Protein And Amino Acid Metabolism

Question 1

What is the breakdown product of creatine and creatine phosphate? In which tissue are they broken down?

Answer 1

Broken down into creatinine in the muscle tissue. Rate of breakdown is dependant on muscle mass (and muscle wastage)
- creatinine urine excretion over 24hrs is proportional to muscle mass

Question 2

What are creatinine levels an indicator of?

Answer 2

Renal function - raised plasma and low urine level upon damage to kidney nephrons

Question 3

What is a positive N balance?

Answer 3

Nitrogen intake > nitrogen output

Increase in total body protein - normal state in growth, pregnancy or in adult recovering from malnutrition

Question 4

What is a negative N balance?

Answer 4

Nitrogen intake < nitrogen output

Net loss of body protein - never normal - trauma, infection or malnutrition

Question 5

Give an example of a glucogenic amino acid

Answer 5

Alanine

Question 6

Give an example of a ketogenic amino acid

Answer 6

Lysine

Question 7

Give an example of an amino acid which is both glucogenic and ketogenic

Answer 7

Threonine

Question 8

What 2 hormones increase protein synthesis and decrease protein degradation?

Answer 8

Insulin and Growth hormone

Question 9

What group of hormones decrease protein synthesis and increase protein degradation?

Answer 9

Glucocorticoids e.g. cortisol

Question 10

What are the 9 essential amino acids

Answer 10

Isoleucine, lysine, threonine, histidine, leucine, methionine, phenylalanine, tryptophan, valine

Question 11

Why are animal proteins considered of higher quality than plant proteins?

Answer 11

Animal proteins contain all essential amino acids, most plant proteins are deficient in one or more essential amino acids

Question 12

Which amino acid contains a sulfhydride group necessary to form disulphide bonds?

Answer 12

Cysteine

Question 13

What are the 2 pathways that facilitate removal of nitrogen from amino acids?

Answer 13

Transamination and Deamination

Question 14

Describe the mechanism of Transamination

Answer 14

The amine group of the amino acid is transferred to a keto acid via an aminotransferase (all aminotransferases require coenzyme pyridoxal phosphate - a vitamin B6 derivative)

Question 15

What is the function of ALT?

Answer 15

Catalyses interconversion of alanine and alphaketoglutarate to pyruvate and glutamate

Question 16

What is the function of AST?

Answer 16

Catalyses interconversion of aspartame and alphaketoglutarate to oxaloacetate and glutamate

Question 17

What are ALT and AST levels (in a blood test) an indicator of?

Answer 17

Liver function - high levels found in conditions that cause extensive cellular necrosis.
E.g. viral hepatitis, autoimmune liver disease, toxic injury

Question 18

What is deamination? Where does most deamination happen?

Answer 18

Deamination is the liberation of the amino group (of an amino acid) as free ammonia.
Most of it happens in the live and kidney

Question 19

Why are ammonia and ammonium ions converted to urea?

Answer 19

Ammonia and NH4+ are very toxic. Urea is inert and non-toxic.
Urea is mostly excreted in urine via kidneys and performs a useful osmotic role in kidney tubes

Question 20

Why can a malnourished person not be given a normal diet immediately?

Answer 20

Can result in refeeding syndrome

Question 21

What happens in genetic disorders caused by deficiency of one of the enzymes in the urea cycle?

Answer 21

Hyperammonaemia and accumulation/excretion of urea cycle intermediates

Requires a low protein diet.
Symptoms include vomiting, lethargy, irritability, mental retardation, seizures, coma

Question 22

Why is ammonia so toxic?

Answer 22

It is readily diffusible and very toxic to the brain. It interferes with amino acid transport and protein synthesis, disruption of cerebral blood flow, alkalinic pH effect, interference with metabolism of excitatory amino acid neurotransmitters, alteration of the blood-brain barrier, interference with the TCA cycle (reacts with alphaketoglutarate to form glutamate)

Question 23

What are the effects of a deficiency in phenylalanine hydroxylase? What is the treatment?

Answer 23

Causes phenylketonuria - accumulation of phenylalanine in tissue, plasma and urine. Phenylketones in urine
Treatment is a low phenylalanine diet enriched with tyrosine, avoiding artificial sweeteners, avoiding high protein foods e.g. meat, milk, eggs

Question 24

What are the symptoms of phenylketonuria (PKU)?

Answer 24

If not detected early, can result in: severe intellectual disability, developmental delay, microcephaly, seizures, hypopigmentation

Question 25

What happens in homocystinurea?

Answer 25

Problem breaking down methionine -> excess homocysteine.
Due to a defect in cystathionine beta-synthase (most commonly)

Question 26

Name 3 diseases tested for with the heel prick test (on newborn infants)

Answer 26

Sickle cell disease; Cystic fibrosis; Congenital hypothyroidism; Phenylketonuria (PKU); Maple syrup urine disease; Isovaleric acidaemia (IVA); Glutaric acid Urania; Homocystinuria