Biochemistry 9: Carbohydrate Metabolism I

Question 1

GLUT 2

Answer 1

low-affinity transporter in hepatocytes and pancreatic cells

high Km - captures the excess glucose for storage after a meal

serves as the glucose sensor for insulin release in B-islet cells

insulin-dependent

Question 2

Km for GLUT 2

Answer 2

15 mM

this means that the liver will pick up excess glucose after a meal and store it

Question 3

GLUT 4

Answer 3

glucose transporter located in adipose tissue and muscle

responds to the glucose concentration in peripheral blood

insulin-dependent transport (increased insulin increases the number of transporters

Question 4

how does insulin affect GLUT4?

Answer 4

stimulates the movement of additional GLUT 4 transporters to the membrane

Question 5

Km for GLUT 4

Answer 5

5 mM (close to normal blood glucose level)

GLUT 4 is saturated when blood glucose levels are just a bit higher than normal

Question 6

how does the liver utilize excess glucose?

Answer 6

uses glycolysis - excess glucose is converted to fatty acids for storage

Question 7

how do the beta-islet cells in the pancrease know when to release insulin?

Answer 7

GLUT2 begins to transport glucose into the cell

glucokinase is induced by insulin; phosphorylates glucose -> G6P

Question 8

what is the first step in Glucose Metabolism?

Answer 8

transport across the membrane (facilitated diffusion/active transport) & phosphorylation by kinase enzymes inside the cell to prevent glucose from leaving via the transporter

glucose —> glucose 6-phosphate

Question 9

Hexokinase

Answer 9

widely distributed enzyme in tissues

glucose –> glucose 6-phosphate

Low Km (reaches vmax at low [glucose])

inhibited by G6P

Question 10

Glucokinase

Answer 10

only in liver cells and pancreatic B-islet cells

glucose —> glucose 6-phosphate

induced by insulin in the liver (acts as a glucose sensor)

High Km (acts proportionally to [glucose])

Question 11

Phosphofructokinase-1

Answer 11

rate-limiting enzyme and main control point in glycolysis

fructose 6-phosphate –> fructose 1,6 biphosphate

inhibited by ATP, citrate, and glucagon (indirectly)

(glycolysis shouldn’t be on if we have enough energy)

activated by AMP and insulin (indirectly)

(glycolysis should be on if we need energy)

Question 12

Phosphofructokinase-2

Answer 12

directly activated by insulin

converts a little fructose 6-phosphate —> fructose 2,6-bisphosphate (which activates PFK-1)

directly inhibited by glucagon, lowering F2,6-BP, inhibiting PFK-1

found mostly in the liver

can override the PFK-1 inhibition caused by ATP so that glycolysis can continue

Question 13

Glyceraldehyde 3-phosphate dehydrogenase

Answer 13

catalyzes an oxidation and addition of Pi to substrate

glyceraldehyde 3-phosphate —>1,3-bisphosphoglycerate (high-energy intermediate)

reduction of NAD+ —> NADH

Question 14

what is the difference between substrate-level and oxidative phosphorylation?

Answer 14

substrate level: ADP is directly phosphorylated to ATP using a high energy intermediate

oxidative: dependent on O2; ATP made from electron transport and chemiosmosis

Question 15

3-Phosphoglycerate kinase

Answer 15

moves high energy Pi from 1,3-biphosphoglycerate (G3P dehydrogenase) to ADP –> ATP and 3-phosphoglycerate

3-phosphoglycerate –> 1,3 bisphosphoglycerate

Question 16

Pyruvate Kinase

Answer 16

activated by fructose 1,6-bisphosphate from the PFK-1 reaction

an example of feed-forward activation

phosphoenolpyruvate (PEP) + ADP —> pyruvate + ATP

Question 17

Lactate Dehydrogenase

Answer 17

key fermentation enzyme in mammalian cells when O2 or mitochondria are absent

oxidizes NADH to NAD+ (replenishing for G3P dehydrogenase)

reduces pyruvate to lactate

Question 18

Dihydroxyacetone phosphate (DHAP)

Answer 18

glycolysis intermediate

used in hepatic and adipose tissue for triacylglycerol synthesis

formed from fructose 1,6-bisphosphate

can be isomerized to glycerol 3-phosphate <—> glycerol (backbone of triacyglycerols)

Question 19

1,3-bisphosphate and phosphoenolpyruvate

Answer 19

high-energy intermediates used to generate ATP by substrate-level phosphorylation

only means of gaining ATP in anaerobic respiration

Question 20

which enzymes in glycolysis are irreversible?

