Protein Metabolism

• Protein and amino acid metabolism

Amino Acid Metabolism

• Amino acids are not stored in the body
• Amino acids are obtained through diet, synthesized de novo, or from normal protein degradation
• Excess amino acids are rapidly degraded
• Amino acid oxidation can provide energy in addition to glucose and fatty acids
• Amino acid oxidation as energy increases with excessive dietary protein or during starvation/disease

Nitrogen Metabolism

• Amino acid catabolism is part of the metabolism of nitrogen-containing molecules
• Nitrogen enters the body in food compounds, mainly amino acids in dietary protein
• Nitrogen leaves the body as urea, ammonia, and other products from amino acid metabolism
• The transformation of body proteins involves the amino acid pool and protein turnover

Amino Acid Pool

• The amino acid pool is supplied by body protein degradation, dietary protein (essential amino acids), and synthesis of nonessential amino acids
• The amino pool is depleted by body protein synthesis, consumption as a precursor of N-containing small molecules, conversion to glucose, glycogen, fatty acids, ketone bodies, CO2, and H2O

Nitrogen Balance

• In healthy, well-fed adults, the amount of amino acids in the pool is constant, that ingested N equals excreted N and that synthesis equals degradation
• Positive nitrogen balance occurs in babies, children, pregnancy, and individuals recovering from illness, where N ingested exceeds N excreted and synthesis exceeds degradation
• Negative nitrogen balance can occur with a lack of essential amino acids, fasting, starvation, malnutrition, and metabolic stress and is categorized by larger excretion than ingestion and catabolism greater than synthesis

Protein Turnover

• Proteins are constantly being synthesized and degraded, the total amount remains constant because synthesis replaces degraded proteins
• The rate of protein turnover varies among individuals

Protein Turnover: Degradation

• Protein degradation occurs via the ubiquitin-proteasome proteolytic pathway
• Proteins covalently attached to ubiquitin (a small, globular non-enzymic protein)
• The consecutive addition of ubiquitin moieties generates polyubiquitin
• Polyubiquitin is recognized by proteasomes
• Proteins unfold, deubiquitinate, cutting into fragments and eventually single amino acids, which then enter the pool

Protein Turnover: Signals

• Proteins chemically altered by oxidation/ubiquitin are preferred for degradation
• The half-life of a protein is influenced by the nature of the N-terminal residue
• Serine as N-terminal is long-lived with a half-life greater than 20 hours
• Aspartate as N-terminal has a half-life of 3 minutes
• PEST (proline, glutamate, serine, threonine)-rich sequence: rapidly degraded

Digestion of Dietary Proteins

• Most nitrogen in the diet is consumed as protein
• Proteins are too large to be absorbed by the intestine (except antibodies from breast milk) and proteins are metabolized to di- or tripeptides and amino acids
• Proteolytic enzymes are produced from different organs including the stomach, pancreas, and small intestine

Organ Specific Digestion

• The stomach secretes gastric juice containing HCl+ pepsinogen
• The pancreas secretes pancreatic enzymes and pancreatic secretion deficiencies can cause steatorrhea and undigested protein in feces
• The small intestine contains aminopeptidase (exopeptidase) in the luminal surface, cleaves N-terminal residues of oligopeptides to produce smaller peptides and free amino acids
• Celiac disease can cause immune-mediated damage to the small intestine in response to the ingestion of gluten found in barley, wheat, and rye

Absorption of Amino Acids and Small Peptides

• Free amino acids enter enterocytes through a sodium-linked secondary transport system of the apical membrane
• Di- and tripeptides are hydrolyzed in the cytosol to amino acids before entering the portal system by facilitated diffusion

Removal of Nitrogen from Amino Acids

• Alpha-amino groups keep amino acids from oxidative breakdown
• Removing the alpha-amino group is obligatory in amino acids that are catabolising
• Once removed, nitrogen can be incorporated into other compounds or excreted

Transamination

• Transamination transfers alpha-amino groups to alpha-ketoglutarate
• The reaction is catalyzed by aminotransferase, found in cytosol and mitochondria, liver, kidney, intestine, and muscle, and uses pyridoxal phosphate as a cofactor
• The product is alpha-ketoacid derived from the original amino acid and glutamate, which then undergoes oxidative deamination
• It is a reversible process involved in the degradation and synthesis of amino acids
• All amino acids except lysine and threonine participate at some point in their catabolism

Aminotransferase: Liver Disease

• Reactions are catalyzed during amino acid catabolism. A. Alanine aminotransferase (ALT). B. Aspartate aminotransferase (AST).
• Alanineaminotransferase (ALT)
• Aspartateaminotransferase (AST)
• Plasma AST and ALT are elevated in the case of liver disease
• In nonhepatic instances of disease, myocardial infarctions and muscle disorders may occur

