Nitrogen Metabolism 1 Flashcards
Is there nitrogen storage in the body?
No
Amino acids in excess of biosynthetic needs are degraded
- amino group is removed and is converted to urea
- carbon skeletons of amino acids are metabolized
What is the amino acid pool?
This is a reserved stock of amino acids in the plasma and tissues
In a healthy, well-fed individual the input to the amino acid pool balances the output (nitrogen balance)
How big the amino ac8d pool?
About 100 g
What can be the amino acid pool be used for?
- Body protein (about 300-400 g a day) used in protein turn over. To other body protein (about 300-400 g a day)
- Dietary protein (about 70-100 g a day) used for synthesis of N-containing compounds
- Synthesis of nonessential amino acids (varies) followed by amino acids in excess of biosynthetic needs are degraded for energy, glucose, glycogen, fatty acids, ketone bodies
What is the rate of protein turnover?
The rate of protein turnover varies for different proteins. —> half life
- Short lived proteins, with half lives of minutes-hours include many regulatory and midfolded proteins.
- Long lived proteins, with half lives of days-weeks constitute majority of proteins in the cell
What is body protein turnover?
Simultaneous synthesis and degradation of protein molecules
Protein turnover about 300-40l g a day
What chemical signals are used in protein turnover?
Chemical signals for degradation depends on structural aspects and post-translational modifications
- PEST -specific amino acids in primary sequence leads to rapid degradation in proteosome
- Oxidation- randomly occurs
- ubiquitination -marks proteins fir degradation in proteosome
What is ubiquitin-proteosome degradation?
Ubiquitination of proteins is a post-translational modification that targets proteins for degradation
-Requires energy in the form of ATP
Outline the process of ubiquitin-proteosome degradation
- Ubiquitination of selected proteins
- Ub-chains tag proteins for destruction by proteosome
- Destroy the old or damaged protein
- Amino acid pool
What is the proteosome?
The proteosome is a large complex that degrades polyubiquitin-tagged proteins
What is ubiquit?
It is a protein released intact from proteosome
What is nitrogen balance ?
Defines balance between intake of N in the form of dietary protein and the loss of nitrogen mainly in the form of urea
- A healthy well-fed adult is in nitrogen balance as input = output
- Body protein remains constant as rate of protein synthesis is sufficient to replace protein degraded
What is positive nitrogen balance?
- Nitrogen intake exceeds nitrogen excretion
- Increased protein synthesis to support tissue growth and development
- occurs in growing kids, pregnant women and during recovery from trauma
Diets low in protein…
- Leads to deficiency of essential amino acids
- Results in a net breakdown of tissue protein
- Can lead to Kwashiorkor
Diets high in protein…
-No storage for amino acids, so excess is catabolized to ammonia (urea) and the carbon skeletons are converted to glucose or fat
What is negative nitrogen balance?
Nitrogen loss is greater than intake
- this puts metabolic stress to use tissue protein to increase amino acid pool
- inadequate dietary protein and lack of essential amino acids reduces the amount of amino acids in amino acid pool goes to tissue protein
What is the role of dietary protein?
- 70-100 g/day in the typical American diet
- Primary role: used as building blocks for biosynthesis. Provides essential amino acids
- Secondary role: used as an energy source
- About 20% of daily energy requirements derived from protein in typical American diey
- Catabolism of amino acids creates a loss of about 30-55 g/day —> RDA is 56 g/day for a 70kg man
What are the major energy sources of the diet?
Carbs- 50%
Fat- 30%
Protein -20%
What fats(lipids) are used for energy?
Triacylglycerol
Fatty acids, phospholipids and cholesterol
What carbs are used for energy?
Polysaccharides (starch)
- disaccharides (sucrose, lactose)
- Monosaccharides (glucose, fructose, galactose)
What are the energy contributions of each of the dietary components?
Carbs- 4 kcal/g
Protein - 4kcal/g
Fat- 9kcal/g
Alcohol- 7kcal/g
What are the major catabolic pathways of carbs?
- glycolysis
- Pyruvate dehydrogenase
- Tricarboxylic acid cycle
What are the major catabolic pathways of fats?
Fatty acid B-oxidation
What are the major catabolic pathways of proteins ?
Carbon skeletons of amino acids are common intermediates of major catabolic pathways
Explain the hydrolysis of proteins in digestion
Begins in the stomach
Stomach- pepsin (low pH)
Pancreas- trypsin, chymotrypsin, carboxypeptidase into duodenum. Also secretes bicarbonate (high pH)
Small intestine- aminopeptidases
Proteases activated from their zymogen form in stomach and small intestine
Explain the absorption of protein
In the small intestine
Free amino acids and dipeptides are absorbed by intestinal epithelial cells
- dipeptides are cleaved in the cytoplasm before entering portal system
- only free amino acids in portal vein, these are metabolized by liver or released into circulation
How is the nitrogen removed from amino acids?
Amino acids are blocked from further oxidation because of the alpha-amino group
- removal of amino group produces ammonia (toxic)
- if all ammonia is excreted in ueine it will need a large amount of water
- urea cycle converts this ammonia into urea
What is transamination?
Funneling alpha-amino groups to glutamates
- amino acid a-keto acid (carbonyl group)
- transfer of amino groups to a-ketoglutarate
- products are Glutamate and an a-keto acid (C-skeleton)
- reversible reaction catalyzed by aminotransferases (transaminases)
What is the function of glutamate in transamination?
Glutamate acts as a collector of nitrogen from most amino acids
-occurs in most tissues
Are aminotransferases specific?
Are specific for one or a few amino acids and are named after the amino group donor
What are the two important aminotransferases?
Two important amino transferases are ALT and AST
What is important about AST + ALT?
Aminotransferases that require pyridixal phosphate (vitamin B6)
All amino acids except lysine and threonine participate in transamination
Describe ALT
Alanine transferase
- also called glutamate pyruvate transaminase
- catalyzes transfer of amino group from alanine to a-ketoglutarate forming Pyruvate and glutamate
- readily reversible
- present in liver in highest concentrations
- glutamate acts as a collector of nitrogen from alanine
What is AST?
Aspartate aminotransferase
- also called glutamate-oxaloacetate
- exception to the rule that amino acids funnel amino group to glutamate
- readily reversible
- during catabolism primarily transfers amino group of glutamate to oxaloacetate forming Aspartate
Aspartate used as a source of nitrogen during the urea cycle
Explain the oxidative deamination of glutamate
- catalyzed by the mitochondrial enzyme glutamate dehydrogenase
- occur primarily in liver and kidneys
- Glutamate rapidly delaminated to liberate ammonia and a-ketoglutarate
Describe the equilibrium and direction Of oxidative deamination of glutamate
Reversible reaction
- direction of reaction depends on concentrations of products and substrates
- ammonia is converted to urea (some excreted in urine)
a-keto glutarate- oxidized in TCA cycle for energy or can be used for gluconeogenesis
The sequential action of transamination and oxidative deamination allows…
Release of amino groups as ammonia which is converted to urea via the urea cycle and excreted in the kidneys
What is the coenzyme of glutamate dehydrogenase?
For oxidative deamination (disposal of nitrogen) NAD+ is used as a coenzyme to maid NADH + NH3
For reductive amination (incorporation of ammonia)
What is D-Amino oxidase?
Allows for metabolism of D-amino acids to alpha-keto acids and eventual deamination to L-amino acids or oxidation via TCA
Requires vitamin B2 (riboflavin)
