Body proteins are continuously degraded to amino acids and re-synthesised.
All proteins have the same half-lives.
Protein cannot be stored in the body.
Why is a high protein intake in an already well-fed individual wasteful?
Excess amino acids are catabolised and excreted as urea
Excess amino acids cause breakdown of glycogen
Excess amino acids cause fat accumulation
Excess protein will not be catabolised so will yield no energy
What is true of healthy adults?
Nitrogen intake = nitrogen excretion
Nitrogen intake > nitrogen excretion
Nitrogen intake < nitrogen excretion
When is a positive nitrogen balance (nitrogen intake > nitrogen excretion) required?
Growth in childhood, recovery after illness, after immobilisation, during pregnancy
During starvation, during serious illness, in injury and trauma
All the time - this is required for healthy adults
When might a negative nitrogen balance (nitrogen intake < nitrogen excretion) occur?
Growth in childhood, recovery after illness, following immobilisation, during pregnancy
In normal circumstances - this is required for healthy adults
How are most cellular proteins degraded?
Ubiquitin breakdown system
Proteolytic enzymes in lysosomes
Proteolytic enzymes in the gut
Dissolve in the cytosol
How are exogenous proteins degraded?
Proteolytic enzymes in gut
Dissolve in cytoplasm
What is oxidative deamination?
The conversion of an amine group to a carbonyl group to form a keto acid from an amino acid
The transfer of an amine group from an amino acid to a keto acid
Which enzymes carry out oxidative deamination?
Dehydrogenases
Carboxylases
Oxidoreductases
Lyases
NADH + H+ are formed from NAD during oxidative deamination.
Oxidative deamination and transamination are reversible reactions.
What is a co-product of oxidative deamination?
Ammonia/ammonium ions
Pyruvate
Amino acids
FADH2
Fill in the blanks below to summarise oxidative deamination. An amino acid reacts with to form a and . This is catalysed by a enzyme specific to that amino acid. This is a reaction that forms NADH and H+ from .
What is transamination?
The transfer of an amino acid to a keto acid to form a keto-carboxylate and another amino acid
The substitution of an amino group for a carbonyl group to form a keto acid and ammonia
What type of enzymes carry out transamination?
Transaminases
Decarboxylases
Fill in the blanks below to describe transamination. An amino acid reacts with a to form a keto-carboxylate such as and the amino acid of the . This is carried out by enzymes in a reaction.
What are glucogenic amino acids?
Amino acids that can be converted back to glucose by the liver during starvation
Amino acids that can be degraded to acetyl CoA
Amino acids that can be incorporated into glycogen
Amino acids that stimulate glycogen degradation
What are ketogenic amino acids?
Amino acids that form ketone bodies in the blood
Amino acids that cause ketoacidosis
Which amino acids are ketogenic only?
Leucine
Lysine
Tyrosine
Iseoleucine
Phenylalanine
Histidine
Methionine
Alanine
Cysteine
Which amino acids are both glucogenic and ketogenic?
Tryptophan
Threonine
Only the liver can convert ammonia to urea for excretion in the urea cycle.
What does glutaminase do?
Form glutamate from glutamine
Form glutamate from alanine
Deaminate glutamate
Deaminate glutamine
What enzyme forms glutamine from glutamate?
Glutamine synthetase
Glutamate deaminase
Glutamate hydrolase
Glutamate carboxylase
When glutamate is formed from glutamine, the enzyme catalyses the reaction. This involves as a reactant and as a product.
When glutamine is formed from glutamate, ATP and are required. is hydrolysed to provide energy for the reaction. catalyses this reaction. Glutamine can carry ammonia equivalents in this way.
Why do we carry ammonia via glutamine to the liver for urea formation?
Carries 2 ammonia equivalents to prevent its toxic effects
The most soluble amino acid
Carries 3 ammonia equivalents to prevent its toxic effects
The most neutral amino acid
When ammonium ions combine with CO2/HCO3- in the mitochondria, what is formed?
Carbamoyl phosphate
L-Citruline
L-Aspartate
Urea
ATP hydrolysis is required for the synthesis of carbamoyl phosphate from CO2/HCO3- and ammonia.
What does carbamoyl phosphate combine with in the mitochondria to form citrulline?
Ornithine
Arginine
Fumarate
Arginosuccinate
What is lost when carbamoyl phosphate and ornithine react to form citrulline?
Inorganic phosphate
ATP
Methyl group
Carbonyl group
Which enzyme catalyses the formation of citrulline from carbamoyl phosphate and ornithine?
Ornithine transcarbamyolase
Ornithine pyrophosphatase
Citrulline synthetase
Carbamoyl pyrophosphatase
Where does citrulline combine with aspartate to form arginosuccinate?
Cytosol
Mitochondria
Plasma
Endoplasmic reticulum
Where does argininosuccinate synthase receive its energy to combine citrulline and aspartate to form argininosuccinate?
ATP hydrolysis
GTP hydrolysis
UTP hydrolysis
Concentration gradient of citrulline
What reaction forms arginine and fumarate from argininosuccinate?
Cleavage
Condensation
Phosphorylation
Dephosphorylation
What does the enzyme argininosuccinase do?
Cleave argininosuccinate into fumarate and arginine
Cleave argininosuccinate into malate and arginine
Dephosphorylate argininosuccinate
Phosphorylate argininosuccinate
What reaction converts arginine to urea and ornithine?
Hydration
What is required to convert arginine to urea and ornithine?
Water
Carbon dioxide
Bicarbonate
Fill in the blanks below to describe the ornithine/urea cycle. 1. Ammonium ions react with c or b in the . This forms . This reaction is catalysed by and requires hydrolysis. 2. combines with to form in the mitochondria. is lost in this reaction. This reaction is catalysed by . 3. is transported into the where it combines with to form . This is catalysed by and requires hydrolysis. 4. is cleaved into and arginine in a reaction catalysed by . 5. is hydrated and splits into urea and . This requires water and is catalysed by . The combines with more in the mitochondria to continue the cycle and the is excreted.
Fill in the blanks below to describe some of the end products of nitrogen metabolism. Protein breakdown results in . Creatine phosphate breakdown results in . DNA and RNA breakdown results in . The control of body pH results in .
Hyperammonaemia is caused by an impaired conversion of to . This can be caused by liver failure, for example in viral , c or other toxins. This can also be caused by genetic defects, for example causing mutations in the enzymes involved in the . The symptoms of this include irratibility, headache and vomiting. In more severe cases hyperammonaemia can result in encephalopathy, seizures and ataxia.
