Protein And Amino Acids

Question 1

Nitrogen balance

N is 16% of proteins - western diet ~ 100g per day

Answer 1

N equilibrium - intake = output 
- normal state in adults 
Positive N balance - intake > output 
- normal state in growth and pregnancy or in an adult recovering from malnutrition 
Negative N balance - intake

Question 2

Protein turnover

Answer 2

Free amino acids are used to produce energy in times of stress e.g. malnutrition

Question 3

Glucogenic amino acids

Answer 3

Alanine
Glycine
Histidine

Question 4

Ketogenic amino acids

Answer 4

Lysine

Leucine

Question 5

Glucogenic and ketogenic amino acids

Answer 5

Tryptophan
Phenylalanine
Tyrosine

Question 6

Mobilisation of protein reserves

Answer 6

• occurs during extreme stress - starvation
• under hormonal control
  Protein synthesis
• inhibited by glucocorticoids e.g. Cortisol
• activated by insulin and growth hormone
  Protein degradation
• inhibited by insulin and growth hormone
• activated by glucocorticoids e.g. Cortisol

Excessive protein breakdown can occur in Cushing’s syndrome (excess cortisol) weaken skin leading to purple striae formation

Question 7

Amino acid structure

Answer 7

H.    H.       OH 
|.       |.         |
N---C----C=O 
|.      |. 
H.    R 

20 different types
Joined by peptide bonds

Question 8

Creatinine

a useful clinical marker

Answer 8

• breakdown product of creatine & creatine phosphate in muscle
• usually produced at a constant ratio and amount in urine is proportional to muscle mass over 24 hours (unless there is muscle wasting)
• men 14-26 mg/kg, women 11-20 mg/kg
• filtered via kidneys into urine - provides an estimate of muscle mass
• used as an indicator of renal function (raised levels - damage to nephrons)

Question 9

Essential amino acids

Answer 9

If - Isoleucine 
Learned - Lysine 
This - Threonine 
Huge - Histidine 
List - Leucine 
May - Methionine 
Prove - Phenylalanine 
Truly - Tryptophan 
Valuable - Valine

Question 10

Amino acid synthesis

Answer 10

C atoms come from 
- intermediates of glycolysis (C3)
- pentose phosphate pathway (C4&C5) 
- kreb's cycle (C4&C5) 
The amino group is provided by the other amino acids by the process of transamination or from ammonia

Question 11

Synthesis of important N containing compounds

Answer 11

Tyrosine
- thyroid hormones - adrenaline and dopamine
Histidine
- histamine
Arginine
- nitric oxide - signalling molecule for vascular dilation
Cysteine
- hydrogen sulphide - signalling molecule
- glutathione

Question 12

Transamination

Answer 12

Most aminotransferase enzymes use alpha-ketoglutarate to funnel the amino acid groups to glutamate
Exception is Aspartate aminotransferase which uses oxaloacetate to funnel amino acid group to Aspartate
- all require coenzyme pyridoxal phosphate which is a derivative of vitamin B6
Amino transferase removes amino group from one amino acid and adds them to a keto acid - glutamate/Aspartate (which can be dealt with in terms of the amino group)

Question 13

Aminotransferase enzymes of clinical importance

Answer 13

Alanine aminotransferase (ALT)
- converts alanine to glutamate
Aspartate aminotransferase (AST)
- converts glutamate to Aspartate
Plasma levels of ALT and AST are measured routinely as part of the liver function test
High levels that cause extensive cellular necrosis
- viral hepatitis
- autoimmune liver diseases
- toxic injury
- death cap mushrooms that cause acute liver failure

Question 14

Deamination

Answer 14

- liberates the amino group as free ammonia 
Occurs mainly in the liver and kidney 
- keto acids can be utilised for energy 
- ammonia (toxic) is converted to urea or excreted directly in the liver
Enzymes 
- amino acid oxidises 
- glutaminase 
- glutamate dehydrogenase

Question 15

Urea

Answer 15

• high nitrogen content
• non-toxic
• extremely water soluble
• inherit in humans
• excreted in urine via the kidneys
• useful osmotic role in kidney tubules

Question 16

Urea cycle

Answer 16

Occurs in liver and involves 5 enzymes

• amount of urea cycle enzymes is related to need to dispose ammonia (can be downgraded in starvation - leads to refeeding syndrome - refeed @ 5-10kcal/kg/day)
• cycle is inducible but not regulated

Question 17

Defects in urea cycle

Answer 17

• autosomal recessive genetic disorders (caused by deficiency of one of enzymes in the urea cycle
  ~ 1 in 30,000 live births
  Mutations cause partial loss of enzyme function - complete loss is incompatible with life
  Leads to -
• hyperammonaemia
• accumulation/excretion of urea cycle intermediates
  Severity depends on nature of defect and amount of protein eaten
• severe show symptoms within a day of birth
• mild show symptoms in early childhood
  LOW PROTEIN DIET, REPLACE AMINO ACIDS WITH KETO ACIDS

Question 18

Ammonia Toxicity

Answer 18

• readily diffusible
• blood levels low (25-40umol/L)
  potential side effects
• interference with aa transport and protein synthesis
• disruption of cerebral blood flow
• pH affects (alkaline)
• interference with TCA cycle (reacts with alpha-ketogluterate to form glutamate)
• interference with the metabolism of excitatory amino acid neurotransmitters (glutamate and Aspartate)

Question 19

Transport of ammonia

Glutamine

Answer 19

Ammonia + glutamate = glutamine

• transported to liver or kidney in blood - cleaved by glutaminase to reform glutamate and ammonia
• in liver ammonia is fed into the urea cycle
• in kidney ammonia is excreted directly as urine

Question 20

Transport of ammonia

alanine cycle

Answer 20

Alanine is formed by transamination of pyruvate in the peripheral tissues

• transported to the liver by the blood where it is converted back to pyruvate by transamination
• amino group is fed via glutamate into the urea cycle
• pyruvate is used to synthesis glucose which can be fed back to the tissues

Question 21

Heel prick test

Answer 21

• every child is screened systematically at birth for a variety of inborn problems of metabolism
• dietary modification can manage disease whereas untreated it could lead to mental retardation

Question 22

Phenylketonuria (PKU)

Answer 22

Most common (~1 in 15,000 births)
- deficiency in phenylalanine hydroxylase
- autosomal recessive gene on chromosome 12
- accumulation of phenylalanine in tissue, plasma and urine
- phenylketones in urine
- musty smell
TREATMENTS
- strictly control phenylalanie in diet (avoid artificial sweetener, avoid high protein foods - meat, milk and eggs)
Not producing enough tyrosine which needed for noradrenaline and adrenaline, dopamine, melanin, thyroid hormones and protein synthesis
SYMPTOMS
- severe intellectual ability and developmental delay
- microcephaly (small head)
- seizures
- hypopigmentation

Question 23

Homocystinurias

Answer 23

Problem breaking down methionine
- excess homocystine (oxidation of homocysteine) is excreted in urine
- autosomal recessive ~ 1 in 344,000
- defect in cystathionine beta-synthase most common
- affects connective tissue, muscles, CNS, CVS
TREATMENT
- low-methionine diet (acid milk, meat, fish, eggs, nuts and peanut butter)
- cysteine, vitamin b6, beating, b12 and folate suppliment
(Accumulation if methionine is less severed than accumulation of homocysteine so supplements are given to promote the conversion of homocysteine to methionine)