Nitrogen Metabolism 1

Question 1

Is there nitrogen storage in the body?

Answer 1

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

Question 2

What is the amino acid pool?

Answer 2

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)

Question 3

How big the amino ac8d pool?

Answer 3

About 100 g

Question 4

What can be the amino acid pool be used for?

Answer 4

1. Body protein (about 300-400 g a day) used in protein turn over. To other body protein (about 300-400 g a day)
2. Dietary protein (about 70-100 g a day) used for synthesis of N-containing compounds
3. 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

Question 5

What is the rate of protein turnover?

Answer 5

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

Question 6

What is body protein turnover?

Answer 6

Simultaneous synthesis and degradation of protein molecules

Protein turnover about 300-40l g a day

Question 7

What chemical signals are used in protein turnover?

Answer 7

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

Question 8

What is ubiquitin-proteosome degradation?

Answer 8

Ubiquitination of proteins is a post-translational modification that targets proteins for degradation

-Requires energy in the form of ATP

Question 9

Outline the process of ubiquitin-proteosome degradation

Answer 9

1. Ubiquitination of selected proteins
2. Ub-chains tag proteins for destruction by proteosome
3. Destroy the old or damaged protein
4. Amino acid pool

Question 10

What is the proteosome?

Answer 10

The proteosome is a large complex that degrades polyubiquitin-tagged proteins

Question 11

What is ubiquit?

Answer 11

It is a protein released intact from proteosome

Question 12

What is nitrogen balance ?

Answer 12

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

Question 13

What is positive nitrogen balance?

Answer 13

• 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

Question 14

Diets low in protein…

Answer 14

• Leads to deficiency of essential amino acids
• Results in a net breakdown of tissue protein
• Can lead to Kwashiorkor

Question 15

Diets high in protein…

Answer 15

-No storage for amino acids, so excess is catabolized to ammonia (urea) and the carbon skeletons are converted to glucose or fat

Question 16

What is negative nitrogen balance?

Answer 16

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

Question 17

What is the role of dietary protein?

Answer 17

1. 70-100 g/day in the typical American diet
2. Primary role: used as building blocks for biosynthesis. Provides essential amino acids
3. Secondary role: used as an energy source
4. About 20% of daily energy requirements derived from protein in typical American diey
5. Catabolism of amino acids creates a loss of about 30-55 g/day —> RDA is 56 g/day for a 70kg man

Question 18

What are the major energy sources of the diet?

Answer 18

Carbs- 50%

Fat- 30%

Protein -20%

Question 19

What fats(lipids) are used for energy?

Answer 19

Triacylglycerol

Fatty acids, phospholipids and cholesterol

Question 20

What carbs are used for energy?

Answer 20

Polysaccharides (starch)

• disaccharides (sucrose, lactose)
• Monosaccharides (glucose, fructose, galactose)

Question 21

What are the energy contributions of each of the dietary components?

Answer 21

Carbs- 4 kcal/g

Protein - 4kcal/g

Fat- 9kcal/g

Alcohol- 7kcal/g

Question 22

What are the major catabolic pathways of carbs?

Answer 22

• glycolysis
• Pyruvate dehydrogenase
• Tricarboxylic acid cycle

Question 23

What are the major catabolic pathways of fats?

Answer 23

Fatty acid B-oxidation

Question 24

What are the major catabolic pathways of proteins ?

Answer 24

Carbon skeletons of amino acids are common intermediates of major catabolic pathways

Question 25

Explain the hydrolysis of proteins in digestion

Answer 25

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

Question 26

Explain the absorption of protein

Answer 26

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

Question 27

How is the nitrogen removed from amino acids?

Answer 27

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

Question 28

What is transamination?

Answer 28

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)

Question 29

What is the function of glutamate in transamination?

Answer 29

Glutamate acts as a collector of nitrogen from most amino acids

-occurs in most tissues

Question 30

Are aminotransferases specific?

Answer 30

Are specific for one or a few amino acids and are named after the amino group donor

Question 31

What are the two important aminotransferases?

Answer 31

Two important amino transferases are ALT and AST

Question 32

What is important about AST + ALT?

Answer 32

Aminotransferases that require pyridixal phosphate (vitamin B6)

All amino acids except lysine and threonine participate in transamination

Question 33

Describe ALT

Answer 33

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

Question 34

What is AST?

Answer 34

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

Question 35

Explain the oxidative deamination of glutamate

Answer 35

• catalyzed by the mitochondrial enzyme glutamate dehydrogenase
• occur primarily in liver and kidneys
• Glutamate rapidly delaminated to liberate ammonia and a-ketoglutarate

Question 36

Describe the equilibrium and direction Of oxidative deamination of glutamate

Answer 36

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

Question 37

The sequential action of transamination and oxidative deamination allows…

Answer 37

Release of amino groups as ammonia which is converted to urea via the urea cycle and excreted in the kidneys

Question 38

What is the coenzyme of glutamate dehydrogenase?

Answer 38

For oxidative deamination (disposal of nitrogen) NAD+ is used as a coenzyme to maid NADH + NH3

For reductive amination (incorporation of ammonia)

Question 39

What is D-Amino oxidase?

Answer 39

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)