Session 5: Protein Metabolism Flashcards
(130 cards)
1
Q
Where does stage 1 of protein catabolism take place?
A
Extracellular: GI tract
2
Q
What enzymes in the GI tract hydrolyse proteins in stage 1 of protein catabolism?
A
Proteases including peptidases hydrolyse the peptide bonds to release free amino acids
3
Q
How are most free amino acids taken up into enterocytes?
A
Sodium-dependent secondary active transport by solute carrier proteins (SLC)
4
Q
Free amino acids are released from enterocytes into the portal system and travel in the blood to other tissues where they are used for…
A
1) Protein synthesis = around ~75% proteins used for this
2) Synthesis of N-containing compounds = (purines, creatine, haem)
3) Oxidation = to produce energy (ATP)
5
Q
What two hormones stimulate the uptake of amino acids into tissues such as skeletal muscle, adipose tissue and liver?
A
Insulin and growth hormone (GH)
6
Q
What are the two mechanisms of protein catabolism?
A
Transamination, deamination
7
Q
How many different types of variable side chain can arise in amino acids?
A
20
8
Q
What are the 9 essential amino acids?
A
Isoleucine, Lysine, Threonine, Histidine, Leucine, Methionine, Phenylalanine, Tryptophan, Valine
9
Q
Creatinine is a useful clinical marker of…
A
Renal function - increased levels in blood indicate damage to nephrons of the kidney
10
Q
Creatinine urine excretion over 24h is proportional to ___ mass (estimate of ___ mass)
A
Creatinine urine excretion over 24h is proportional to muscle mass (estimate of muscle mass)
11
Q
What is creatinine?
A
Product of muscle breakdown of creatine and creatine phosphate, measurement of renal function
12
Q
What is transamination?
A
The transfer of an amino group from one amino acid to a keto acid, producing a new nonessential amino acid and a new keto acid
13
Q
When does mobilisation of protein reserves take place?
A
During extreme stress (starvation)
14
Q
In what disease does excessive breakdown of protein occur?
A
Cushing’s syndrome (excess cortisol)
Striae of the skin due to excessive protein breakdown causing weaknesses in the skin
15
Q
Insulin and growth hormone stimulates ___ synthesis
A
Insulin and growth hormone stimulates protein synthesis
16
Q
___ (e.g., cortisol) decreases protein synthesis
A
Glucocorticoids (e.g., cortisol) decreases protein synthesis
17
Q
What is the name of the enzyme involved in transamination?
A
Aminotransferases
18
Q
Aminotransferase enzymes require the coenzyme pyridoxal phosphate - a derivative of vitamin ___
A
Aminotransferase enzymes require the coenzyme pyridoxal phosphate - a derivative of vitamin B6
19
Q
What are two important diagnostic markers in the blood test to indicate liver function?
A
Plasma alanine aminotransferase (ALT) and plasma aspartate aminotransferase (AST)
20
Q
Plasma ALT and AST at heightened levels in the blood in conditions that cause extensive cellular necrosis such as…
A
Viral hepatitis, autoimmune liver diseases, toxic injury (death cap mushroom)
21
Q
Where does deamination mainly occur?
A
Liver and kidney
22
Q
At what range does ammonia need to be kept in the blood to be considered safe?
A
25-40 umol/L
23
Q
Where does the urea cycle take place?
A
Occurs in the liver
- A high protein diet activates enzymes
- A low protein diet inhibits enzymes (refeeding syndrome)
24
Q
What is refeeding syndrome?
A
Can occur when nutritional support is suddenly given to those who are severely malnourished patients e.g., anorexic patients
25
What are the symptoms of ornithine transcarbamylase (OCT) deficiency?
**Lethargy** → Ammonia is neurotoxic, depressing brain function.
**Confusion** → Ammonia disrupts neurotransmitters and causes cerebral edema.
**Vomiting** → The brainstem's vomiting center is triggered by toxins.
**Poor feeding** → The body naturally avoids protein to reduce ammonia production.
Rapid breathing (Respiratory alkalosis) → The body hyperventilates to remove excess carbon dioxide and balance blood pH.
**Seizures and coma** → Severe ammonia toxicity leads to brain swelling and dysfunction.
26
What are the risk factors for refeeding syndrome?
BMI <16
Unintentional weight loss >15% in 3-6 months
10 days or more with little/no nutritional intake
27
Management of ornithine transcarbamylase (OCT) deficiency?
Low protein diet, replace amino acids in the diet with keto-acids
28
What test is used for diagnosing a number of paediatric diseases?
Heel prick test
29
What is the most common defect/enzyme deficiency in the urea cycle?
