Chapter 4
Carbohydrate Metabolism
1
ATP and Energy
Adenosine triphosphate (ATP)
• Is the energy form stored in cells.
• Is obtained from the oxidation of food.
• Consists of adenine (nitrogen base), a ribose
sugar, and three phosphate groups.
• Requires (31 kJ) per mole to convert ADP + Pi
to ATP.
2
ATP and Energy
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
3
Hydrolysis of ATP
• The hydrolysis of ATP to ADP releases 7.3 kcal (31
kJ/mole).
ATP
ADP + Pi + 7.3 kcal (31 kJ/mole)
• The hydrolysis of ADP to AMP releases 7.3 kcal (31
kJ/mole).
ADP
AMP + Pi + 7.3 kcal (31 kJ/mole)
4
Learning Check
Match the following:
1) ATP
2) ADP + Pi
A.
B.
C.
D.
Used in anabolic reactions.
The energy-storage molecule.
Coupled with energy-requiring reactions.
Hydrolysis products.
5
Solution
Match the following:
1) ATP
2) ADP + Pi
A.
B.
C.
D.
1
1
1
2
Used in anabolic reactions
The energy-storage molecule.
Coupled with energy-requiring reactions.
Hydrolysis products.
6
Carbohydrate Metabolism
and
Regulation of glucose
7
Classes of carbohydrates
Stage 1: Digestion of Carbohydrates
In Stage 1, the digestion of carbohydrates
• Begins in the mouth where salivary amylase breaks down
polysaccharides to smaller polysaccharides (dextrins), maltose, and
some glucose.
• Continues in the small intestine where pancreatic amylase hydrolyzes
dextrins to maltose and glucose.
• Hydrolyzes maltose, lactose, and sucrose to monosaccharides, mostly
glucose, which enter the bloodstream for transport to the cells.
9
Digestion of Carbohydrates
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
10
Glycemic index
• Is defined as the increase in blood glucose level over the baseline
level during a 2-hour period following the consumption of a defined
amount of carbohydrate (usually 50 g) compared with the same
amount of carbohydrate in a reference food
• white bread and glucose are mostly used as a reference food
• With glucose as the reference food white bread has a glycemic index
of about 71.
• The use of white bread as the standard causes some foods to have a
glycemic index of greater than 100.
Glycemic load (GL)
• equals the glycemic index times the grams of
carbohydrate in a serving of the food.
• The higher the GL, the greater the expected elevation in
blood glucose
• Long-term consumption of a diet with a relatively high
GL is associated with an increased risk of type 2
diabetes and coronary heart disease
Metabolic Pathways for Carbohydrates
• Glycogenesis - the making of glycogen
• glycogenolysis - the breakdown of glycogen
• Glycolysis - the oxidation of glucose
• Hexose monophosphate shunt - the production of 5-carbon
monosaccharides and NADPH
• Tricarboxylic acid cycle (TCA cycle) - oxidation of pyruvate
and acetyl CoA. AKA ‘the Krebs cycle’ or the citric acid cycle,
• gluconeogenesis - the making of glucose from
noncarbohydrate precursors
Integrated overview of carbohydrate metabolic pathway
Glycolysis: Oxidation of Glucose
17
Overview
• Come from two Greek workds
• 'glykos' means sweet, and 'lysis' means splitting
• So glycolysis is splitting/breaking of sweets/sugar
• Is a central pathway for glucose catabolism
• Happens in the cytoplasm of cell
….Overview con
• Special transporter to allow glucose to inter the cell (GLUT)
• Is a sequence of 10 enzyme catalyzed reactions
• Converts glucose to pyruvate
• 1 mol of glucose is partially oxidized to 2 mol of pyruvate
• Has two phase
• Preparatory phase (the 1st 5 steps)/ investment phase
• Pay-off phase
Phase 1/ preparatory phase
Phase 2/pay off phase
Reaction 1: phosphorylation
pg 526
Reaction 2: isomerization
aldose
ketose
Reaction 3: phosphorylation
Reaction 4: cleavage
Reaction 5: isomerization
Reaction 6: oxidation
Reaction 7: substrate level phosphorylation
Reaction 8: shift of phosphoryl group
Reaction 9: dehydration
Reaction 10: substrate level phosphorylation
Summary
Energy
investment
Cleavage
Energy
Harvest
https://www.youtube.com/watch?v=EfGlznwfu9U
Important steps in glycolysis
• Important steps in glycolysis are those that regulate the pathway
• Two types of regulation
• Allosteric
• Hormonal
• Three important steps
• The making of glucose 6 phosphate
• F6P F1,6BP
• PEP pyruvate
The making of G6P
• the first committed step
• Glucose stimulate Hexokinase
• Glucose 6 phosphate inhibit Hexokinase (feedback‐inhibition)
• The work of Hexokinase is hormonally regulated by insulin and
glucagon
Fructose 6 phosphate  fructose1,6 bisphosphate
• The most regulating reaction of the pathway
• The slowest reaction in the pathway
• PFK1 is inhibited by high levels of ATP, low PH, high levels of citrate
• PFK1 is stimulated by high levels of AMP and F2,6BP
• PFK1 is hormonally regulated by insulin and glucagon
Phosphoenol pyruvate  pyruvate
• The third regulatory step in the pathway
• High amount of F1,6BP stimulate pyruvate kinase
(feed‐forward activation)
• High amount of Acetyl-CoA inhibit pyruvate kinase
• High amount of Alanin and ATP inhibit pyruvate kinase
Energy from glycolysis
• Glycolysis: Glucose in 2 Pyruvate
• 2 ATP
• 2 NADH