate
Phosphofructokinase-1: fructose-6-phosphate –> fructose 1,6-bisphosphate
Pyruvate kinase: PEP –> pyruvate

Question 29

glucokinase

Answer 29

(Liver)
a high Km enzyme (unlike hexokinase) not inhibited by glucose-6-phosphate (also unlike hexokinase)
-when glucose levels are low, glycolysis is low so that there is sufficient glucose for other functions/cells

Question 30

Major site of regulation in glycolysis

Answer 30

Phosphofructokinase-1
Senses energy status (ATP vs. AMP)
High ATP, High Citrate, and low pH inhibit PFK1
High AMP and high fructose 2, 6 bisphosphate stimulate

Question 31

Fructose 2,6-bisphosphate

Answer 31

Responds to glucose levels and when glucose levels are high, it stimulates PFK-1 and inhibits fructose 1,6-bisphosphatase
Promotes glycolysis
**is a regulatory molecule, not a glycolytic pathway intermediate
Regulated by 2 enzyme activities, a kinase and a phosphatase, that reside on the same polypeptide, PFK-2

Question 32

Phosphofructokinase-2

Answer 32

Has kinase and phosphatase activities

• The Kinase domain phosphorylates fructose-6-phosphate for form F-2,6-BP (increases levels)
• The phosphatase domain dephosphorylates F-2,6,-BP to form fructose-6-P (lower levels of F-2,6-BP)

Question 33

Insulin and PFK2 Relationship

Answer 33

High Glucose Levels/Insulin INHIBIT PFK2 and thus PROMOTE GLYCOLYSIS

• Insulin signaling activates PFK2 phosphatase activity, removing the phosphate from the PFK2 kinase domain - this ACTIVATES the kinase and INHIBITS phosphatase
• Increases fructose 2,6-BP, stimulates PFK-1, and glycolysis predominates

Question 34

Pyruvate Kinase Regulation

Answer 34

In normal cells:
ATP allosterically inhibits pyruvate kinase - this makes sense, because when ATP is high you want to slow glycolysis

In liver:
When glucose levels are low, glucagon signaling leads to the phosphorylation of pyruvate kinase and reduced activity, so that the other cells can have the glucose

Question 35

Hemolytic anemia

Answer 35

Caused by pyruvate kinase deficiency

Results in lysis of RBCs because these cells need glycolysis for ATP. Other cells are usually okay.