**Ornithine transcarbamylase deficiency** (OTC deficiency)
30
How many inherited diseases of amino acid metabolism does the heel prick test screen for?
>50 inherited diseases involving defects in amino acid metabolism
31
What is phenylketonuria (PKU)?
Most common inborn error of amino acid metabolism (~1 in 15,000 births)
**Deficiency in phenylalanine hydroxylase (PAH) enzyme**
32
What does phenylketonuria (PKU) cause?
Accumulation of phenylalanine in tissue, plasma and urine.
Phenylketones in urine
33
What causes Phenylketonuria (PKU)?
**Autosomal recessive condition** (**chromosome 12**) leading to deficiency of phenylalanine hydroxylase (PAH)
34
What is the treatment of phenylketonuria (PKU)?
Low-phenylalanine diet + tyrosine supplementation
Avoid high protein foods
35
What are the symptoms of phenylketonuria (PKU)?
Severe intellectual disability, developmental delay, microcephaly, seizures, hypopigmentation
36
What is the most common cause of homocystinurias?
Defect in **cystathionine beta-synthase (CBS)** is most common
Defect in methionine synthase is also possible
37
What is homocystinuria?
Autosomal recessive disorder leading to excess homocysteine (oxidised form of homocysteine) excreted in urine
38
What causes disease symptoms in homocystinuria?
Accumulation of homocysteine and methionine
39
What is treatment of homocystinuria?
Low-methionine diet
Avoiding milk, meat, fish, cheese, eggs
Cysteine, vitamin B6, Betaine, B12 and folate supplementation
40
What are the symptoms of Homocystinuria?
**Marfan-like body stature** (long limbs and fingers), dislocation of the lens, intellectual disability
41
Elevated plasma homocysteine shown to be associated with ___ disease
Elevated plasma homocysteine shown to be associated with cardiovascular disease
42
What are some key features of urea?
**High nitrogen content**, **non-toxic**, extremely water-soluble, **most urea excreted in urine via kidneys**
43
Where is urea synthesised by the urea cycle?
Liver
44
Which enzymes can deaminate amino acids?
- Amino acid oxidases
- Glutaminase
- **Glutamate dehydrogenase**
45
What is deamination?
**Removal of an amino group** - removes ammonia (NH3)
46
What are major nitrogen containing compounds in the body?
Proteins, amino acids, purines & pyrimidines (DNA/RNA)
47
What are minor nitrogen containing compounds in the body?
Porphyrins (haem), creatine, neurotransmitters (e.g., dopamine), some hormones (e.g., adrenaline)
48
What are the conditionally essential amino acids?
Cysteine, Arginine, Tyrosine (CAT)
49
What is the reference range of average excretion of creatine in urine per day for men and women?
Men = 14-26 mg/kg
Women = 11-20 mg/kg
50
All 20 amino acids are required for ___ synthesis
All 20 amino acids are required for protein synthesis
51
Phenylketonuria (PKU)
- Autosomal recessive
- Deficiency in phenylalanine hydroxylase (PAH) enzyme
- Accumulation of phenylalanine in tissue, plasma, urine
- **Phenylketones in urine**
- Phenylketones oxidised to **phenylacetate** = **musty smell excreted in urine**
52
Symptoms of phenylketonuria
Severe intellectual disability, developmental delay, microcephaly, seizures, hypopigmentation
53
Treatment of phenylketonuria
- Low phenylalanine diet
- Supplement with tyrosine
- Avoid artificial sweeteners containing phenylalanine
- Avoid high protein food (milk, meat, egg)
54
Diagnosis of phenylketonuria
- **Detection of phenylketones in the urine**
- Elevated blood serum phenylalanine concentration [normally less than 0.1mmol/L; in PKU can exceed >1.0mmol/L]
- **Heel prick test** in babies
55
What is phenylketonuria an example of?
A defect in amino acid metabolism (metabolism of phenylalanine in this case)
56
Homocystinuria
- Autosomal recessive
- Defect in cystathione B-synthase (CBS) enzyme OR defect in methionine synthase
- Excess homocysteine excreted in urine
- Excess homocysteine & methionine leading to symptoms
57
Symptoms of homocystinuria
- Dislocation of the lens
- Marfan-like stature (long limbs & fingers)
- Intellectual disability
58
Treatment of homocystinuria
- Low methionine diet (avoid nuts/peanut butter)
- Cysteine, Vit. B6, Betaine, B12 and Folate supplementation
- Avoid high protein food (milk, meat, fish, cheese, egg)
59
Diagnosis of homocystinuria
- Elevated plasma homocysteine + methionine
- Presence of oxidised homocysteine in urine
- Heel prick test in babies
60
In healthy adults, there is a steady state in which the amount of nitrogen taken into the body equals the amount of nitrogen lost. This is known as...
