Proteolysis

By Carl Page

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.

Proteolysis is the

breakdown of proteins

or peptides into amino

acids.

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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