Nitrogen Metabolism

Question 1

Describe how nitrogen enters and leaves the body.

Answer 1

• Nitrogen enters the body from
  dietary protein
• Dietary proteins are digested to
  amino acids
• Nitrogen leaves the body as
  urea, ammonia and other
  products derived from amino
  acid metabolism. because ammonium is highly toxic :(

Question 2

Explain the digestion of proteins, where it occurs, and the processes associated with it.

Answer 2

Proteins are too large to be absorbed by the intestine. They must be hydrolyzed to yield individual amino acids.

Digestion begins in the stomach. HCl kills some bacteria and denatures (unfolds) proteins so that they are more easily hydrolyzed.

Pepsin (an acid stable endopeptidase) is activated to pepsin by HCl and it releases peptides and amino acids from protein.

In the small intestine, polypeptides are further digested by pancreatic enzymes: (trypsin, chymotrypsin, elastase, and carboxypeptidase A/B). These enzymes are produced as inactive zymogens.

Trypsinogen is activated to trypsin by enteropeptidase, and trypsin then activates all the other proteases.

Question 3

What is the role of peptidases and proteases?

Answer 3

To break down proteins

Question 4

What is a zymogen?

Answer 4

an inactive substance which is converted into an enzyme when activated by another enzyme.

Question 5

What is protein turn over, and how is it controlled?

Answer 5

It is the recycling of amino acids in the body. The proteins in the body are constantly being synthesized and degraded, and turnover permits the removal of abnormal or unneeded proteins.

Protein turnover is determined by the rates of synthesis and degradation, and cells can therefore govern protein abundance by controlling these opposing cellular processes.

Question 6

What is the rate of protein turnover for the following proteins?

Regulatory proteins
Structural proteins
Majority of other proteins

Answer 6

Regulatory proteins - quick
Structural proteins - very long
Majority of other proteins - long

Question 7

What are the two steps of amino acid degradation?

Answer 7

1. Removal and disposal of nitrogen
2. The carbon skeleton is converted into metabolic intermediates

Question 8

Explain how the urea cycle safely disposes of the amino group.

Answer 8

Ammonia is transported to the liver, and is converted to urea via the urea cycle. Urea is the major disposal form of amino groups from amino acids. Once urea is formed in the liver it is transported to the kidneys to be excreted in the urine.

Question 9

Explain the process of amino acid degradation.

Answer 9

(catabolism)
1. removal of a-amino groups
2. breakdown of the resulting carbon skeletons

catabolism of carbon skeletons generates: oxaloacetate, pyruvate, fumarate, acetyl coA, and succinyl coA. These products enter into the synthesis of glucose or lipids and the production of energy

Question 10

What are glucogenic and ketogenic amino acids?

Answer 10

Glucogenic (able to make glucose) amino acids:
- through catabolism they yield a pyruvate or an intermediate of TCA cycle
- these intermediates are also substrates of gluconeogenesis

Ketogenic amino acids:
- their catabolism can make acetoacetate or one of its precursors (acetyl coA or acetoacetyl coA)
- These may be converted to ketone
bodies but are not substrates for
gluconeogenesis

Question 11

What are the ketogenic amino acids?

Answer 11

Leucine and Lysine

Question 12

What are the amino acids that are both ketogenic and glucogenic?

Answer 12

Tyrosine, isoleucine, phenylalanine, tryptophan

Question 13

What are the glucogenic amino acids?

Answer 13

Basically just remember what are the ketogenic ones and both, then everything else is glucogenic (easier this way)

Question 14

What are ketone bodies, where are they produced, and what are they used for?

Answer 14

They are made from acetyl coA or ketogenic amino acids

They are produced in the mitochondria of the liver, and used in the body everywhere (e.g, heart, kidney, muscle, fasting brain) EXCEPT for the liver!!! Liver lacks thiolase therefore cannot use ketone bodies.

They are an important energy source for peripheral tissues because:
- they are water soluble (no need for lipoproteins)
- When it has enough acetyl coA, the liver can change it into ketone bodies and ship it to the rest of the body to be used there
- ketone bodies can replace glucose; particularly important during long periods of fasting

Question 15

When are ketone bodies used?

Answer 15

They are a backup energy source when glucose levels are low.

During fasting, their production increases.
- due to increased breakdown of FAs in the liver leading to increased acetyl coA production

Ketone bodies are transported to the exrahepatic tissues in the blood, and taken up to generaye NADH and acetyl coA.

Question 16

Describe diabetic ketosis.

Answer 16

Diabetes = lack of insulin = lack of glucose uptake in cells (glucose accumulates in blood

Oxaloacetate consumed to make glucose in the liver, therefore TCA is slowed down.

Lack of insulin = more FA degradation (liver relies more heavily on fatty acid for its energy source since less citric acid cycle)

More fatty acid degradation = more acetyl coA which means increase in ketone body production in the liver.

Ketones are acidic, and over production leads to acidosis (denaturation and inactivation of proteins)

Question 17

Describe PKU deficiency, the characteristics, and treatment.

Answer 17

PKU is deficient, which means phenylalanine is not being converted to tyrosine, so it builds up in the tissues, blood and urine.

Due to lack of tyrosine (which is used in making melanin) there is hyperpigmentation to the skin (fair hair, light skin, blue eyes)

Phenylylacetate, phenylacetate and phenylpyruvate are elevated. Phenylpyruvate is excreted in the urine.

Also symptoms can be disabilities in learning, microcephaly, and seizures.

TREATMENT: diet low with phenyl alanine

Question 18

What happens in maple syrup urine disease?

Answer 18

Deficiency of enzyme branched-chain α-keto acid dehydrogenase enzyme involved in degradation of branched chain amino acids (Leu, Val, Ile)
* Rare inborn error (1:250,000)
* Elevated of branched-chain amino acids (ile, val and leu) and their α-keto acids in the blood and urine
* Severe metabolic acidosis results in maple syrup odor to the urine.
* Treatment: low protein diet and reduced levels of branched-chain amino acids

Question 19

What is albinism caused by? (a defect)

Answer 19

A defect in tyrosine metabolism, because this means a lack of production of melanin.

It is a partial or full absence of pigment from the skin, hair and eyes.

Question 20

What are three things that contribute to the amino acid pool and three things that take away from it?

Answer 20

Contribute to it are:
- degradation of body proteins
- digestion of dietary proteins
- synthesis of amino acids

Take away from it
- synthesis of body proteins
- degradation of amino acids
- synthesis of nitrogen containing compounds

Question 21

What are porphyrins and an example?

Answer 21

Porphyrins are N containing, cyclic compounds that readily bind to metal ions.

The most common metalloporphyrin in humans is heme. This is a prosthetic group found in hemoglobin, myoglobin and cytochromes. It is produced from glycine.