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Chapter 21 Amino Acid Metabolism
Biochemistry/Lab (CHEM 3650)
Nova Southeastern University
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####### The components of living cells are constantly turning
####### over. Proteins have lifetimes that range from as short as
####### a few minutes to weeks or more. In any case, cells
####### continuously synthesize proteins from and degrade
####### them to amino acids. This seemingly wasteful process
####### has three functions:
####### 1. to store nutrients in the form of proteins and to break
####### them down in times of metabolic need, processes
####### that are most significant in muscle tissue.
####### 2. to eliminate abnormal proteins whose accumulation
####### would be harmful to the cell.
####### 3. to permit the regulation of cellular metabolism by
####### eliminating superfluous enzymes and regulatory
####### proteins.
Protein Degradation
A lysosome is a membrane-bound cell organelle found in animal cells. They are structurally and chemically spherical
vesicles containing hydrolitic enzymes, which are capable of breaking down virtually all kinds of biomolecules, including
proteins, nucleic acids, carbohydrates, lipids, and cellular debris. They are known to contain more than fifty different
enzymes which are all active at an acidic environment of about pH 5. Thus they act as waste disposal system of the cell by
digesting unwanted materials in the cytoplasm, both from outside of the cell and obsolete components inside the cell.
Lysosome
####### Free amino acids originate from the degradation of cellular proteins and from the digestion of
####### dietary proteins. The gastric protease pepsin, the pancreatic enzymes trypsin, chymotrypsin,
####### and elastase. degrade polypeptides to oligopeptides and amino acids. These substances are
####### absorbed by the intestinal mucosa and transported via the bloodstream to be absorbed by other
####### tissues. These enzymes are first made in inactive form. For example, pancreatic trypsinogen
####### and chymotrypsinogen are inactive form of trypsin and chymotrypsin. Synthesis of the
####### enzymes as inactive precursors and the presence of a protein called pancreatic trypsin
####### inhibitor protects the pancreas from proteolytic attack.
Amino Acid Deamination
Deamination
####### Excess dietary amino acids are not simply excreted but are
####### converted to common metabolites that are precursors of glucose,
####### fatty acids, and ketone bodies and are therefore metabolic fuels.
Most amino acids are deaminated by transamination, the transfer of their amino group to an α-keto acid to yield the α-keto acid of the original amino acid and a new amino acid. The predominant amino group acceptor is α-ketoglutarate, producing glutamate and the new α-keto acid: Glutamate's amino group, in turn, can be transferred to oxaloacetate in a second transamination reaction, yielding aspartate and reforming α-ketoglutarate:
Transamination
####### Transamination, of course, does not result in any net deamination. Glutamate,
####### however, can be oxidatively deaminated by glutamate dehydrogenase (GDH),
####### yielding ammonia and regenerating α-ketoglutarate for use in additional
####### transamination reactions.
####### Thus, the glutamate dehydrogenase reaction functions to eliminate amino groups
####### from amino acids that undergo transamination reactions with α-ketoglutarate.
Glutamate Can Be Oxidatively Deaminated
####### urea
Ammonia is quite toxic to animal tissues. The ammonia intoxication in humans is
characterized by an increase in the brain’s water content. This may have been caused
by the glutamine synthetase reaction making a high levels of glutamine, which acts
as an osmotically active solute.
If not reused for the synthesis of new amino acids or other nitrogenous products,
excess NH 4 + is excreted as ammonia (microbes and fish), urea (most terrestrial
vertebrates), or uric acid (birds and terrestrial reptiles). Urea is synthesized in the
liver by the enzymes of the urea cycle. It is then secreted into the bloodstream
and sequestered by the kidneys for excretion in the urine.
Notice that the carbon atoms of
urea and uric acid are highly
oxidized.
Ammonia & Less Toxic Waste Products
Chapter 21 Amino Acid Metabolism
Course: Biochemistry/Lab (CHEM 3650)
University: Nova Southeastern University
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