Chapter 9 - Carbohydrate Metabolism 1

Question 1

What is normal blood glucose concentration?

Answer 1

around 100 mg/dL (5.6 mM), and the normal range is 4-6 mM

Question 2

GLUT 2

Answer 2

low-affinity transporter in hepatocytes and pancreatic cells

Question 3

GLUT 4

Answer 3

in adipose tissue and muscle, responds to the glucose concentration in peripheral blood; rate of glucose transport in these two tissues is increased by insulin

Question 4

What happens to GLUT 4 transporters when insulin levels are decreased?

Answer 4

the number of GLUT 4 transporters on the plasma membranes decreases

Question 5

What is the rate-limiting enzyme of glycolysis?

Answer 5

phosphofructokinase-1

Question 6

What is the rate-limiting enzyme of fermentation?

Answer 6

lactate dehydrogenase

Question 7

What is the rate-limiting enzyme of glycogenesis?

Answer 7

glycogen synthase

Question 8

What is the rate-limiting enzyme of glycogenolysis?

Answer 8

glycogen phosphorylase

Question 9

What is the rate-limiting enzyme of gluconeogenesis?

Answer 9

fructose-1,6-bisphosphatase

Question 10

What is the rate-limiting enzyme of pentose phosphate pathway?

Answer 10

glucose-6-phosphate dehydrogenase

Question 11

What is the importance of hexokinase?

Answer 11

in most tissues, inhibited by its product glucose-6-phosphate, low Km (reaches max velocity at low [glc]

Question 12

What is the importance of glucokinase?

Answer 12

in hepatocytes and pancreatic beta-islet cells (along with GLUT 2), high Km, induced by insulin in hepatocytes

Question 13

Phosphofructokinase-1 (PFK-1)

Answer 13

PFK-1 is rate-limiting enzyme and main control point in glycolysis, it’s inhibited by ATP and citrate, activated by AMP; insulin stimulates and glucagon inhibits PFK-1 in hepatocytes by an indirect mechanism involving PFK-2 and fructose 2,6-bisphosphate,

Question 14

Phosphofructokinase-2 (PFK-2)

Answer 14

glucagon inhibits PFK-2, which lowers F2,6-BP and inhibits PFK-1, PFK-2 is mostly in the liver

Question 15

Glyceraldehyde-3-Phosphate Dehydrogenase

Answer 15

catalyzes an oxidation and addition of inorganic phosphate (Pi) to its substrate; creates a high-energy intermediate 1,3-bisphosphoglycerate + reduction of NAD+ to NADH

Question 16

3-PG Kinase

Answer 16

transfers high-energy phosphate from 1,3-BPG to ADP to form ATP and 3-PG

Question 17

Substrate-Level Phosphorylation

Answer 17

ADP is directly phosphorylated to ATP using a high-energy intermediate

Question 18

Pyruvate Kinase

Answer 18

catalyzes a substrate-level phosphorylation of ADP using the high-energy substrate PEP

Question 19

Fermentation

Answer 19

key enzyme is lactate dehydrogenase which oxidizes NADH to NAD+, replenishes oxidized coenzyme for G3P; main objective is to replenish NAD+

Question 20

Dihydroxyacetone phosphate (DHAP)

Answer 20

used in hepatic and adipose tissue for triacylglycerol synthesis; DHAP is formed from F1,6-BP, can be isomerized to G3P which can convert to glycerol

Question 21

How is ATP gained during anaerobic respiration?

Answer 21

1,3-BPG and PEP are high-energy intermediates used to generate ATP by substrate-level phosphorylation

Question 22

How Glycolysis Pushes Forward the Process: Kinases

Answer 22

Hexokinase, Glucokinase, PFK-1, Pyruvate Kinase

Question 23

How does adaptation to high altitudes (low pO2) work?

Answer 23

Increased respiration, increased O2 affinity for Hb (initial), increased rate of glycolysis, increased [2,3-BPG] in RBCs (12-24 hr period), normalized O2 affinity for Hb restored by the increased level of 2,3-BPG, increased Hb (over days to weeks)

Question 24

Glycolysis in Erythrocytes

Answer 24

anaerobic glycolysis represents the only pathway for ATP production, yielding a net 2 ATP per glucose

Question 25

Why do red blood cells have bisphophoglycerate mutase?

Answer 25

to produce 2,3-BPG from 1,3-BPG in glycolysis

Question 26

2,3-BPG

Answer 26

binds allosterically to the beta-chains of hemoglobin A (HbA), decreases affinity for O2, goal is to unload O2 in tissues

Question 27

What physiological changes promote a rightward shift of the oxygen dissociation curve (the Bohr effect)?

Answer 27

High 2,3-BPG, low pH, high [H+], high pCO2

Question 28

The Bohr Effect

Answer 28

increases in CO2 partial pressure of blood or decreases in blood pH result in a lower affinity of Hb for O2

Question 29

Galactose Metabolism

Answer 29

lactose is a disaccharide that is hydrolyzed to galactose and glucose by lactase

Question 30

Pyruvate Dehydrogenase Complex (PDH)

Answer 30

irreversible, cannot be used to convert acetyl-CoA to pyruvate or glucose, activated by insulin in the liver, in the nervous system it is not responsive to hormones

Question 31

Glycogenesis

Answer 31

synthesis of glycogen granules, begins with a core protein called glycogenin

Question 32

Glycogen Synthase

Answer 32

rate-limiting enzyme of glycogen synthesis, and forms the alpha-1,4 glycosidic bond found in the linear glucose chains of the granule, stimulated by G6P and insulin, it is inhibited by epinephrine and glucagon through a protein kinase cascade that phosphorylates and inactivates the enzyme

Question 33

Branching Enzyme (Glycosyl alpha-1,4:alpha-1,6 Transferase)

Answer 33

introduces alpha-1,6-linked branches into the granule; hydrolyze one of the alpha-1,4 bonds to release a block of oligoglucose, which is moved and added to a slightly different spot via an alpha-1,6 bond to make a branch

Question 34

Glycogen Phosphorylase

Answer 34

breaks alpha-1,4 glycosidic bonds, releasing G1P from periphery of granule

Question 35

Debranching Enzyme (Glucosyl alpha-1,4:alpha-1,4 Transferase and alpha-1,6 Glucosidase)

Answer 35

two-enzyme complex: one moves the terminal end of a glycogen chain to the branch point (alpha-1,4:alpha-1,4 transferase), and one removes glucose monomer at branch point (alpha-1,6)

Question 36

What are important substrates of gluconeogenesis?

Answer 36

Glycerol-3-phosphate Lactate (from anaerobic glycolysis) Glucogenic amino acids (from muscle proteins)

Question 37

What are glucogenic amino acids?

Answer 37

All AAs except leucine and lysine, can be converted to intermediates that feed into gluconeogenesis

Question 38

What are ketogenic amino acids?

Answer 38

can be converted into ketone bodies, can be used as an alternative fuel mainly during prolonged starvation

Question 39

Pentose Phosphate Pathway (PPP) or Hexose Monophosphate (HMP) Shunt

Answer 39

two major functions: production of NADPH and serve as a source of ribose 5-phosphate for nucleotide synthesis

Question 40

Glucose-6-phosphate Dehydrogenase

Answer 40

rate-limiting enzyme of the PPP which is activated by NADP+ and insulin; inhibited by NADPH