Protein and amino acid metabolism Flashcards
1
Q
stage 1 catabolism of protein
A
- proteases and peptidases hydrolyse peptide bonds to release free amino acids in GI tract
- amino acids absorbed into circulation and used for synthesis of proteins and various nitrogen-containing compounds (purines, pyrimidines, haem, creatine)
2
Q
stage 2 catabolism of amino acids
A
- removal of amino group -NH2
- converted to urea and excreted from body in urine
- remaining C-skeletons converted to one or more of following: pyruvate, oxaloacetate, fumarate, α-ketoglutarate, succinate, acetyl Co-A
- acetyl Co-A enters stage 3 of catabolism - TCA cycle
3
Q
products at the end of stage 2 catabolism
A
- glucose, galactose, fructose > pyruvate > acetyl Co-A
- glycerol > pyruvate > acetyl Co-A
- fatty acids and ketone bodies > acetyl Co-A
- amino acids > acetyl Co-A
4
Q
N compounds in the body
A
total amount of N in 70kg male ~2kg
- 90% major N compounds: amino acids, proteins, purines+pyrimidines (DNA + RNA)
- smaller amounts of porphyrins, creatine phosphate, neurotransmitters, hormones, carnitine
5
Q
Nitrogen balance (N-balance)
A
amount of N taken into body equals amount of N lost from body
6
Q
positive N balance
A
intake > output
- increase in total body protein
- normal state in periods of active growth, pregnancy, tissue repair, convalescence
7
Q
negative N balance
A
intake < output
- net loss of body protein
- never normal
- causes include trauma, infection, starvation, malnutrition
8
Q
how does nitrogen leave the body
A
- 85% as urea
- 5% as creatinine
- 3% as ammonia
- uric acid in urine, sweat and faeces
- direct loss of protein (skin, hair, nails)
9
Q
mobilisation of protein reserves
A
insulin and growth hormone
- stimulate uptake of amino acids into tissues such as skeletal muscle, adipose tissue and liver and their incorporation into proteins
- decrease protein degradation
glucocorticoids e.g.cortisol
- promoting breakdown of muscle proteins (proteolysis) and release of amino acids
- decrease protein synthesis
10
Q
protein turnover
A
continuous breakdown and resynthesis of body proteins
- rate depends on protein and varies during growth and ageing
- average half-life of body protein ~80 days
- total protein turnover ~300-400g a day
- rate of protein breakdown normally equals rate of protein resynthesis
11
Q
what are ketogenic amino acids
A
amino acids that produce acetyl Co-A as they can produce ketone bodies e.g.leucine, lysine
12
Q
what are glucogenic amino acids
A
amino acids that give rise to products other than acetyl Co-A as they can be used for gluconeogenesis e.g. glycine, alanine
13
Q
examples of both ketogenic and glucogenic amino acids
A
isoleucine, threonine, phenylalanine, tyrosine, tryptophan
14
Q
what are conditionally essential amino acids
A
- need dietary input when demand exceeds ability to synthesise them
- children and pregnant women need arginine, tyrosine and cysteine in diet
15
Q
what are essential amino acids
A
- cannot be synthesised in body so taken in through diet
- isoleucine, lysine, threonine, histidine, leucine, methionine, phenylalanine, tryptophan, valine
16
Q
what is the amino acid pool
A
- total amount of free amino acids in the body (intracellular and extracellular)
- ~100g in 70kg male
- normal fasting concentration ~3mmol/L
17
Q
amino acid reutilisation
A
- 75% of amino acids released during protein breakdown are reused for synthesis
- 25% oxidised to release energy or used in synthesis of other N-containing compounds
17
Q
amino acid reutilisation
A
- 75% of amino acids released during protein breakdown are reused for synthesis
- 25% oxidised to release energy or used in synthesis of other N-containing compounds
18
Q
where do carbon atoms for non-essential amino acid synthesis come from
A
- intermediates of glycolysis (C3)
- pentose phosphate pathway (C4 & C5)
- Krebs cycle (C4 & C5)
19
Q
where do amino groups for non-essential amino acid synthesis come from
A
- other amino acids by transamination
- ammonia
20
Q
main functions of amino acids
A
- protein synthesis (all 20)
- synthesis of other N-compounds (specific amino acids)
21
Q
2 signalling molecules synthesised from amino acids
A
- nitric oxide from L-argining
- hydrogen sulphide from L-cysteine
22
Q
synthesis of N-compounds
A
- tryptophan: 5HT, nicotinamide, melatonin
- histidine: histamine (local mediator)
- glycine: purines, glutathione, porphyrins, creatine
- tyrosine: melanin, thyroid hormones, catecholamines
23
Q
amino acid breakdown
A
liver is major site of breakdown
- C-atoms converted to intermediates of carbohydrate and lipid metabolism
- removal of -NH2 group (transamination or deamination)
- N-atoms converted to urea
24
fate of the C-atoms of amino acids
