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CHAPTER 22
Metabolic Pathways for Carbohydrates
What is this chapter about?
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When you eat food, what happens to it?
The sum total of chemical reactions in the body to break down
or build molecules = metabolism
Metabolic pathway = reactions linked together in a series
This chapter introduces metabolism, then focuses on
carbohydrates. Other macromolecules will come in future
chapters.
A. Metabolism and Cell Structure
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Catabolic reactions: breaking down complex molecules
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Anabolic reactions: building up larger molecules from simple
ones
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Releases energy
Requires energy
And remember that we have two cell types:
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Prokaryotic: simple
Eukaryotic: more complex
Cell Structure for Metabolism
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Eukaryotic cells are more complex, having
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Nucleus
Organelles in the cytoplasm, such as
 Ribosomes
 Mitochondria
 Lysosomes
 Rough and smooth endoplasmic reticulum
 Golgi complex
Prokaryotes don’t have any of those – not even a nucleus.
B. ATP and Energy
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Energy obtained from the food we eat is used to form ATP
(adenosine triphosphate)
Hydrolysis of ATP = energy
ATP + H2O  ADP + Pi + 7.3 kcal/mole
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When we take in food, the reverse reaction occurs.
ATP is Coupled with Reactions
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There are certain reactions in our body that need to happen,
but require energy.
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Example: first step to break down glucose in the cell is to add a
phosphate group. But this requires 3.3 kcal/mol.
How do these reactions happen???
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Couple the endothermic reaction with the hydrolysis of ATP -- this will
drive the reaction requiring energy.
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Example on next page!
Example: Coupling ATP Hydrolysis with a
Reaction
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Glucose + Pi + 3.3 kcal/mol --> glucose-6-phosphate
ATP --> ADP + Pi + 7.3 kcal/mol
------------------------------------------------------------Glucose + ATP --> ADP + glucose-6-phosphate + 4.0 kcal/mol
C. Important Coenzymes in Metabolic Pathways
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Reminder: What is oxidation?
What is reduction?
NAD+ (nicotinamide adenine dinucleotide)
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Formed from a derivative of the vitamin niacin bonded to an
ADP
NAD+ gets reduced to NADH, generally participating in
reactions producing a carbon-oxygen double bond
FAD (flavin adenine dinucleotide)
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Derived from riboflavin plus an ADP
FAD gets reduced to FADH2 (2 nitrogens in the flavin accept
hydrogens)
Typically participates in reactions that produce a carboncarbon double bond
Coenzyme A (CoA)
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Made of several components…
Main function: activation of acyl groups, producing a thioester
D. Digestion of Carbohydrates
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Digestion: conversion of larger molecules into smaller ones that
the body can absorb
First step: chewing the food in the mouth. Enzyme in saliva -salivary amylase -- breaks the larger polysaccharides into
smaller units.
Disaccharides are broken into monosaccharides in the small
intestine.
Still in the small intestine, these monosaccharides are absorbed
through the intestinal wall into the bloodstream. The liver
converts fructose and galactose to glucose.
E. Glycolysis: Oxidation of Glucose
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There are 10 steps in glycolysis. You do not need to memorize
them for this class! Hooray!
But I will give you a general overview (there are three main
phases).
No oxygen is required for glycolysis (anaerobic).
For one glucose molecule through glycolysis, net result 2 ATP
and 2 NADH.
What happens in glycolysis?
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1.
2.
3.
2 ATP are invested
Glucose (a 6 carbon sugar) is split into 2 3 carbon pieces
(glucose is lysed… hence the name glycolysis!)
Energy payout -- 2 NADH and 4 ATP (which gives you the
net result of 2)
Regulation of Glycolysis?
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When there are large amounts of glucose-6-phosphate
(product of step 1) present, hexokinase (enzyme for 1st step) is
inhibited.
High cellular AMP/ADP levels indicate that much of the cellular
ATP has been used up.
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Conversely, when cellular ATP is plentiful, several of the glycolytic
enzymes (phosphofructokinase, pyruvate kinase) are inhibited by ATP
-- the cell does not need to make any more ATP.
When ATP levels drop, the enzymes are activated once again.
F. Pathways for Pyruvate
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Pyruvate = end product of glycolysis
What happens to pyruvate after glycolysis?
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Depends on if there’s oxygen present.
Aerobic conditions (oxygen): pyruvate oxidized (requiring NAD+),
CO2 removed, the remaining acetyl group attached to coenzyme A - called acetyl CoA
Anaerobic conditions (no oxygen): pyruvate reduced to lactate (in
muscle tissue), or reduced to ethanol via the fermentation process (in
microorganisms)
 Both of the anaerobic pathways recycle NADH back to NAD+
 Both of the anaerobic pathways produce a very small amount of ATP
So, let’s think.
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When you’re working out really hard, why might lactate
accumulate in the muscles?
Fermentation
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Conversion of pyruvate first to acetaldehyde, then to ethanol
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A CO2 is first removed from pyruvate (decarboxylation) then
acetaldehyde is reduced to ethanol with the help of NADH
This CO2 is the bubbles produced in beer and champagne.
Thus, this process is performed primarily by yeast.
G. Glycogen Metabolism
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Glycogen: long-term storage of glucose. Stored in skeletal
muscles and liver.
When the amount of glucose we consume exceeds our
immediate needs, the excess can be synthesized into glycogen
for later use.
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This is used when we have insufficient blood glucose
When our glycogen stores are full, any remaining blood
glucose is converted to triglycerides and stored as body fat.
Glycogenesis (glycogen synthesis)
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Glycogen contains glucose units linked by  1,4 and 1,6
glycosidic bonds.
When we eat polysaccharides, they are digested and provide
us with individual glucose units to synthesize glycogen.
Regarding the reactions themselves, know that the process
requires ATP.
Glycogenolysis
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The breakdown of glycogen -- provides us with glucose when
blood glucose is depleted
Many organs (red blood cells, brain, skeletal muscles) require
glucose to function properly
Regulation of Glucose Metabolism
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When blood glucose levels are low, the hormone glucagon is
released. This hormone accelerates the rate of glycogenolysis
and inhibits the synthesis of glycogen.
When blood glucose levels are high (after a meal), the
hormone insulin is released. This hormone accelerates
glycogen synthesis and glycogen degradation through
processes such as glycolysis. Insulin also inhibits glucose
synthesis.
H. Gluconeogenesis: Glucose Synthesis
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Not all of our glucose comes from glycogen stores -- some of it
comes from carbon/hydrogen/oxygen atoms available to the
cell.
Gluconeogenesis reactions are, for the most part, the reverse
of glycolysis (except for three out of the 10 steps).
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By not making it the exact reverse, we bypass three reactions that
require energy, making gluconeogenesis energetically favorable.
The Cori Cycle
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Glucose can also be made from lactate, a byproduct of
vigorous exercise.
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Lactate builds up in the muscles and is transported to the liver, where
gluconeogenesis occurs.
Meanwhile, glucose (being formed in the liver) is transported to the
muscle to rebuild glycogen stores.
The flow of lactate and glucose between the muscles and liver
is known as the Cori cycle.
Regulation of Gluconeogenesis
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If you eat a high carbohydrate diet, not much gluconeogenesis
occurring.
When you are eating a low carbohydrate diet, the
gluconeogenesis pathway is very active.
At any point, if the cell is in need of glucose, glycolysis is
turned off.