Zero nitrogen balance or N-balance
61
The nitrogen balance in periods of active growth, pregnancy, tissue growth, convalescence (muscle atrophy repair)
Positive N-balance (intake exceeds excretion)
62
The nitrogen balance in periods of starvation, malnutrition and trauma
Negative N-balance
63
Non-essential amino acids
Body can synthesise these
64
Most nitrogen leaves the body as...
- Urea (85%)
- Creatinine (5%)
- Ammonia (3%)
- Uric acid in urine (sweat & faeces)
- Direct loss of protein from body (skin, hair, nails)
65
Most nitrogen enters the body as...
Protein (>90%)
66
Where does the urea cycle take place?
Liver cytosol and mitochondria
67
Describe urea cycle steps in this diagram
**Reaction 1** = **carbamoyl phosphate** is formed which is catalysed by carbamoyl synthetase enzyme.
**Reaction 2** = carbamoyl portion of this product is **transferred to ornithine** .
**Reaction 3** = citrulline is released to cytoplasm - **combined with aspartate** to form **argininosuccinate**.
**Reaction 4** = argininosuccinate cleaved by lyase (ASL enzyme) to form **arginine + fumarate**.
**Reaction 5** = arginine is cleaved by arginase to **urea + ornithine**.
68
Name the most common urea cycle disorder
**Ornithine transcarbamylase deficiency (OCT)**
69
Ornithine transcarbamylase deficiency (OTC)
- Example of urea cycle defect
- X-linked
70
Treatment of urea cycle defects (OTC)
- Low protein diet
- Replace amino acids in diet with keto acids
71
Symptoms of urea cycle defects (OTC)
Vomiting, lethargy, irritability, developmental delay, seizures, coma
72
Creatinine is a clinical marker of...
Renal function
73
Increased creatinine in the blood indicates...
**Nephron (renal) damage**
74
Which enzymes deaminate amino acids (get rid of excess ammonia)?
- Amino acid oxidases
- Glutaminase
- **Glutamate dehydrogenase**
75
Where does deamination occur?
Liver and kidney
76
Normal range of ammonia
25-40uM/L
77
In transamination - the NH2 from one amino acid is transferred to a keto-acid (a-ketogutarate) to produce a new amino acid (glutamate). This reaction is catalysed by...
Aminotransferases
78
In what disease processes may you expect to observe elevated plasma Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST)?
Conditions that cause extensive cellular necrosis such as...
- Viral hepatitis
- Autoimmune liver disease
- Toxic injury
79
Alanine Aminotransferase (ALT)
Catalyses alanine -> glutamate [transamination]
80
Aspartate Aminotransferase (AST)
Catalyses aspartate -> glutamate [transamination]
81
Some amino acids cannot be synthesised in the body and are therefore essential and must be obtained within a diet. True or false?
True
82
The only two ketogenic amino acids are...
Lysine and leucine
83
A negative N-balance occurs in...
Burns, kwashiorkor, trauma, illness
84
Ammonia is ___ and it has to be converted to ___ and then excreted in the ___
Ammonia is toxic and it has to be converted to urea and then excreted in the urine
85
The urea cycle takes place in
Liver
86
What is the **most common and safe form of ammonia which is transported** from tissue via blood to the liver?
A) Glutamine
Ammonia converted to glutamine by glutamine synthase
87
The main site of urea synthesis is...
Liver
88
The most common defect in the urea cycle arises from the deficiency of the enzyme...
Ornithine transcarbamylase
89
Homocystinuria enzyme
**Cystathione beta-synthase deficiency (CBS)**
90
Methionine synthase
91
Phenylketonuria enzyme
Phenylalanine hydroxylase (PAH) deficiency
92
What enzyme deficiency is cause of most common urea cycle disorder (X-linked)?
Ornithine transcarbamylase (OTC) deficiency
93
Free amino acids in the body are used for...
A) Protein synthesis (~75%)
B) Synthesis of nitrogen-containing compounds (e.g., purines, creatine & haem)
C) Oxidation to energy
D) Synthesis of important signalling molecules from amino acids...
L-arginine → nitric oxide
L-cysteine → hydrogen sulphide
94
What hormone stimulates the breakdown of muscle protein in skeletal muscle (proteolysis)?
Cortisol
95
What amino acids are both glucogenic and ketogenic?
Isoleucine, threonine, phenylalanine, tyrosine and tryptophan
96
The two fates of N atoms in amino acids...
1) Transferred to other molecules (transamination)
2) Removed (deamination)
97
Free ammonia NH3 (NH4+) from the deamination of amino acids has two fates...
