Carbohydrate metabolism I

Question 1

This glucose transporter:
-Found in hepatic and pancreatic cells
-Has a low affinity for glucose
-Appears when glucose levels are high (Km~15 mM)
-Acts as sensor for insulin release by interacting with pancreatic B-cells
-Has a low affinity for glucose

Answer 1

GLUT 2

Question 2

This glucose transporter:
-Found in adipose tissue and skeletal muscles
-Responds to glucose levels in peripheral blood
-Is saturated when glucose levels are only slightly elevated (Km~5 mM)
-Is modulated by insulin, where insulin increases the amount of transporters brought to the cell membrane

Answer 2

GLUT 4

Question 3

The end products of glycolysis are: __ ATP, __ pyruvate and __ NADH

Answer 3

2, 2, 2

Question 4

This glycolysis enzyme:
-Prevents glucose from leaving the cell by phosphorylating it
-Is irreversible
-Is widely distributed in tissue and has a high affinity for glucose
-Is inhibited by it’s product glucose-6-phosphate

Answer 4

Hexokinase

Question 5

This glycolysis enzyme:
-Prevents glucose from leaving the cell by phosphorylating it
-Is irreversible
-Is found only in liver and pancreatic B-cells, and has a low affinity for glucose
-Is induced by insulin (in the liver)

Answer 5

Glucokinase

Question 6

Compare and contrast hexokinase and glucokinase?

Answer 6

Both are involved in the phosphorylation of glucose so that it cannot leave once it enters the cell

Hexokinase is found in every cell because it helps with normal glycolysis and is inhibited by glucose-6-phosphate, its product. Associate it with GLUT 4, in that it’s saturated at normal glucose levels and has a high affinity for glucose

Glucokinase is found only in liver and pancreatic B-cells. It only helps with glycolysis in the liver and is stimulated by the presence of insulin (which indicates the levels of glucose are increased). Think of it like GLUT 2, where it isn’t saturated at normal glucose levels and has a low affinity for glucose

Question 7

This phosphofructokinase:
-Is a rate limiting enzyme
-Is irreversible
-Turns fructose-6-phosphate into fructose-1,6-biphosphate
-Is inhibited by ATP and citrate
-Is activated by AMP

Answer 7

PFK 1

Essentially determines whether or not glycolysis needs to proceed or if the cell has enough energy for the time being

Question 8

Why is PFK-1 activated by AMP but inhibited by citrate and ATP?

Answer 8

PFK-1 is involved in an energy making process, so if the cell already has enough energy (indicated by levels of ATP and citrate) there’s no need for glycolysis to continue. Similarly, if the cell only has AMP, it’s lacking in energy and glycolysis is necessary

Question 9

This phosphofrutokinase:
-Activates PFK-1
-Allows PFK-1 to override the presence of ATP and citrate and keep glycolysis active

Answer 9

PFK-2

Question 10

This dehydrogenase:
-Adds phosphate to glyceraldehyde-3-phosphate to form 1,3-biphosphoglycerate
-Forms NADH from NAD+, which is then brought into the electron transport chain

(Has a name based on the molecule it’s adding to)

Answer 10

Glyceraldehyde-3-phosphate dehydrogenase

Question 11

This kinase:
-Forms ATP from ADP and 1,3-biphosphoglycerate
-Is the only means of ATP generation in anaerobic tissue
-Is irreversible

Answer 11

3-Phosphoglycerate kinase

Question 12

This kinase:
-Creates ATP from ADP and phosphoenolpyruvate
-Is activated by PFK-1
-Is irreversible

Answer 12

Pyruvate kinase

Question 13

_____ occurs when ADP is directly phosphorylated to ATP via a high energy intermediate

Answer 13

Substrate-level phosphorylation

Question 14

What are the four irreversible enzymes of glycolysis?

