Module 9 - biochem of simple sugars and the glycolytic pathway

Question 1

Sucralose and aspartame are…

Answer 1

not sugars, but mimic the chemical structure

Question 2

Why is aspartame so much sweeter than real sugars?

Answer 2

higher binding affinity, slower off rate

Question 3

How does a glucose monitor detect blood glucose levels?

Answer 3

glucose oxidizes to gluconate and hydrogen peroxide

hydrogen peroxide oxidizes the dye in the test strip, turning it a different color

oxidized dye is measured by the device

Question 4

Glucose and fructose have the same chemical formula, so what is the difference?

Answer 4

glucose is a six membered ring, fructose is a five membered ring

glucose is a aldose and fructose is a ketose

Question 5

When glucose and fructose are drawn in chair or boat formations, they…

Answer 5

pucker

Question 6

Some monosaccharides have an —–.
How can you tell the difference between the L and D forms?

Answer 6

enantiomers

L - OH is on the left
R - OH is on the right

Question 7

What is an epimer?

Answer 7

the position of one hydroxyl group is flipped

Question 8

Monosaccharides are —- stable as cyclic structures than as linear structures. When drawn as cyclic, what is the difference between a and B structures?

Answer 8

more stable as cyclic

a: right hand OH is on the bottom
B: right hand OH is on the top

Question 9

What is Benedict’s test?

Answer 9

an alkaline solution of copper is used as an oxidizing agent

the reduction of Cu2+ to Cu+ changes the color from blue to red, indicating the oxidation of a sugar took place

Question 10

What are the bonds for lactose, sucrose, and trehalose?

Answer 10

lactose:
Gal(B1–>4)Glc

sucrose:
Glc(a1–>B2)Fru

trehalose:
Glc(a1–>a1)Glc

Question 11

Describe the structure and formation of maltose

Answer 11

consists of two glucose units attached by a
a1–>4. linkage at the O-glycosidic bond

produced by the hydrolysis of amylose by the enzyme amylase

Question 12

Describe the structure and formation of sucrose

Answer 12

links a glucose to fructose through an
a1–>B2 glycosidic bond

formed by a condensation reaction (removal of water)

Question 13

What is glycolysis?

Answer 13

primary pathway for ATP generation under anaerobic conditions

cleaves glucose into two molecules of pyruvate with a net yield of 2 ATP

Question 14

What does glycolysis accomplish for the cell?

Answer 14

a small amounbt of ATP, and pyruvate which is a precursor to acetyl CoA, lactate, and ethanol in yeast

Question 15

What is the overall net reaction of glycolysis?

Answer 15

Glucose + 2NAD+ + 2ADP + 2 Pi →

2 pyruvate + 2NADH + 2H+ + 2ATP + 2H2O

ΔGº’ = -35.5 kJ/mol

Question 16

What are the key regulated enzymes in glycolysis?

Answer 16

hexokinase, phosphofructokinase-1, pyruvate kinase

Question 17

When is glycolysis used in everyday chemistry?

Answer 17

intense exercise, making alcoholic beverages

Question 18

At some point in glycolysis, everything is doubled (there is 2 of everything). What step does this split happen?

Answer 18

when aldolase breaks fructose-1,6-BP into 2 glyceraldehyde-3-P

Question 19

Some steps of glycolysis have a positive dG’o but a negative dG. Why?

Answer 19

substrate concentration makes the reaction favorable

Question 20

Why do some reactions in glycolysis require the investment of ATP? How does it affect the reaction?

Answer 20

ATP phosphoryl transfer is highly favorable, so linking it to other unfavorable reactions gives the overall reaction a net negative dG

Question 21

How does hexokinase deficiency effect oxygen transport?

Answer 21

2,3 BPG is formed from 1,3 BPG. 1,3 BPG is formed from glucose via a series of reactions, but the first one that converts glucose uses the enzyme hexokinase. Therefore, hexokinase deficiency would lead to decreased 2,3 BPG.
With less 2,3 BPG there will be higher affinity for oxygen, and less delivery of oxygen to tissues.

Question 22

When aldolase converts fructose-1,6-BP to dihydroxyacetone-P and glyceraldehyde-3-P, what happens to the products and why is it important?

Answer 22

the direction of this reaction is reversible, and depends on metabolite concentrations. to keep the reaction moving forward, the products get pulled out of the cell, keeping the concentration low and making the reaction favorable.

Question 23

What keeps reaction 6 (glyceraldehyde-3-P to 1,3 BPG) moving in a forward direction?

Answer 23

a constant supply of oxidized NAD+ provides redox energy to move the reaction forward

Question 24

In reactions 7 and 10, why is the reaction able to produce ATP (even though ADP—>ATP is unfavorable)?

Answer 24

1,3 BPG and phosphoenolpyruvate have a lot of phosphoryl transfer energy so the dG is ««0 (more negative than ATP dephos.)

