Module 9 - biochem of simple sugars and the glycolytic pathway Flashcards
Sucralose and aspartame are…
not sugars, but mimic the chemical structure
Why is aspartame so much sweeter than real sugars?
higher binding affinity, slower off rate
How does a glucose monitor detect blood glucose levels?
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
Glucose and fructose have the same chemical formula, so what is the difference?
glucose is a six membered ring, fructose is a five membered ring
glucose is a aldose and fructose is a ketose
When glucose and fructose are drawn in chair or boat formations, they…
pucker
Some monosaccharides have an —–.
How can you tell the difference between the L and D forms?
enantiomers
L - OH is on the left
R - OH is on the right
What is an epimer?
the position of one hydroxyl group is flipped
Monosaccharides are —- stable as cyclic structures than as linear structures. When drawn as cyclic, what is the difference between a and B structures?
more stable as cyclic
a: right hand OH is on the bottom
B: right hand OH is on the top
What is Benedict’s test?
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
What are the bonds for lactose, sucrose, and trehalose?
lactose:
Gal(B1–>4)Glc
sucrose:
Glc(a1–>B2)Fru
trehalose:
Glc(a1–>a1)Glc
Describe the structure and formation of maltose
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
Describe the structure and formation of sucrose
links a glucose to fructose through an
a1–>B2 glycosidic bond
formed by a condensation reaction (removal of water)
What is glycolysis?
primary pathway for ATP generation under anaerobic conditions
cleaves glucose into two molecules of pyruvate with a net yield of 2 ATP
What does glycolysis accomplish for the cell?
a small amounbt of ATP, and pyruvate which is a precursor to acetyl CoA, lactate, and ethanol in yeast
What is the overall net reaction of glycolysis?
Glucose + 2NAD+ + 2ADP + 2 Pi →
2 pyruvate + 2NADH + 2H+ + 2ATP + 2H2O
ΔGº’ = -35.5 kJ/mol
What are the key regulated enzymes in glycolysis?
hexokinase, phosphofructokinase-1, pyruvate kinase
When is glycolysis used in everyday chemistry?
intense exercise, making alcoholic beverages
At some point in glycolysis, everything is doubled (there is 2 of everything). What step does this split happen?
when aldolase breaks fructose-1,6-BP into 2 glyceraldehyde-3-P
Some steps of glycolysis have a positive dG’o but a negative dG. Why?
substrate concentration makes the reaction favorable
Why do some reactions in glycolysis require the investment of ATP? How does it affect the reaction?
ATP phosphoryl transfer is highly favorable, so linking it to other unfavorable reactions gives the overall reaction a net negative dG
How does hexokinase deficiency effect oxygen transport?
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.
When aldolase converts fructose-1,6-BP to dihydroxyacetone-P and glyceraldehyde-3-P, what happens to the products and why is it important?
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.
What keeps reaction 6 (glyceraldehyde-3-P to 1,3 BPG) moving in a forward direction?
a constant supply of oxidized NAD+ provides redox energy to move the reaction forward
In reactions 7 and 10, why is the reaction able to produce ATP (even though ADP—>ATP is unfavorable)?
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
What is special about the structure/function of phosphoglycerate kinase in reaction 7?
it has open and closed configurations via the induced fit mechanism which maximize the accessibility of the active site without sacrificing the hydrophobic environment
Which reactions in glycolysis are dependent on metabolite concentrations?
2, 4, 5, 8, 9
What happens when the body does not have lactase to break down lactose?
bacteria convert lactose to gas via fermentation
What is a critical part of pyruvate metabolism that keeps the reaction going over and over again?
regenerating NAD+ for the GAP dehydrogenase reaction to maintain flux through the glycolytic pathway, either through aerobic or anaerobic conditions
What are the “valves” in metabolic pathways?
rate-limiting enzymes that are stimulated or inhibited in response to cellular conditions
How is glucokinase (hexokinase IV) different from hexokinase I?
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)
What are the two roles of glucokinase?
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)
Why is glucokinase an ideal glucose sensor capable of stimulating insulin releasE?
- high Km (only active when glucose is high)
- allosterically activated by glucose in pancreatic B cells
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?
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
How can fructose, galactose, and glycerol enter the glycolytic pathway?
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
What happens to people with a fructose intolerance?
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)
Intermediates of glycolysis are also used for…
amino acid synthesis, pentose phosphate pathway, and triacylglyceride synthesis
(also forming 2,3 BPG for oxygen delivery)
What happens to pyruvate under aerobic conditions?
the citrate cycle produces 2 ATP, followed by oxidative phosphorylation which produces 28 ATP
end products are CO2 and H2O
What happens to pyruvate under anaerobic conditions?
2 pathways:
fermentation -
- CO2 and ethanol produced, NAD+ regenerated
lactate dehydrogenase -
- lactate produced, NAD+ regenerated
Why does NAD+ need to be regenerated during pyruvate metabolism?
NADH —-> NAD+ is critical for use in the glyceraldehyde-3P dehydrogenase reaction under anaerobic conditions (allows the reaction to keep going)
What is LDHA?
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
How can mitochondria regenerate NAD+?
when pyruvate is oxidized in the mitochondria, mitochondrial shuttles regenerate NAD+ in the cytosol
During the brewing process, when is most ethanol produced?
When yeast are incubated under anaerobic conditions
