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Chapter 18 Metabolic Pathways and
Energy Production
1
18.8
Degradation of Amino Acids
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Degradation of Amino Acids
2
Proteins provide
• energy when carbohydrate and lipid resources are not
available
• carbon atoms to be used in the citric acid cycle
• carbon atoms to be used in synthesis of fatty acids, ketone
bodies, and glucose
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Transamination
3
In transamination,
• amino acids are degraded in the liver
• an amino group is transferred from an amino acid to an keto acid, usually -ketoglutarate
• a new amino acid and -keto acid are formed
• when alanine combines with -ketoglutarate, pyruvate and
glutamate are produced
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
A Transamination Reaction
4
NH3+
|
CH3—CH—COO– +
Alanine
transaminase
O
||
–OOC—C—CH —CH —COO–
2
2
-Ketoglutarate
Alanine
O
||
CH3—C—COO–
Pyruvate
(new -ketoacid)
+
NH3+
|
–OOC—CH—CH —CH —COO–
2
2
Glutamate
(new amino acid)
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Oxidative Deamination
5
Oxidative deamination
• removes the ammonium group (−NH3+) from glutamate as
NH4+
• regenerates -ketoglutarate for transamination
NH3+
Glutamate
│
dehydrogenase
–OOC—CH—CH —CH —COO– + NAD+ + H O
2
2
2
Glutamate
O
||
–OOC—C—CH —CH —COO– + NH + + NADH
2
2
4
-Ketoglutarate
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Learning Check
6
Write the structures and names of the products for the
transamination of -ketoglutarate by aspartate.
NH3+
|
–OOC—CH—CH —COO–
2
Aspartate
+
O
||
–OOC—C—CH —CH —COO–
2
2
-Ketoglutarate
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Solution
7
Write the structures and names of the products for the
transamination of -ketoglutarate by aspartate.
O
||
–OOC—C—CH —COO–
2
Oxaloacetate
+
NH3+
|
–OOC—CH—CH —CH —COO–
2
2
Glutamate
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Urea Cycle
8
The urea cycle
• removes toxic ammonium ions from amino acid
degradation
• converts ammonium ions to urea in the liver
O
||
+
2NH4 + CO2
H2N—C—NH2
Ammonium ions
Urea
• produces 25–30 g of urea daily for urine formation in the
kidneys
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
ATP Energy from Amino Acids
9
Carbon skeletons of amino acids
• form intermediates of the citric acid cycle
• produce energy
• enter the citric acid cycle at different places depending on the
amino acid
Three-carbon skeletons:
alanine, serine, and cysteine
pyruvate
Four-carbon skeletons:
aspartate, asparagine
oxaloacetate
Five-carbon skeletons:
glutamine, glutamate, proline,
arginine, histidine
glutamate
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Intermediates of the Citric Acid Cycle
from Amino Acids
10
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Learning Check
11
Match each the intermediate with the amino acid that
provides its carbon skeleton: pyruvate, fumarate, or ketoglutarate.
A.
B.
C.
D.
cysteine
glutamine
aspartate
serine
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Solution
12
Match each the intermediate with the amino acid that
provides its carbon skeleton: pyruvate, fumarate, or ketoglutarate.
A.
B.
C.
D.
cysteine
glutamine
aspartate
serine
pyruvate
-ketoglutarate
fumarate
pyruvate
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Overview of Metabolism
13
In metabolism,
• catabolic pathways degrade large molecules
• anabolic pathways synthesize molecules
• branch points determine which compounds are degraded to
acetyl CoA to meet energy needs or converted to glycogen
for storage
• excess glucose is converted to body fat
• fatty acids and amino acids are used for energy when
carbohydrates are not available
• some amino acids are produced by transamination
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Overview of Metabolism
14
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Metabolic Pathways Concept Map
15
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.