Therefore, dephosphorylation pushes the reaction forward and outweighs the positive dG of ATP formation

Question 25

What is special about the structure/function of phosphoglycerate kinase in reaction 7?

Answer 25

it has open and closed configurations via the induced fit mechanism which maximize the accessibility of the active site without sacrificing the hydrophobic environment

Question 26

Which reactions in glycolysis are dependent on metabolite concentrations?

Answer 26

2, 4, 5, 8, 9

Question 27

What happens when the body does not have lactase to break down lactose?

Answer 27

bacteria convert lactose to gas via fermentation

Question 28

What is a critical part of pyruvate metabolism that keeps the reaction going over and over again?

Answer 28

regenerating NAD+ for the GAP dehydrogenase reaction to maintain flux through the glycolytic pathway, either through aerobic or anaerobic conditions

Question 29

What are the “valves” in metabolic pathways?

Answer 29

rate-limiting enzymes that are stimulated or inhibited in response to cellular conditions

Question 30

How is glucokinase (hexokinase IV) different from hexokinase I?

Answer 30

hexokinase I:
- high affinity for sunstrate/low Km
- expressed in all tissues
- phosphorylates a variety of hexose sugars
- inhibited by the products of the reaction (glucose-6-P)

glucokinase (hexokinase IV):
- low affinity for substrate/high Km
- expressed primarily in liver and pancreatic cells
- highly specific for glucose
- not inhibited by glucose-6-P (does not turn off)

Question 31

What are the two roles of glucokinase?

Answer 31

liver:
when blood glucose is high, it differentially traps glucose inside liver cells

pancreatic B cells:
sequesters glucose in the pancreatic B cells initiating a signaling pathway that releases insulin into the blood (aka, a glucose sensor)

Question 32

Why is glucokinase an ideal glucose sensor capable of stimulating insulin releasE?

Answer 32

• high Km (only active when glucose is high)
• allosterically activated by glucose in pancreatic B cells

Question 33

What allosteric effectors increase and decrease the activity of phosphofructokinase-1 in converting fructose-6-P + ATP —> fructose-1,6-BP + ADP?

Why do each of them act in the way that they do?

Answer 33

positive effectors (increase activity):
AMP, ADP, and Fructose-2,6-BP
- stabilize the R state to promote production of fructose-1,6-BP
- F-2,6-BP is a super activator

negative effectors (decrease activity):
ATP, citrate
- high energy charge, don’t need more glucose
- citrate build up indicates that citrate cycle is terminated

Question 34

How can fructose, galactose, and glycerol enter the glycolytic pathway?

Answer 34

fructose - hexokinase converts to fructose-6-P

galactose - galactokinase phosphorylates galactose to galactose-1-P, which through a few more reactions enters pathway as glucose-6-P

glycerol - glycerol kinase and glycerol-3-P dehydrogenase form dihydroxyacetone-P in the pathway

Question 35

What happens to people with a fructose intolerance?

Answer 35

fructose is still converted to fructose-1-P by fructokinase (which uses an ATP)

the enzyme to convert fructose-1-P is defective but there is no other metabolic route, so fructose-1-P builds up

liver cells keep using ATP to convert fructose but it never reaches glycolysis, so the ATP is not reformed (toxic to liver cells)

Question 36

Intermediates of glycolysis are also used for…

Answer 36

amino acid synthesis, pentose phosphate pathway, and triacylglyceride synthesis
(also forming 2,3 BPG for oxygen delivery)

Question 37

What happens to pyruvate under aerobic conditions?

Answer 37

the citrate cycle produces 2 ATP, followed by oxidative phosphorylation which produces 28 ATP

end products are CO2 and H2O

Question 38

What happens to pyruvate under anaerobic conditions?

Answer 38

2 pathways:

fermentation -
- CO2 and ethanol produced, NAD+ regenerated

lactate dehydrogenase -
- lactate produced, NAD+ regenerated

Question 39

Why does NAD+ need to be regenerated during pyruvate metabolism?

Answer 39

NADH —-> NAD+ is critical for use in the glyceraldehyde-3P dehydrogenase reaction under anaerobic conditions (allows the reaction to keep going)

Question 40

What is LDHA?

Answer 40

Lactate dehydrogenase deficiency

• inability to utilize glycolysis to produce ATP needed for muscle contraction under anaerobic conditions
• NAD+ cannot be regenerated during conversion of pyruvate to lactate
• When NAD+ is not regenerated by LDH, glyceraldehyde-3P dehydrogenase is responsible for decreased flux

Question 41

How can mitochondria regenerate NAD+?

Answer 41

when pyruvate is oxidized in the mitochondria, mitochondrial shuttles regenerate NAD+ in the cytosol

Question 42

During the brewing process, when is most ethanol produced?

Answer 42

When yeast are incubated under anaerobic conditions