Lecture 2 - Protein and Amino Acid Metabolism

Question 1

explain nitrogen balance

do not need numerical values

Answer 1

amino acids, proteins, prurines and pymramidines, creatine phosphate all contain nitrogen

in a 70kg male we have 2kg of N in proteins, 16g in amino acids and 60g in N containing compounds

we take in 16g of N every day

loose 14g of N through urine, faeces
2g lost to hair and nails

this is N equilibrium

Positive N balance - we are taking in more N than we are loosing - normal in growth, pregnancy ect

Negative N balance - loosing more N than we take in

Net loss of body protein - not normal - from trauma, infection or malnutrition

Question 2

explain protein turnover

Answer 2

we digest proteins into amino acids
some amino acids we also synthesise

these are free amino acids

they are used to build cellular proteins like muscle
or broken down into amino acids via proteolysis

OR free amino acids enter the liver

amino groups could convert to ammonia (very toxic) so we convert to urea and expel in the urine

the carbon skeleton is used in gluconeogenisis (glucogenic aa’s) or ketogenisis (ketogenic aa’s) - these can be used for energy

Question 3

explain creatinine as a clinical marker

Answer 3

creatine and creatine phosphate breakdown into creatinine

this is produced at a constant rate unless we are wasting away

filtered via the kidneys into urine - provides an estimate of muscle mass
and indicator of renal function

Question 4

what is the role of creatine phosphate

Answer 4

will store Phospahte in muscles

convert to creating to make an ATP

rapid source of ATP

Question 5

give an example each of the types of amino acids

Answer 5

glucogenic - glycine - used to synthesise things like pyrivate, can be used for gluconeogenisis - glucose synthesis

Ketogenic - lysine - feeds in at acetyl CoA - can be use to synthesise ketone bodies

Both glucogenic and ketogenic - tyrosine

they will all feed into somewhere in TCA cycle

Question 6

Mobilisation of protein reserves - when and why

Answer 6

• Occurs during extreme stress (starvation)
• Under hormonal control -

insulin and growth hormone will increase protein synthesis and decrease protein degradation

glucocorticoids - cortisol - will decrease protein synthesis and increase protein degradation

Question 7

what is Cushing’s syndrome

Answer 7

Excessive breakdown of protein can occur in Cushing’s syndrome (excess cortisol). Weakens skin structure leading to striae formation.

Question 8

give two uses of specific amino acids outside of protein synthesis

Answer 8

Histidine to make Histamine

Tyrosine to make
• Catecholamines
• Thyroid hormones

Question 9

why do we want to remove the nitrogen group on amino acids

what are the two methods for doing this ?

Answer 9

• Removal of amino group is essential to allow carbon
skeleton of amino acids to be utilised in oxidative
metabolism

• Once removed nitrogen can be incorporated into
other compounds or excreted from body as urea

• Two main pathways facilitate removal of nitrogen from
amino acids

• Transamination
• Deamination

Question 10

what is transamination ?

Answer 10

N containing NH2 group is transferd to a ketone, to create ie glutamate

• Most aminotransferase enzymes use α-ketoglutarate to funnel the amino group to glutamate.

Not vital • Exception to rule is aspartate aminotransferase which uses oxaloacetate to funnel amino group to aspartate

• All aminotransferases require the coenzyme pyridoxal phosphate which is a derivative of vitamin B6

Question 11

Key aminotransferase enzymes you need to know

Answer 11

Alanine aminotransferase (ALT)
Converts alanine to glutamate

Aspartate aminotransferase (AST)
Converts glutamate to aspartate

Plasma ALT and AST levels measured routinely as part of liver function test

Levels particularly high in conditions that cause extensive cellular necrosis such as:
• Viral hepatitis
• Autoimmune Liver Diseases
• Toxic injury

Question 12

what is Deamination ?

Answer 12

• Liberates amino group as free ammonia
• Mainly occurs in liver & kidney
• Keto acids can be utilised for energy

• Also important in deamination of dietary D-amino
acids (found in plants and microorganisms)

• Ammonia (and ammonium ions) very toxic and
must be removed. Ultimately converted to urea or
excreted directly in urine

an example enzyme is glutaminase, glutamine dehydrogenase

Question 13

what is urea

Answer 13

it is a nitrogen containing compound that is non toxic, and water soluble - so can be excreted in urine

Question 14

Summarise the urea cycle

Answer 14

Occurs in liver and involves 5 enzymes
• Amount of urea cycle enzymes normally related to need to dispose of ammonia
• High protein diet induces enzyme levels
• Low protein diet or starvation represses levels
• Cycle is inducible but not regulated

it is for the safe disposal of amino acids N via conversion into urea, for urine excretion

Question 15

what is refeeding syndrome ?