- **glucogenic** converted to pyruvate, oxaloacetate, fumarate, α-ketoglutarate or succinyl Co-A
- **ketogenic** converted to acetyl Co-A or acetoacetyl Co-A
- normally products oxidised to **CO2 and H2O** and energy released
- in starvation and diabetes products can be used to produce **glucose** and **ketone bodies**
25
removal of nitrogen from amino acids
- removing amino group essential to allow **carbon skeleton** to be utilised in oxidative metabolism
- nitrogen incorporated into **compounds** or excreted in **urea**
- amino groups transferred to other molecules (**transamination**) or removed (**deamination**)
26
transamination
- **aminotransferases** (transaminases) specific for individual/groups of amino acids
- use **α-ketoglutarate** to funnel amino group to **glutamate**
- aminotransferases require coenzyme **pyridoxal phosphate** which is a derivative of vitamin B6
- **cortisol** stimulates transaminase synthesis in liver
27
clinically important aminotransferases
**alanine aminotransferase (ALT)**
alanine + α-ketoglutarate to pyruvate + glutamate
**aspartate aminotransferase (AST)**
aspartate + α-ketoglutarate to oxaloacetate + glutamate
28
plasma ALT and AST levels
- measured routinely as part of **liver function test**
- **high levels** in conditions causing extensive **cellular necrosis** (viral hepatitis, autoimmune liver diseases, toxic injury)
29
deamination
enzymes in liver and kidney react with amino acids to remove the -NH2 group as free NH3
30
what are D-amino acids
- found in **plants and microorganisms** so enter through diet
- not used for protein synthesis as proteins would be structurally **abnormal** and **non-functional**
- D-amino acid oxidase converts them to **keto acids** that aren't optically active
31
enzymes involved in deamination
**amino acid oxidases**
- low specificity enzymes that convert amino acids to keto acids and ammonia
**glutaminase**
- high specificity enzyme that converts glutamine to glutamate and ammonia
**glutamate dehydrogenase**
- high specificity enzyme that catalyses interconversion of glutamate and α-ketoglutarate
- involved in disposal of amino acids and synthesis of non-essential amino acids
32
disorders of amino acid metabolism
- over **50 inherited disorders** due to specific enzyme defects in amino acid metabolism
- either **total or partial loss** of enzyme activity
- amino acid/products of breakdown accumulate which may be **toxic** or metabolised to toxic products
- significant portion of **paediatric genetic disease**
- **mental retardation** and **developmental abnormalities**
- treatment involves **restricting** specific amino acids in diet
33
what diseases can heel prick test screen for
- sickle cell disease
- cystic fibrosis
- congenital hypothyroidism
**inborn errors of metabolism**
- phenylketonuria (PKU)
- maple syrup urine disease
- isovaleric acidaemia (IVA)
- glutaric aciduria
- homocystinuria
34
what is phenylketonuria (PKU)
- most common inborn error of amino acid metabolism (~1 in 15000)
- large amounts of **phenylketones in urine**
- deficiency in **phenylalanine hydroxylase**
- **autosomal recessive** (gene on Chr12)
35
how does PKU occur
- deficiency in phenylalanine hydroxylase so phenylalanine can't be oxidised to **tyrosine**
- **phenylalanine accumulates** in tissues and blood so metabolised by other pathways
- converted to **phenylpyruvate** by transamination and phenylketones which are excreted in urine
36
how can PKU be diagnosed
- detection of **phenylketones** in urine
- measurement of **blood phenylalanine** concentration (normally <0.1mmol/L)
- **heel prick test**
37
what is the treatment for PKU
- strictly controlled low phenylalanine diet enriched with tyrosine
- avoid artificial sweeteners
- avoid high protein foods like meat, milk and eggs
38
symptoms of PKU
- severe intellectual disability
- developmental delay
- microcephaly
- seizures
- hypopigmentation
**can be avoided with early intervention**
39
what is homocystinuria
- **autosomal recessive** disorder
- problem breaking down **methionine**
- ~1 in 344000
- excess **homocystine** (oxidised form of homocysteine) in urine
- defect in **cystathione β-synthase (CBS)** most common
40
how does homocystinuria occur
- methionine converted to **homocysteine**
- **CBS enzyme deficient** so can't convert homocysteine to **cystathione** which would be converted to cysteine
- levels of **homocysteine increase** in blood and some converted to **methionine** (promoted by betaine, Vit B12 + folate)
- **accumulation** of homocysteine and metabolites causes disease symptoms
41
how is homocystinuria diagnosed
- elevated levels of **homocysteine** and **methionine** in plasma
- presence of **homocystine** in urine
42
symptoms of homocystinuria