Answer 20

Hexokinase/Glucokinase, PFK-1, and Pyruvate kinase

How Glycolysis Pushes Forward the Process: Kinases

Question 21

Bisphosphoglycerate Mutase

Answer 21

rearranges the phosphate in 1,3-BPG

1,3-bisphosphoglycerate —> 2,3 bisphosphoglycerate

Question 22

Mutases

Answer 22

enzymes that move a functional group from one place in a molecule to another

Question 23

2,3 bisphosphoglyerate (2,3-BPG)

Answer 23

binds allosterically to hemoglobin

decreases its affinity for oxygen

creates rightward shift in O2 disaccosication curve for hemoglobin

does not bind to fetal hemoglobin

Question 24

how is fermentation in mammals different from fermentation in yeast?

Answer 24

in mammals, pyruvate is reduced to lactate

in yeast, pyruvate is converted to ethanol and CO2

Question 25

Galactokinase

Answer 25

**phosphorylates** galactose and traps it in the cells galactose ---\> **galactose 1-phosphate**

Question 26

Galactose 1-phosphate Uridyltransferase

Answer 26

**galactose 1-phosphate** --\> **glucose 1-phosphate** this reaction also requires an **epimerase** links galactose metabolism to glycolysis

Question 27

Epimerase

Answer 27

enzymes that catalyze the **conversion of one sugar epimer to another** (differ at exactly one chiral center)

Question 28

Fructokinase

Answer 28

phosphorylates **fructose** ----\> **fructose 1-phosphate**, trapping it in the cell

Question 29

Aldolase B

Answer 29

cleaves **fructose 1-phosphate** ---\> **glyceraldehyde** and **DHAP** links fructose metabolism to glycolysis

Question 30

Pyruvate Dehydrogenase Complex

Answer 30

**pyruvate** + CoA + NAD+ ---\> **acetyl-CoA** + **NADH** + **CO2** irreversible enzyme present in the **liver** **activated by insulin** (high insulin = well-fed state. liver should make energy, make fat, or store fat) **inhibited** by its product **acetyl-CoA** hight levels of acetyl-CoA implies that cell is satisified and need not enter citric acid cycle (eventual buildup causes a shift from entering citric acid cycle or fatty acid oxidation to produce oxaloacetate for gluconeogenesis)

Question 31

Three possible fates of pyruvate:

Answer 31

conversion to **acetyl-CoA** by **pyruvate dehydrogenase** conversion to **lactate** by **lactate dehydrogenase** conversion to **oxaloacetate** by **pyruvate carboxylate**

Question 32

Glycogen

Answer 32

**storage** form of glucose **synthesized and degraded** primarily by **liver and skeletal muscle** stored in the **cytoplasm as granules** granules have **central protein core** with **polyglucose chains** radiating outward to form a sphere can be **branched or linear chains**

Question 33

how is the structure of glycogen granules different when they are composed of **linear chains** and **branched**?

Answer 33

when the chains are **linear**, the highest density of glucose is near the **core** when the chains are **branched**, the highest density of glucose is near the **periphery**, allowing more rapid release of glucose

Question 34

Glycogenesis

Answer 34

**synthesis** of glycogen granules using **glycogen synthase** and **branching enzyme**

Question 35

what is the mechanism of glycogenesis?

Answer 35

begins with a core protein, **glycogenin**, to which glucose is added **glucose** ---\> **glucose 6-phosphate** ---\> **glucose 1-phosphate** glucose 1-phosphate is **activated** by coupling to a molecule of uridine diphosphate UDP glucose 1-phosphate + UTP ---\> **UDP-glucose** + pyrophosphate (PPi) once activated, glucose can be added to glycogen chain

Question 36

Glycogen synthase

Answer 36

rate-limiting enzyme of glycogen synthesis UDP-glucose ---\> glycogen forms **alpha-1,4** glycosidic bond found in the **linear glucose chains** of the glycogen granule **activated** by glucose 6-phosphate and **insulin** in liver and skeletal **inhibited** by epinephrine and **glucagon**

Question 37

Branching enzyme

Answer 37

responsible for introducing **alpha-1,6** linked **branches** into the glycogen granule as it grows 1. hydrolyzes one of the **alpha-1,4** bonds to release a block of oligoglucose (relocated to another position) 2. forms an **alpha-1,6** bond to create a branch

