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Test Bank - Lehninger Principles of Biochemistry, 8th Edition (Nelson, 2022), Chapter 1-28 | All Chapters
TEST BANK FOR LEHNINGER PRINCIPLES OF BIOCHEMISTRY NELSON 6TH EDITION
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Amino Acid Degradation:
Remember : the breakdown of amino acids is the last fuel in the body – under starvation conditions
The pathways of amino acid catabolism, taken together, normally account for only 10% to 15% of the human body’s energy
production; these pathways are not nearly as active as glycolysis and fatty acid oxidation.
The 20 catabolic pathways converge to form only six major products, all of which enter the citric acid cycle (Fig. 18–15). From
here the carbon skeletons are diverted to gluconeogenesis or ketogenesis or are completely oxidized to CO2 and H2O.
The catabolism of the amino acids involves the removal of α-amino groups, followed by the breakdown of the resulting carbon
skeletons. These pathways converge to form seven intermediate products: oxaloacetate, pyruvate, α-ketoglutarate, fumarate,
succinyl coenzyme A (CoA), acetyl CoA, and acetoacetate. These products directly enter the pathways of intermediary
metabolism, resulting either in the synthesis of glucose or lipid or in the production of energy through their oxidation
to CO2 by the citric acid cycle.
Nonessential amino acids (Figure 20.2) can be synthesized in sufficient amounts from the intermediates of metabolism or, as in
the case of cysteine and tyrosine, from essential amino acids. In contrast, the essential amino acids cannot be synthesized (or
produced in sufficient amounts) by the body and, therefore, must be obtained from the diet in order for normal protein synthesis to
occur. Genetic defects in the pathways of amino acid metabolism can cause serious disease.
The Liver is the major source of N metabolism in the body.
In times of dietary surplus, the potentially toxic N of amino acids is eliminated via transaminations, deamination and urea
formation. Excess AA’s are not stored as AA or eliminated as AA, instead, excess AA’s are degraded by 20 different pathways.
These 20 pathways converge on 7 common intermediates that feed into the TCA cycle:
1. pyruvate,
2. α-ketogluterate,
3. succinyl CoA,
4. fumarate,
5. oxaloacetate (all glucogenic),
6. acetyl CoA and
7. acetoacetate (both ketogenic as neither can bring about net glucose production, only Lys and Leu are solely ketogenic).
What happens to the C skeletons? They are generally conserved as CHO via gluconeogenesis or as fatty acids via fatty acid
synthesis pathways. In this respect, amino acids fall into 3 categories: glucogenic, ketogenic or glucogenic and ketogenic.
All or part of the carbon skeletons of seven amino acids are ultimately broken down to acetyl-CoA. Five amino acids are
converted to _-ketoglutarate, four to succinyl-CoA, two to fumarate, and two to oxaloacetate. Parts or all of six amino acids are
converted to pyruvate, which can be converted to either acetyl-CoA or oxaloacetate.
A. Glucogenic amino acids
Amino acids whose catabolism yields pyruvate or one of the intermediates of the citric acid cycle are termed glucogenic. These
intermediates are substrates for gluconeogenesis (see p. 117) and, therefore, can give rise to the net formation of glucose in the
liver and kidney. The amino acids that are degraded to pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, and/or oxaloacetate
can be converted to glucose and glycogen by pathways described.
B. Ketogenic amino acids
Amino acids whose catabolism yields either aceto acetate or one of its precursors (acetyl CoA or aceto acetyl CoA) are termed
ketogenic (see Figure 20.2). Aceto acetate is one of the ketone bodies, which also include 3-hydroxybutyrate and acetone (see p.
195). Leucine and lysine are the only exclusively ketogenic amino acids found in proteins. Their carbon skeletons are not
substrates for gluconeogenes is and, therefore, cannot give rise to the net formation of glucose.
, PITTT-
both glucogenic and ketogenic Phe, Thr,
Trp, Tyr, Iso
KiLL-
ketogenic Leucine and Lysine.
Glucogenic-
The remainder!!
A third possible fate during times of
starvation is the breakdown of C skeleton
for energy via reduction. The RXN generates CO2 and H20. Catabolism of amino acids is particularly critical to the survival of
animals with high-protein diets or during starvation. Leucine is an exclusively ketogenic amino acid that is very common in
proteins. Its degradation makes a substantial contribution to ketosis under starvation
conditions.
Cofactors:
A variety of interesting chemical rearrangements occur in the catabolic pathways of amino acids. It is useful to begin our study of
these pathways by noting the classes of reactions that recur and introducing their enzyme cofactors. We have already considered
one important class: transamination reactions requiring pyridoxal phosphate. Another common type of reaction in amino acid
catabolism is one-carbon transfers, which usually involve one of three cofactors: biotin, tetrahydrofolate, or S-
adenosylmethionine.
These cofactors transfer one-carbon groups in different oxidation states:
1. biotin transfers carbon in its most oxidized state, CO2
2. tetrahydrofolate transfers one-carbon groups in intermediate oxidation states and sometimes as methyl groups; and
3. S-adenosylmethionine transfers methyl groups, the most reduced state of carbon.
The latter two cofactors are especially important in amino acid and nucleotide metabolism.
Some synthetic pathways require the addition of single carbon groups that exist in a variety of oxidation states, including formyl,
methenyl, methylene, and methyl. These single carbon groups can be transferred from carrier compounds such as THF and SAM
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