- elevated plasma homocysteine cause disorders of **connective tissue, muscle, CNS and cardiovascular system**
- in children, symptoms similar to **Marfan's** (Marfan's = lack of expression of fibrillin-1 protein whereas homocystinuria is disrupted protein)
43
what is the treatment for homocystinuria
- low methionine diet
- avoid milk, meat, fish, eggs, cheese, nuts
- cysteine, Vit B6 (cofactor for CBS enzyme), betaine, B12 + folate supplement
44
what is the peripheral blood concentration of ammonia
25-40 µmol/L
45
toxic effects of ammonia
**readily diffusable and extremely toxic to brain**
- interference with amino acid transport and protein synthesis
- disruption of cerebral blood flow
- pH effects (alkaline)
- interference with metabolism of excitatory amino acid neurotransmitters
- alteration of blood-brain barrier
- interference with TCA cycle - reacts with α-ketoglutarate to form glutamate)
46
ammonia detoxification using glutamine
- in tissues ammonia combined with **glutamate** to form **glutamine**
- glutamine transported in blood to **liver** or **kidneys**
- cleaved by **glutaminase** to reform glutamate and ammonia
- ammonia fed into **urea cycle** in liver and excreted directly in **urine** in kidney
47
how are amino acid nitrogen transported using alanine
- amine groups transferred to **glutamate** by **transamination**
- **pyruvate transaminated** by glutamate to form **alanine**
- alanine transported in blood to **liver**
- converted back to **pyruvate** by transamination (**alanine aminotransferase**)
- amino group fed via **glutamate** into **urea cycle** for disposal as urea
- pyruvate used to **synthesise glucose** which is fed back to tissues (**glucose-alanine cycle**)
48
glutamine synthesis
- normal blood concentration is **0.5mmol/L**
- synthesised from **ammonia and glutamate** via **glutamine synthetase**
- used by cells to **reduce ammonia toxicity** as hydrolysed in liver and kidneys by **glutaminase** to release ammonia to be disposed of in urine (kidney) or converted to urea (liver)
49
what is urea
- extremely **water soluble** so excreted in urine
- **non-toxic**, **metabolically inert** and **high nitrogen content** so effective way of disposing of nitrogen
- synthesised in **liver** by urea cycle
- transported via blood to **kidneys** for excretion in urine
- useful **osmotic role** in kidney tubules
50
urea synthesis (urea cycle)
**NH2 groups come from ammonia and aspartate**
- ammonia comes from enzymes in liver that **deaminate** amino acids releasing NH3 and from NH3 produced by **gut bacteria** that enters liver via portal circulation
- aspartate formed from **oxaloacetate** by **transmaination**
51
what happens if urea diffuses across intestinal wall and enters intestine
- gut bacteria break down urea, releasing ammonia that can be reabsorbed
- contributes to hyperammonaemia in kidney failure when concentration of urea in blood is high
52
regulation of urea synthesis
- urea cycle involves **5 enzymes** which are not subjected to feedback inhibition by end product of cycle as function is to dispose of ammonia as urea
**enzymes of the cycle are inducible but not regulated**
- high protein diet induces enzyme levels
- low protein diet or starvation represses enzyme levels
53
defects in the urea cycle
- **autosomal recessive** genetic disorders caused by deficiency in each one of the five **enzymes** discovered
- complete loss of enzyme is fatal but **partial loss** occurs ~1 in 30000
- leads to **hyperammonaemia** and **accumulation/excretion of urea cycle intermediates**
54
symptos of diseases of urea cycle
- vomiting
- lethargy
- irritability
- mental retardation
- seizures
- coma
- eventually death
55
treatment of diseases of urea cycle
- low protein diet
- replace amino acids in diet with keto acids
56
severity of diseases of urea cycle
- severity depends on **nature of defect** and **amount of protein eaten**
- **severe** urea cycle disorders show symptoms within **1 day** after birth
- **mild** urea cycle enzyme deficiencie may not show symptoms until **early childhood**
57
what is hyperammonaemia
- **high levels of ammonia in blood**
- blurred vision, tremors, slurred speech, coma, eventually death
- secondary consequence of **liver disease** (cirrhosis) where ability to remove NH3 from portal blood impaired
58
overview of protein metabolism
- **free amino acids** from digestion, de novo amino acid synthesis and proteolysis of cellular proteins
- **synthesis** of cellular proteins from free amino acids
- **breakdown** of free amino acids in liver
- **carbon skeleton used for energy** (glucogenic for gluconeogenesis and ketogenic for ketone bodies)
- **amino group converted to urea** and excreted in urine