1) Excreted in the urine via urea cycle
2) Used to produce alanine and glutamine (ammonia detoxification)
98
Ammonia can be detoxified by converting the ammonium ion to ___ using the enzyme ___ ___
Ammonia can be detoxified by converting the ammonium ion to glutamine using the enzyme glutamine synthetase
99
Most N leaves the body as ___ (~85%), ___ (~5%), ammonia (~3%), uric acid (sweat, faeces),
100
What are the two fates of ammonia from the deamination of amino acids?
1) Excreted in the urine via urea cycle
2) Used to produce alanine and glutamine (ammonia detoxification)
101
How can ammonia be detoxified?
By converting the **ammonium ion to glutamine** using the **enzyme glutamine synthetase**.
102
What is the most common form of nitrogen leaving the body?
Most N leaves the body as urea (~85%), creatine (~5%), ammonia (~3%), uric acid (sweat, faeces), direct protein loss (hair, skin & nails).
103
How does most nitrogen enter the body?
As protein (>90%).
104
What is transamination?
A process involving the transfer of an amino group from one amino acid to a keto acid.
105
What is the clinical importance of transamination?
**ALT and AST are important diagnostic markers**
* ALT (Alanine Aminotransferase) and AST (Aspartate Aminotransferase) are key **diagnostic markers for liver damage**. They are enzymes that are found inside liver cells. When liver cells are damaged or inflamed, these enzymes leak into the bloodstream, leading to elevated levels of ALT and AST in blood tests.
106
What enzyme converts glutamate to alpha-ketoglutarate and ammonium ion (NH4+)?
Glutamate dehydrogenase.
107
What is the difference between a peptide/polypeptide and a protein?
Polypeptide = single linear chain of many amino acids held together by amide bonds.
Protein = generally >50 amino acids long.
108
What is the minimum categorization of proteins in terms of amino acid length?
Proteins can range from 40 to 100 amino acids long.
109
Where do carbon atoms for the synthesis of non-essential amino acids come from?
- Intermediates of glycolysis (C3)
- Pentose phosphate pathway (C4, C5)
- Krebs cycle (C4, C5)
110
What are the sources of the amino group for amino acid synthesis?
- Other amino acids by process of transamination
- From ammonia
111
What are conditionally essential amino acids?
Cysteine, Arginine, and Tyrosine are required by children and pregnant women due to high rates of protein synthesis.
112
What are major nitrogen-containing compounds?
- Proteins
- Amino acids
- Purines & Pyrimidines
113
What are minor nitrogen-containing compounds?
- Porphyrins (haem)
- Creatine
- Neurotransmitters (e.g., dopamine)
- Some hormones (e.g., adrenaline)
114
What is zero nitrogen balance?
Nitrogen intake = nitrogen output.
115
What is positive nitrogen balance?
Nitrogen intake > nitrogen output, indicating an increase in total body protein.
116
What is negative nitrogen balance?
Nitrogen intake < nitrogen output, indicating a net loss of body protein.
117
What are some causes of negative nitrogen balance?
Trauma, illness, burns, and malnutrition.
118
What is the breakdown product of creatine and creatine phosphate in the muscle?
Creatinine.
119
What are glucogenic amino acids?
All except leucine and lysine; can be converted into intermediates that feed into gluconeogenesis.
120
What are ketogenic amino acids?
Lysine and leucine.
121
Which amino acids are both glucogenic and ketogenic?
- **Phenylalanine**
- **Tyrosine**
- Tryptophan
- Threonine
- Isoleucine
122
What is a useful investigation that estimates muscle mass?
**Creatinine urine excretion over 24h** is proportional to muscle mass.
123
What is the largest store of fuel in a 70kg man?
Triacylglycerol (~15kg, 600,000kJ).
124
What is the smallest store of fuel in a 70kg man?
Glycogen (~0.4kg, ~4,000kJ).
125
How much does muscle protein make up in a 70kg man?
~6kg, ~100,000kJ.
126
What are the two main pathways by which nitrogen is **removed from amino acids**?
- Transamination
- Deamination
127
Why is the removal of nitrogen from amino acids essential?
- To allow the carbon skeleton of amino acids to be used in oxidative metabolism.
- Removed nitrogen can be incorporated into other compounds.
- Removed nitrogen is excreted from the body as urea.
128
What are aminotransferases?
Enzymes which catalyse transamination and are important diagnostic markers of liver function test (LFT).
129
What are two examples of amino acid metabolism disorders?
1) Phenylketonuria (PKU)
2) Homocystinuria.
130
What are the effects of defects in the urea cycle?
1) Hyperammonaemia
2) Accumulation/excretion of urea cycle intermediates.