Answer 14

The glucokinases (hexo and gluco)
PFK-1
3-phosphoglycerate kinase
Pyruvate kinase

Question 15

This pathway occurs anaerobically to replenish NAD+ that can then be used to continue glycolysis

Answer 15

Fermentation

Question 16

This glycolysis intermediate:
-Is nicknamed DHAP
-Is found in hepatic and adipose tissue
-Forms the backbone of triacylglycerol

Answer 16

Dihydroxyacetone

Question 17

These two glycolysis intermediates:
-Form ATP from substrate level phosphorylation
-Are the only anaerobic source of ATP

Answer 17

1,3-biphosphoglycerate (1,3-BPG) and phosphoenolpyruvate

Question 18

In the galactose metabolism, galactose is transformed into ____

Answer 18

Glucose-1-phosphate

Question 19

In the fructose metabolism, fructose is transformed into ____

Answer 19

DHAP and glyceraldehyde

Question 20

This complex of enzymes:
-Can enter the citric acid cycle as either acetyl-CoA if ATP is needed, or become fatty acids if there’s already enough ATP
-Is activated by insulin (when it’s in the liver)
-Is inhibited by acetyl CoA

Answer 20

Pyruvate dehydrogenase

Question 21

Glycogen is synthesized and degraded in ____ and ___ muscle, and is stored in the ____ as granules

Answer 21

Liver and skeletal muscle

Cytoplasm

Question 22

This pathway creates new glycogen

Answer 22

Glycogenesis

Question 23

This glycogenesis enzyme:
-Forms the linear alpha 1,4 bond in glycogen
-Is the rate limiting enzyme
-Is stimulated by insulin and glucose-6-phosphate
-Is inhibited by ephinephrine and glucagon

Answer 23

Glycogen synthase

Question 24

This glycogenesis enzyme:
-Creates the alpha 1,6 branches in glycogen by hydrolyzing areas of the linear 1,4 chain and moving them into the branched position at carbon 6

Answer 24

Branching enzyme

Question 25

This pathway breaks down glycogen

Answer 25

Glycogenolysis

Question 26

This glycogenolysis enzyme:
-Breaks alpha 1,4 bonds but cannot break 1,6 branches
-Is the rate limiting step
-Is activated by glucagon in the liver and by AMP and epinephrine in the skeletal muscle
-Is inhibited by ATP

Answer 26

Glycogen phosphorylase

Question 27

This glycogenolysis enzyme breaks the 1,6 branching in two steps: first by breaking the 1,4 bond closest to the branch and placing it back on the linear chain and second by hydrolyzing the 1,6 bond to release the single free glucose left from the branch

Answer 27

Debranching enzyme

Question 28

This pathway:
-Forms new glucose
-Is inhibited by insulin
-Is activated by glucagon and epinephrine
-Activates or increases during the the first 12 hours of fasting

Answer 28

Gluconeogenesis

Question 29

____ amino acids can turn into intermediates for gluconeogenesis, except for lysine and leucine

Answer 29

Glucogenic AA’s

Question 30

____ amino acids can turn into ketone bodies and serve as alternative fuel during starvation

Answer 30

Ketogenic AA’s

Question 31

This important gluconeogenesis enzyme:
-Is mitochondrial
-Forms Oxaloacetate as a product
-Is activated by the beta oxidation of acetyl-CoA

Answer 31

Pyruvate carboxylase

Question 32

This important gluconeogenesis enzyme:
-Is in the cytoplasm
-Is induced by glucagon and cortisol
-Circumvents pyruvate kinase by converting pyruvate into phosphoenolpyruvate

Answer 32

Phosphoenolpyruvate carboxykinase

Question 33

This important gluconeogenesis enzyme:
-Is the rate limiting step
-Circumvents PFK-1 by creating fructose-6-phosphate

Answer 33

Fructose-1,6-biphosphatase

Question 34

This important gluconeogenesis enzyme:
-Is only found in the lumen of the rough ER
-Circumvents hexo and glucokinase by tranposting glucose-6-phosphate to the ER and releasing it as glucose

Answer 34

Glucose-6-phosphatase

Question 35

This pathway:
-Occurs in the cytoplasm of all cells
-Has two major functions: to act as a source of NADPH and ribose-5-phosphate for nucleotide synthesis

Answer 35

Pentose phosphate pathway

(Also called the hexose monophosphate shunt)

Question 36

This enzyme of the pentose phosphate pathway:
-Is the rate limiting enzyme
-Is induced by insulin
-Is activated by NADP+
-Is inhibited by NADPH

Answer 36

Glucose-6-phosphate dehydrogenase

Question 37

What is the difference between NADH and NADPH?

Answer 37

NADH indirectly produces ATP by feeding into the electron transport chain. As NAD+, it’s a powerful oxidizing agent and electron acceptor

NADPH acts an electron donor and reducing agent, and is required for fatty acid and cholesterol synthesis, bleach production in some white blood cells, and maintaining glutathione to prevent free radicals