Protein Background
Definition Of Proteolysis
How/Where Proteins
Breakdown
Enzyme Precursor
Proteolytic Enzymes
Protein To Energy
Pathway
Summary
Bibliography
Body proteins are broken down
when dietary supply of energy is
inadequate during illness or
prolonged starvation.
The proteins in the liver are
utilised in preference to those of
other tissues such as the brain.
The gluconeogenesis pathway is
present only in liver cells and in
certain kidney cells.
Disorders of amino acid metabolism
include phenylketonuria , albinism,
alkaptonuria, type 1 tyrosinaemia,
nonketotic hyperglycaemia,
histidinaemia, homocystinuria, and
maple syrup urine disease.
The hydrolytic breakdown of
proteins into simpler, soluble
substances such as peptides and
amino acids, by the action of
enzymes: occurs especially during
digestion.
Proteins in food are attacked in
the stomach by pepsin and in the
small intestine mainly by trypsin
and chymotrypsin from the
pancreas.
The proteolytic enzymes are
secreted as zymogens, which are
themselves converted by
proteolysis to their active forms.
Many other zymogens or
precursors undergo proteolysis to
form active enzymes or proteins
for example fibrinogen to fibrin.
The proteolytic enzymes have an
essential function in all cells.
In cells, proteolytic degradation
of old proteins is part of cellular
maintenance.
Their activities are regulated by
the rate of synthesis, activation of
proenzymes and by the rate of
synthesis of their inhibitors.
Digestion breaks protein down to amino
acids. If amino acids are in excess of the
body's biological requirements, they are
metabolised to glycogen or fat and
subsequently used for energy metabolism.
If amino acids are to be used for energy
their carbon skeletons are converted to
acetyl CoA, which enters the Krebs cycle
for oxidation, producing ATP.
The final products of protein catabolism
include carbon dioxide, water, ATP, urea,
and ammonia.
Protein is metabolised into amino
acids and peptides.
With normal anabolic hormone
activity, most of the proteins by
products are used for protein
synthesis, not for energy.
However, energy is required for
the synthesis process. Not all
amino acids can be used.
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