Glucose Transport

Question 1

GLUT 2

Answer 1

A low-affinity transporter in hepatocytes and pancreatic cells. After a meal, blood traveling through the hepatic portal vein from the intestine is rich in glucose – captures the excess glucose primarily for storage.

Question 2

GLUT 4

Answer 2

In adipose tissue and muscle and responds to the glucose concentration in peripheral blood.
The transporter is saturated when blood glucose levels are just a bit higher than normal.

Question 3

Glycolysis

Answer 3

A cytoplasmic pathway that converts glucose into 2 pyruvate molecules, releasing a modest amount of energy captured in 2 substrate-level phosphorylations and one oxidation reaction.

Question 4

Hexokinase

Answer 4

• • Present in most tissues
• • Low Km (reaches max velocity at low [glucose])
• • Inhibited by glucose-6-phosphate

Question 5

Glucokinase

Answer 5

• • Present in hepatocytes and pancreatic Beta-islet cells
• • High Km
• • Induced by insulin in hepatocytes

Question 6

Phosphofructokinase-1 (PFK-1)

Answer 6

The rate-limiting enzyme and main control point in glycolysis: inhibited by ATP and citrate, and activated by AMP.
Insulin stimulates and glucagon inhibits PFK-1 in hepatocytes by an indirect mechanism involving PFK-2 and fructose 2,6-biphosphate.

Question 7

Why is activation of PFK-1 by insulin important

Answer 7

It allows those cells to override the inhibition caused by ATP so that glycolysis can continue, even when the cell is energetically satisfied.

Question 8

Glyceraldehyde-3-phosphate dehydrogenase

Answer 8

Catalyzes an oxidation and addition of inorganic phosphate (Pi) to its substrate, glyceraldehyde 3-phosphate. Results in high-energy intermediate 1,3-bisphosphoglycerate and the reduction of NAD+ to NADH.

Question 9

3-phosphoglycerate kinase

Answer 9

Transfers the high-energy phosphate from 1,3-bisphoglycerate to ADP, forming ATP and 3-phosphoglycerate –> this step is substrate-level phosphorylation

Question 10

Pyruvate kinase

Answer 10

Catalyzes substrate-level phosphorylation of ADP using high-energy substrate phosphoenolpyruvate (PEP).–> activated by fructose 1,6-biphosphate from the PFK-1 reaction

Question 11

Why is activation of pyruvate kinase feed-forward activation

Answer 11

Product of an earlier reaction of glycolysis stimulates, or prepares, a later reaction in glycolysis.

Question 12

Lactate dehydrogenase

Answer 12

Key fermentation enzyme in mammalian cells: reduces pyruvate to lactate and oxidizes NADH to NAD+, replenishing NAD+ supply needed for glycolysis.

Question 13

Dihydroxyacetone phosphate (DHAP)

Answer 13

Used in hepatic and adipose tissue for triacylglycerol synthesis.
Formed from fructose 1,6-bisphosphate –> can be isomerized to glycerol 3-phosphate –> converted to glycerol –> backbone of triacylglycerols

Question 14

What are the irreversible enzymes of glycolysis

Answer 14

How Glycolysis Pushes Forward the Process: Kinases
Hexokinase
Glucokinase
PFK-1
Pyruvate Kinase

Question 15

What is the only pathway for ATP production in erythrocytes

Answer 15

Anaerobic glycolysis, yielding a net 2 ATP per glucose.

Question 16

Effect of 2,3 - BPG on red blood cells

Answer 16

Binds allosterically to the beta-chains of hemoglobin A (HbA) and decreases its affinity for oxygen.

Question 17

Adaptation to high altitudes involve:

Answer 17

• Increased respiration
• Increased oxygen affinity for hemoglobin (initial)
• Increased rate of glycolysis
• Increased [2,3-BPG] in RBC (over a 12-24 hour period)
• Normalized oxygen affinity for hemoglobin restored by the increased level of 2,3-BPG
• Increased hemoglobin

Question 18

What 2 monosaccharides make up lactose

Answer 18

Glucose and galactose

Question 19

What 2 monosaccharides make up sucrose

Answer 19

Glucose and fructose

Question 20

Glycogenesis

Answer 20

The synthesis of glycogen granules.

Question 21

Glycogen synthase

Answer 21

The rate-limiting enzyme of glycogen synthesis and forms the alpha-1,4 glycosidic bond found in the linear glucose chains of the granule.

Question 22

Branching enzyme

Answer 22

Responsible for introducing the alpha-1,6-linked branches into the granule as shown.

Question 23

Glycogenolysis

Answer 23

The process of breaking down glycogen

Question 24

Glycogen phosphorylase

Answer 24

Rate-limiting enzyme of glycogenolysis. Breaks alpha-1,4, glycosidic bonds, releasing glucose 1-phosphate from the periphery of the granule. Activated by glucagon in the liver

Question 25

Debranching enzyme

Answer 25

A 2-enzyme complex that deconstructs the branches in glycogen that have been exposed by glycogen phosphorylase.

Question 26

Important substrates for gluconeogenesis

Answer 26

• Glycerol 3-phosphate (from stored fats, or triacylglycerols, in adipose tissue)
• Lactate (from anaerobic glycolysis)
• Glucogenic amino acids (from muscle proteins)

Question 27

Glucogenic amino acids (all except leucine and lysine0

Answer 27

Can be converted into intermediates that feed into gluconeogenesis

Question 28

Ketogenic amino acids

Answer 28

Can be converted into ketone bodies, which can be used as an alternative fuel, particularly during periods of prolonged starvation

Question 29

Fructose-1,6-bisphosphatase

Answer 29

In the cytoplasm: a key control point of gluconeogenesis and represents the rate-limiting step of the process. It reverses the action of phosphfructokinase-1, the rate-limiting step of glycolysis, by removing phosphate from fructose 1,6-bisphosphate to produce fructose 6-phosphate.

Question 30

Pentose phosphate pathway (PPP)

Answer 30

Occurs in the cytoplasm of all cells, where it serves 2 major purposes: production of NADPH and serving as a source of ribose 5-phosphate for nucleotide synthesis

Question 31

Glucose-6-phosphate deyhdrogenase (G6PD)

Answer 31

Rate-limiting enzyme in the PPP: produces NADPH.
Induced by insulin because the abundance of sugar entering the cell under insulin stimulation will be shunted into both fuel utilization pathways (glycolysis and aerobic respiration) as well as fuel storage pathways (fatty acid synthesis, glycogenesis, and the PPP).

Question 32

Difference between NADPH and NADH

Answer 32

NAD+ is an energy carrier; NADPH is used in biosynthesis, in the immune system, and to help prevent oxidative damage.
NAD+ acts as a high-energy electron acceptor from a number of biochemical reaction –> can be thought of as a potent oxidizing agent
NADPH acts as an electron donor: can be thought of as a potent reducing agent