Proteins & Amino Acid Metabolism

Question 1

Why is creatinine useful as a clinical marker?

Answer 1

Breakdown product of creatine + creatinine phosphate in muscle = provides estimate of muscle mass, also indicator or renal function (higher = worse)

Question 2

Explain nitrogen balance

Answer 2

Amount entering (dietary protein 16g) = amount exiting (16g),

AA not incorporated into protein enter AA pool which can be used for protein synthesis or N-containing synthesis

Question 3

When does nitrogen balance change?

Answer 3

+N balance = preg: more in than out.

–N balance = malnutrition: less in then out

Question 4

Describe protein turnover

Answer 4

Free AA pool = syntheses proteins, last resort with no energy = liver utilises AA pool = broken to C skeleton + amino group

Question 5

How are amino acids catabolised?

Answer 5

AA broken to C skeleton + amino group (excreted as urea).

C skeleton is either:

1) glucogenic (gluconeogenesis)
2) ketogenic (ketone bodies)

Question 6

Give an example of a glucogenic AA

Answer 6

Valine

Question 7

Give an example of a ketogenic AA

Answer 7

Lysine

Question 8

Give an example of an AA that is both glucogenic and ketogenic

Answer 8

Phenylalanine

Question 9

Outline defects in amino acid metabolism

Answer 9

‘inborn errors of met’ = 50 inherited types, treatment = restricting specific AA in diets. Found via heel prick test.

Question 10

What is PKU?

Answer 10

Phenylketonuria = phenylalanine hydroxylase def

= accum phenylalanine + phenylketones (urine) = no prod of tyrosine,

affected pathways = NA, dopamine, melanin, thyroid hormone, protein synthesis

Question 11

Outline homocystinurias

Answer 11

Defect in cystathionine beta-synthase = prob breaking down methionine = excess homocysteine

= affects CT, muscle, CNS, CVS

Question 12

Define glucogenic amino acids

Answer 12

More carbons present = amino acid that can be converted into glucose through gluconeogenesis, and other intermediates of the TCA cycle

Question 13

Define ketogenic amino acids

Answer 13

When broken down there is only sufficient carbons to form Acetyl-CoA (pre-cursor of ketone bodies)

Question 14

How is nitrogen removed from AA?

Answer 14

Need to remove so the carbon skeleton can be used for energy prod:

Transamination = swap amine from AA with oxygen of keto group (requires α-ketoglutate = aminotransferase).

Deamination = removes amine as free ammonia (must be able to remove ammonia straight away)

Question 15

What are ALT and AST?

Answer 15

Alanine aminotransferase (ALT) = converts alanine to glutamate

Aspartate aminotransferase (AST) = Converts glutamate to aspartate

Question 16

What is the urea cycle?

Answer 16

Liver = allowing amino groups of AA to be disposed of via urea

by the input of CO2, aspartate, glutamate,

controlled by 5 enzymes

Question 17

What is the link between the urea cycle and refeeding syndrome?

Answer 17

Starved = down regulation of 5 urea cycle enzymes,

sudden feeding = urea cycle now unable to cope with amino groups = toxicity of ammonia

Question 18

Why is ammonia toxic to cell?

Answer 18

Readily diffusible and toxic to the brain = effects AA transport, pH, neurotransmitters, TCA cycle

Question 19

How is ammonia metabolised

Answer 19

Glutamate + ammonia = Glutamine = excreted in urea cycle.

Pyruvate + ammonia = alanine = broken back down, pyruvate for energy, ammonia to urea cycle

Question 20

From which AA is adrenaline synthesised from?

Answer 20

Tyrosine

Question 21

Which keto acid is used by aminotransferase enzymes to funel the amino group of other AA to glutamate?

Answer 21

Alpha-ketoglutarate

Question 22

Which aminotransferase enzymes are routinely measured as part of LFTs?

Answer 22

ALT = alanine aminotransferase

AST = aspartate amino transferase

Question 23

Which AA transports ammonia from peripheral tissue to the liver?

Answer 23

Alanine

Question 24

What is the diff between transamination and deamination?

Answer 24

Transamination = swap amine from AA with oxygen of keto group (requires α-ketoglutate).

Deamination = removes amine as free ammonia (must be able to remove ammonia straight away)

Question 25

Why do we need to remove N from AA?

Answer 25

Remove as amino group

To allow C skelton to be used in oxidatve metabolism

Question 26

What is the purpose of glutamine?

Answer 26

Glutamate + ammonia = glutamine

1) Glutamine by blood to liver/kidneys
2) glutamine cleaved by glutaminase
3) ammonia into urea cycle