Question 38

Glycogenolysis

Answer 38

process of **breaking down glycogen** using **glycogen phosphorylase** and **debranching enzyme**

Question 39

Glycogen phosphorylase

Answer 39

rate-limiting enzyme of glycogenolysis **glycogen** ---\> **glucose 1-phosphate** glucose 1-phosphate is converted to glucose 6-phosphate by mutase **breaks alpha-1,4 glycosidic bonds**, releasing glucose 1-phosphate from the periphery of the granule **cannot break alpha-1,6 bonds** so it can only degrade linear chains activated by glucagon (liver), AMP (skeletal), and epinephrine inhibited by ATP

Question 40

Debranching enzymes

Answer 40

two-enzyme complex that **deconstructs the branches in glycogen** that have been exposed by glycogen phosphorylase 1. **breaks an alpha-1,4** bond adjacent to the branch point 2. **moves** the small oligoglucose chain that is released **to the exposed end of the other chain** 3. forms a **new alpha-1,4 bond** 4. **hydrolyzes the alpha-1,6 bond**, releasing the **single residue** at the branch point as free glucose

Question 41

Important substrates for gluconeogenesis:

Answer 41

**glycerol 3-phosphate** (from triacylglycerols) **lactate** (from anaerobic glycolysis) glucogenic **amino acids** (from muscle proteins) (also dietary fructose and glucose)

Question 42

which enzymes in **gluconeogenesis** are **irreversible**?

Answer 42

pyruvate carboxylase phosphoenolpyruvate carboxykinase fructose-1,6-bisphosphatase glucose-6-phosphatase

Question 43

Pyruvate Carboxylase

Answer 43

mitochondrial gluconeogenic enzyme **pyruvate** ---\> **oxaloacetate** (citric acid cycle intermediate stuck in mitochondria, reduced to malate) once in the cytoplasm, oxaloacetate is made again from malate works with phosphoenolpyruvate carboxykinase to **replace pyruvate kinase** **activated** by **acetyl-CoA** (fom beta-oxidation) high levels of acetyl-CoA suggest cell satisified, so pyruvate shunted to pyruvate carboxylase to generate more glucose for the rest of the body

Question 44

Phosphoenolpyruvate carboxykinase

Answer 44

gluconeogenic enzyme converts **OAA** --\> **phosphoenolpyruvate** (rxn requires GTP) works with pyruvate carboxylase to **replace pyruvate kinase** PEP later forms fructose 1,6-bisphosphate induced by **glucagon and cortisol** to raise blood glucose levels

Question 45

Frutose 1,6-bisphosphatase

Answer 45

key control point of gluconeogenesis; **rate-limiting step** of the process **fructose 1,6 bisphosphate** --\> **fructose 6-phosphate** (opposite of PFK-1) **activated by ATP** (high levels imply cell is energetically satisified and doesn't need to break down glucose) **inhibited by AMP** and **fructose 2,6 bisphosphate** high levels of AMP mean cell needs to break down glucose F2,6-BP is produced by PFK-2 to activate PFK-1 (levels increased with insulin, decreased with glycogen)

Question 46

Fructose 2,6 bisphosphate

Answer 46

marker for satisfactory energy levels in liver cells low levels signals to the liver that it should shift its function from burning energy to storing energy controls both gluoconeogenesis and glycolysis **glucagon lowers levels to stimulate gluconeogensis** **insulin increases levels to stimulate glycolysis**

Question 47

Glucose 6-phosphatase

Answer 47

gluconeogenic enzyme found only in the **ER** of liver cells **glucose 6-phosphate** ---\> **glucose** diffuses into cytoplasm is used to **circumvent glucokinase and hexokinase**

Question 48

Pentose phosphate pathway / hexose monophosphate (HMP) shunt

Answer 48

occurs in the cytoplasm of all cells produces **NADPH** and serves as a source of **ribose 5-phosphate** for nucleotide synthesis

Question 49

Glucose-6-phosphate dehydrogenase

Answer 49

irreversible rate-limiting enzyme involved in the first step of the pentose phosphate pathway glucose 6-phoshate to **make NADPH** **induced by insulin** (high [glucose] entering the cell) and NADP+ **inhibited by NADPH**

Question 50

Functions of NADPH

Answer 50

biosynthesis of **fatty acids and cholesterol** assisting in **cellular bleach production** in certain **white blood cells** maintenance of a **supply or reduced glutathione** to **protect against reactive oxygen species**