utritional Biochemistry İlker GÖÇHAN (M.D) Clinical Biochemistry Specialist

Protein metabolism WEEK 13

Protein Metabolism Protein metabolism is an essential part of metabolism. Since amino-acid metabolism is closely connected with the metabolism of other nitrogen compounds, protein metabolism is often included in the more general concept of nitrogen metabolism. In autotrophic organisms—that is, plants (except fungi) and chemo-synthesizing bacteria—protein metabolism begins with the assimilation of inorganic nitrogen and synthesis of amino acids and amides.

In man and animals, only a portion of the amino acids—the so-called nonessential ones—can be synthesized in the organism from simpler organic compounds. The other portion—the essential amino acids—must be obtained from food, usually as protein.

Proteins contained in various foods are broken down by cleavage under the action of such proteolytic enzymes as pepsin, trypsin, and chymotrypsin into amino acids, which are absorbed into the blood and carried to organs and tissues.

A considerable portion of amino acids are used in the formation and completion of various proteins in the body, including functionally active proteins (enzymes, hormones, antibodies, and so forth), plastic proteins, structural proteins, and others.

At the same time, the body’s proteins undergo constant breakdown and renewal, replenishing the reserve of free amino acids. The other portion of the amino acids is used in the formation of a number of low-molecular hormones, biologically active peptides, amines, pigments, and other substances necessary for the maintenance of life.

For example, the amino acid glycine is used to form purine bases, and aspartic acid is used to synthesize pyrimidine bases.

The mutual transformation of amino acids is, in significant measure, produced by a process that is widespread in all organisms—the enzyme process, involving the transfer of amino groups. This process, called transamination, was discovered by the Soviet scientists A. E. Braunshtein and M. G. Kritsman. Excess amino acids undergo enzyme processes of decomposition.

The most common initial reaction of amino-acid decomposition is deamination, primarily oxidative deamination, after which the nitrogen-free remainder of the amino-acid molecule degrades to the end products—carbon dioxide, water, and nitrogen that splits off in the form of ammonia.

The transformation and fate of food proteins from their ingestion to the elimination of their excretion products: Proteins are of exceptional importance to organisms because they are the chief constituents, aside from water, of all the soft tissue of the body.

Special proteins have unique roles as structural and functional elements of cells and tissues. Examples are keratin of skin, collagen of tendons, actin and myosin of muscle, the blood proteins, enzymes in all tissues, and protein hormones of the hypophysis.

Protein is digested to amino acids in the gastrointestinal tract. These are absorbed and distributed among the different tissues, where they form a series of amino acid pools that are kept equilibrated with each other through the medium of the circulating blood. The needs for protein synthesis of the different organs are supplied from these pools.

Excess amino acids in the tissue pools lose their nitrogen by a combination of transamination and deamination. The nitrogen is largely converted to urea and excreted in the urine. The residual carbon products are then further metabolized by pathways common to the other major foodstuffs—carbohydrates and fats.

Protein digestion occurs to a limited extent in the stomach and is completed in the duodenum of the small intestine. The main proteolytic enzyme of the stomach is pepsin, which is secreted in an inactive form, pepsinogen.

Its transformation to the active pepsin, initiated by the acidity of the gastric juice, involves liberation of a portion of the pepsinogen molecule as a peptide. Pepsin preferentially hydrolyzes peptide bonds containing an aromatic amino acid, and it requires an acid medium to function.

The acid chyme is discharged from the stomach, containing partially degraded proteins, into a slightly alkaline fluid in the small intestine. This fluid is composed of pancreatic juice and succus entericus, the intestinal secretion.

The pancreas secretes three known proteinases, trypsin, chymotrypsin, and carboxypeptidase. All three are secreted as inactive zymogens. Activation starts with the transformation of the inactive trypsinogen into the active trypsin. Trypsin, in turn, activates chymotrypsin and carboxypeptidase.

Trypsin and chymotrypsin are endopeptidases; that is, they cleave internal peptide bonds. The so-called peptidases are exopeptidases; they cleave terminal peptide bonds. Trypsin has a predilection for those containing the basic amino acid residues of lysine and arginine.

These two proteinases perform the major share in hydrolyzing proteins to small peptides. Digestion to amino acids is completed by the exopeptidases. Carboxypeptidase acts on peptides from the free carboxyl end; aminopeptidases from the free amino end. Other peptidases act on di- or tripeptides, or peptides containing such special amino acids as proline.