Oxidative Deamination

• Oxidative deamination is the oxidative removal of the amino group from amino acids, resulting in ketoacids and ammonia
• Reactions occur in the liver and kidney, catalyzed by: Glutamate dehydrogenase (GD)
• The coenzyme is NAD+ or NADP+ and has allosteric regulators
• GTP: allosteric inhibitors of GD, ADP: allosteric activators of GD, when ATP decreases, amino acid degradation by GD increases

Transport of Ammonia

• The first mechanism involves most tissues, ammonia is used with glutamine synthetase to combine with glutamate to form glutamine, which is transported in the blood to the liver
• Once at the liver, glutamine is cleaved by glutaminase to produce glutamate and free NH3
• The second mechanism is for muscle
• It involves transamination of pyruvate to alanine
• Alanine is transported by blood to the Liver, then the pathway of gluconeogenesis is able to synthesize glucose (glucose-alanine cycle)

Urea Cycle

• Urea is the major form of amino group derived from amino acids, accounting for 90% of N-containing components of urine
• A urea molecule consists of 2 nitrogen atoms and 1 carbon and oxygen, and it is produced in the liver, then transported in the blood to the kidneys for excretion in the urine

Fates of Urea

• Urea diffuses from the liver and is transported in the blood to the kidneys-excreted in urine
• Urea can diffuse from the blood into the intestine, is cleaved to CO2 + NH3 by bacterial urease, and ammonia is then partly lost in the feces or reabsorbed into the blood
• Hyperammonaemia in kidney failure is characterized by elevated plasma urea levels, higher amounts of urea in the gut, and greater urease activity
• Treatment: Neomycin can cause decreased intestinal bacteria, and thus decreased ammonia

Fasting

• During fasting, the liver maintains blood glucose levels by utilizing amino acids from muscle protein that are a major carbon source for the production of glucose, or gluconeogenesis
• As amino acid carbons are converted to glucose, the nitrogens are converted to urea, and urinary excretion of urea is elevated as a result
• As fasting progresses, ketone bodies allow the brain to use ketone bodies and spare blood glucose
• Less muscle protein is cleaved to provide amino acids for gluconeogenesis, there is decreased production of glucose from amino acids is accompanied by decreased production of urea

Metabolism of Ammonia

• Ammonia levels must be kept low because hyperammonemia is toxic to the central nervous system
• There must be a metabolic mechanism by which nitrogen is moved from peripheral tissue to the liver and converted to urea, this occurs while maintaining the low levels of circulating ammonia

Sources of Ammonia

• Glutamine
• Bacterial action in intestine
• Amines
• Purine and pyrimidines

Additional Sources of Ammonia

• Ammonia is generated from glutamine by kidney/renal and intestinal glutaminase and glutamate dehydrogenase and it is excreted in urine as NH4+
• Bacterial urease in the lumen of the intestine
• Diet and hormones and neurotransmitter monoamines, from amine oxidase
• Catabolism of purines and pyrimidines is released when the amino group attached to the rings

Transport of Ammonia in Circulation

• Ammonia is constantly produced in tissues and is present at very low levels in blood
• Rapid removal of blood ammonia by the liver, and tissues including muscle, release amino acid nitrogen in form of glutamine or alanine, rather than free ammonia

Ammonia Transport Methods

• Urea is produced in the liver, is the disposal route for ammonia, and is created from liver→kidney→urine
• Glutamine is nontoxic, occurs primarily in muscle and liver, important for removal of ammonia in the brain, plasma concentrations are higher than other amino acids, is removed by liver and kidney, and deaminated by glutaminase,

Hyperammonemia

• Hyperammonemia is neurotoxic on the central nervous system
• Serum ammonia levels range from (5-35 M), elevated levels can cause tremors, slurring of speech, somnolence (sleepy, drowsy), vomiting, cerebral oedema, and blurring of vision, high concentrations can cause coma and death
• This occurs in acquired instances, such as liver disease (viral hepatitis) and hereditary conditions, such as ornithine transcarbamoylase deficiency (X-linked) and arginase deficiency
• Treatment includes the restriction of dietary protein and/or administration of phenylbutyrate
• Phenylbutyrate is converted to phenylacetate, which combines with glutamine to form phenylacetylglutamine, containing two atoms of nitrogen and causing excretion through urination.

Explores protein degradation, amino acid catabolism. Discusses digestion, absorption, and the urea cycle. Key enzymes and metabolic processes are highlighted.