Amino Acid Metabolism

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?

What is true of healthy adults?

When is a positive nitrogen balance (nitrogen intake > nitrogen excretion) required?

When might a negative nitrogen balance (nitrogen intake < nitrogen excretion) occur?

How are most cellular proteins degraded?

How are exogenous proteins degraded?

What is oxidative deamination?

Which enzymes carry out oxidative deamination?

NADH + H+ are formed from NAD during oxidative deamination.

Oxidative deamination and transamination are reversible reactions.

What is a co-product of oxidative deamination?

Fill in the blanks below to summarise oxidative deamination. An amino acid reacts with [blank_start]water[blank_end] to form a [blank_start]keto acid[blank_end] and [blank_start]ammonia[blank_end]. This is catalysed by a [blank_start]dehydrogenase[blank_end] enzyme specific to that amino acid. This is a [blank_start]reversible[blank_end] reaction that forms NADH and H+ from [blank_start]NAD[blank_end].

What is transamination?

What type of enzymes carry out transamination?

Fill in the blanks below to describe transamination. An amino acid reacts with a [blank_start]keto acid[blank_end] to form a keto-carboxylate such as [blank_start]pyruvate[blank_end] and the amino acid of the [blank_start]keto acid[blank_end]. This is carried out by [blank_start]transaminase[blank_end] enzymes in a [blank_start]reversible[blank_end] reaction.

What are glucogenic amino acids?

What are ketogenic amino acids?

Which amino acids are ketogenic only?

Which amino acids are both glucogenic and ketogenic?

Only the liver can convert ammonia to urea for excretion in the urea cycle.

What does glutaminase do?

What enzyme forms glutamine from glutamate?

When glutamate is formed from glutamine, the enzyme [blank_start]glutaminase[blank_end] catalyses the reaction. This involves [blank_start]water[blank_end] as a reactant and [blank_start]ammonia[blank_end] as a product.

When glutamine is formed from glutamate, ATP and [blank_start]ammonia[blank_end] are required. [blank_start]ATP[blank_end] is hydrolysed to provide energy for the reaction. [blank_start]Glutamine synthetase[blank_end] catalyses this reaction. Glutamine can carry [blank_start]two[blank_end] ammonia equivalents in this way.

Why do we carry ammonia via glutamine to the liver for urea formation?

When ammonium ions combine with CO2/HCO3- in the mitochondria, what is formed?

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?

What is lost when carbamoyl phosphate and ornithine react to form citrulline?

Which enzyme catalyses the formation of citrulline from carbamoyl phosphate and ornithine?

Where does citrulline combine with aspartate to form arginosuccinate?

Where does argininosuccinate synthase receive its energy to combine citrulline and aspartate to form argininosuccinate?

What reaction forms arginine and fumarate from argininosuccinate?

What does the enzyme argininosuccinase do?

What reaction converts arginine to urea and ornithine?

What is required to convert arginine to urea and ornithine?

Fill in the blanks below to describe the ornithine/urea cycle. 1. Ammonium ions react with c[blank_start]arbon dioxide[blank_end] or b[blank_start]icarbonate[blank_end] in the [blank_start]mitochondria[blank_end]. This forms [blank_start]carbamoyl phosphate[blank_end]. This reaction is catalysed by [blank_start]carbamoyl phosphate synthase I[blank_end] and requires [blank_start]ATP[blank_end] hydrolysis. 2. [blank_start]Carbamoyl phosphate[blank_end] combines with [blank_start]ornithine[blank_end] to form [blank_start]citrulline[blank_end] in the mitochondria. [blank_start]Inorganic phosphate[blank_end] is lost in this reaction. This reaction is catalysed by [blank_start]ornithine transcarbamyolase[blank_end]. 3. [blank_start]Citrulline[blank_end] is transported into the [blank_start]cytosol[blank_end] where it combines with [blank_start]aspartate[blank_end] to form [blank_start]argininosuccinate[blank_end]. This is catalysed by [blank_start]argininosuccinate synthase[blank_end] and requires [blank_start]ATP[blank_end] hydrolysis. 4. [blank_start]Argininosuccinate[blank_end] is cleaved into [blank_start]fumarate[blank_end] and arginine in a reaction catalysed by [blank_start]argininosuccinase[blank_end]. 5. [blank_start]Arginine[blank_end] is hydrated and splits into urea and [blank_start]ornithine[blank_end]. This requires water and is catalysed by [blank_start]arginase[blank_end]. The [blank_start]ornithine[blank_end] combines with more [blank_start]carbamoyl phosphate[blank_end] in the mitochondria to continue the cycle and the [blank_start]urea[blank_end] is excreted.

Fill in the blanks below to describe some of the end products of nitrogen metabolism. Protein breakdown results in [blank_start]urea[blank_end]. Creatine phosphate breakdown results in [blank_start]creatinine[blank_end]. DNA and RNA breakdown results in [blank_start]uric acid[blank_end]. The control of body pH results in [blank_start]ammonia[blank_end].

Hyperammonaemia is caused by an impaired conversion of [blank_start]ammonia[blank_end] to [blank_start]urea[blank_end]. This can be caused by liver failure, for example in viral [blank_start]hepatitis[blank_end], c[blank_start]irrhosis[blank_end] or other toxins. This can also be caused by genetic defects, for example causing mutations in the enzymes involved in the [blank_start]urea cycle[blank_end]. The symptoms of this include irratibility, headache and vomiting. In more severe cases hyperammonaemia can result in encephalopathy, seizures and ataxia.