Answer 15

Can occur when nutritional support given to severely malnourished patients

• Ammonia toxicity significant factor (urea
cycle down regulated) - urea cycle is down regulated so , we cant safely remove nitrogen, ammonia will build up to toxic levels if too much protein rich food is eaten quickly
• Re-feed @ 5 to 10 kcal/kg/day. Raise
gradually to full needs within a week

Question 16

summarise defects in the urea cycle and what this can cause

what are some symptoms , and what is the solution ?

Answer 16

autosomal reccesive disorders caused by deficiency of enzymes within the cycle

leads to

hyperammoaemia - ammonia toxicity

accumulation and excretion of intermediates within the cycle

Clinical picture (NH3 toxicity)
• Severity depends on:
• nature of defect
• amount of protein eaten
• Severe urea cycle disorders show symptoms within 1 day after birth. If untreated, child will die.
• Mild urea cycle enzyme deficiencies may not show
symptoms until early childhood

Management:
• Low protein diet
• Replace amino acids in diet with keto acids

Symptoms 
• Vomiting
• Lethargy
• Irritability
• Mental retardation
• Seizures
• Coma

Question 17

biochemically , why is ammonia considered toxic ?

Answer 17

it is readily diffusable, and very toxic to the brain

levels need to be kept low : 25-40 umol/L

may be becuase - 
PH effects
Alters blood brain barrier
interferes with amino acids transport and protein synthesis
interferes with TCA cycle

Question 18

how is ammonia removed from the body ?

have a go at drawing the diagram - lec 2, page 24

Answer 18

Two mechanisms are utilised for the safe transport of amino acid nitrogen from tissues to the liver for disposal

Glutamine
• Ammonia combined with glutamate to form
glutamine
• Glutamine transported in blood to liver or
kidneys where it is cleaved by glutaminase to
reform glutamate and ammonia.

In liver ammonia fed into urea cycle. In kidney excreted directly in urine

Alanine
• Amine groups transferred to glutamate by transamination
• Pyruvate then transaminated by glutamate to form alanine
• Alanine transported in blood to liver where it is converted back to pyruvate by transamination.
• Amino group fed via glutamate into urea cycle for disposal as urea whereas pyruvate is used to synthesise glucose which can be fed back to tissues

Question 19

what are the clinical problems with amino acid metabolism

Answer 19

many inherited diseases

involve a total or partial loss of enzyme activity

left untreated they can lead to intellectual impairment

treatment will involve restricting specific amino acids within the diet

we do the heel prick test with infants - checks for sickle cell, cystic fibrosis, and errors in metabolism

the two you need to know are

Phenylketonuria (PKU)
Homocystinurias

Question 20

what is Phenylketonuria (PKU)

what enzyme does it effect ?

what are the symptoms and treatments ?

what pathways does it affect ?

Answer 20

most common problem with amino acid metabolism

Deficiency in phenylalanine hydroxylase
Autosomal recessive.

Accumulation of phenylalanine in tissue, plasma & urine
this will be transaminated to phenylpyrivate - this will be converted to phenyl ketones, which accumulate

body cannot produce tyrosine - this effects noradrenaline and adrenaline , dopamine, melanin, thyroid hormones and protein synthesis

Phenylketones in urine
• Musty smell

Symptoms*
• Severe intellectual disability
• Developmental delay
• Microcephaly (small head)
• Seizures
• Hypopigmentation

Treatment
• Strictly controlled low phenylalanine diet enriched with tyrosine
• Avoid artificial sweeteners (contain phenylalanine)
• Avoid high protein foods such as meat, milk, and eggs

Question 21

what are Homocystinurias ?

what enzyme does it effect ?

what are the symptoms and treatments ?

Answer 21

• Problem breaking down methionine
• Excess homocystine (oxidised form of homocysteine) excreted in urine
• Autosomal recessive disorders.
• Defect in cystathionine β-synthase most common
• Affects connective tissue, muscles, CNS,
CVS

Treatment
• Low-methionine diet
• Avoid milk, meat, fish, cheese, eggs
• Nuts, and peanut butter also
contain methionine
• Cysteine, Vit B6, Betaine, B12 &
Folate supplement

Accumulation ofHomocysteine &
methionine causes disease symptoms such as cardiovascualr disease