The amino acid digestion products of the proteins are absorbed by the small intestine as rapidly as they are liberated. The absorbed amino acids are carried by the portal blood system to the liver, from which they are distributed to the rest of the body.

Small amounts of the peptides formed during digestion escape further hydrolysis and may also enter the circulation from the intestine. This is shown by a rise in the peptide nitrogen in the blood.

PROTEIN IS A major component of foods. It is digested firstly in the stomach, and then in the duodenum to dipeptides and amino acid. Absorbed using symport active transport with sodium. Stored in liver and muscles.

Uses Protein synthesis : The synthesis of new proteins is very important during growth. In adults new protein synthesis is directed towards replacement of proteins as they are constantly turned over. Synthesis of a variety of other compounds :Examples of compounds synthesized from amino acids include purines and pyrimidines (components of nucleotides), catecholamines (adrenaline and noradrenalin) & neurotransmitters (serotonin)

Amino acid catabolism The other biological fuels discussed (carbohydrates & fats) contain only the elements carbon, hydrogen and oxygen. Amino acids contain nitrogen as well. The first step in amino acid catabolism is the removal of the nitrogen (the amino group).

Nitrogen removal from amino acids Aminotransferase PLP

Transamination it is a process of transferring amino groups from one molecule to another. There is no formation and no exceretion of ammonia, thusly no net change in the nitrogen amount of body.

It is a process involved in amino acids in which the amino group is transferred from the amino acid to a certain α-ketoacid with the consequant formation of a second α-ketoacid and amino acid. The reaction is catalyzed by the enzyme aminotranferase (aka transaminase) which requires pyridoxal phosphate as a prosthetic group.

All transaminases contain this prosthetic group which derives from pyridoxine a water soluble vitamin also known as vitamin B6. The amino group from amino acids is temporarily uptaken by the pyridoxal phosphate as pyridoxamine phosphate prior to its donation to an α-ketoacid. All aminoacids except lysine, threonine, proline and hydroxyproline participate in transamination process.

Deamination it is a process of removing amino groups from one molecule in order to reduce the amount of nitrogen of the body through ammonia synthesis and elimination. It is a process occurring in the liver during the metabolism of amino acids. The amino group is removed from the amino acid and converted to ammonia-NH3 whose toxic activity is canceled by conversion into urea which is eventually excreted.

The glutamate dehydrogenase-GDH enzyme occupies a central role in nitrogen metabolism. Glutamate amino acid is cleaved into α-ketoglutarate and ammonia a reaction catalyzed by GDH in a process called deamination.

Glutamate is the only amino acid that undergoes oxidative deamination at a relatively high rate. The formation of ammonia from the amino group thusly occurs mainly via the amino group of glutamate.

Once the amino groups have all been "collected“ in the form of the one amino acid, glutamate, this amino acid has its amino group removed (termed "oxidative deamination"). This reaction reforms alpha-ketoglutarate with the other product being ammonia (NH4 +).

Ammonia is toxic to the nervous system and its accumulation rapidly causes death. Therefore it must be detoxified to a form which can be readily removed from the body. Ammonia is converted to urea, which is water soluble and is readily excreted via the kidneys in urine.

Unlike glucose, there is no storage form of amino acids. Amino acids are degraded into free ammonia (NH4+) and the carbon skeleton. Living organisms excrete excess nitrogen as ammonia, uric acid, and urea.

Excretory forms of nitrogen

Excretory forms of nitrogen

Excretory forms of nitrogen

Excretory forms of nitrogen Excess NH4 + is excreted as ammonia (microbes, aquatic vertebrates or larvae of amphibia), b) Urea (many terrestrial vertebrates) c) or uric acid (birds and terrestrial reptiles)

Nitrogen removal from amino acids Step 1: Remove amino group Step 2: Take amino group to liver for nitrogen excretion Step 3: Entry into mitochondria Step 4: Prepare nitrogen to enter urea cycle Step 5: Urea cycle

The urea cycle takes place in the mitochondria and the cytosol. There are four enzymes involved, three of which are cytosolic and one is mitochondrial.

Sources Lippincott’s Illustrated Reviews: Biochemistry, 3rd edition.