Biosynthesis of nutritionally essential and non-essential amino acids

Question 1

What are the nutritionally essential amino acids?

Answer 1

• isoleucine
• leucine
• lysine
• threonine
• valine
• methionine
• phenylalanine
• tryptophan
• histidine
• arginine

Question 2

Glutamate is an nutritionally non-essential amino acid; the body makes this using what substrate and what enzyme?

Answer 2

Glutamate is made from alpha-keto glutarate (a substrate in the TCA cycle) using the enzyme glutamate dehydrogenase.

Question 3

How is glutamine made in the body? What are the steps involved?

Answer 3

Glutamine is made from glutamate using the enzyme glutamine synthetase.

Formation of glutamine involves the following steps:
- amidation of glutamate

Question 4

List some enzymes where the amino acid selenocysteine is found.

Answer 4

• thioredoxin reductase
• glutathione peroxidase
• deiodinase (this converts T4 to T3)

Question 5

Alanine is a nutritionally non-essential amino acid, from what substrate is it synthesised? What is the amine group donor for the synthesis of alanine?

Answer 5

• Alanine is synthesised from pyruvate via transamination
• unlike the synthesis of glutamine, where NH4+ is the amino group donor, in the synthesis of alanine the amino group donor is either glutamate or aspartate

Question 6

What is the difference between the biosynthesis of asparagine from aspartate and glutamine from glutamate?

Answer 6

In the biosynthesis of glutamine, glutamate is the precursor and the amino group - for glutamine - is provided by NH3; however, in the biosythesis of asparagine, even though aspartate is the precursor, the NH2 donor is glutamine (think of it as though the glutamine is sacrificing itself for asparagine)

Question 7

Serine is synthesised from a glycolysis intermediate, what is it? What are the steps involved in serine biosynthesis?

Answer 7

Serine is synthesised from the glycolysis intermediate 3-phosphoglycerate

The steps involved in serine synthesis are:

• oxidation of glycolytic intermediate 3-phosphoglycerate
• transamination
• hydrolysis of phosphate group

Question 8

Is glycine a nutritionally essential or non-esential amino acid? How is it synthesised in the body?

Answer 8

Glycine is a nutritionally non-essential amino acid.
Glycine can be made from choline (and other sources).

Its synthesis entails the following:

• choline is oxidised to form betain aldehyde
• betaine aldehye then goes on to form betaine
• betaine loses a methyl group, leaving imethylglycine
• imethylglycine loses another methyl group to form sarcosine (N-methylglycine)
• sarcosine loses its final methyl group, giving glycine

Question 9

Proline is a nutritionally essential amino acid. True or False? How is proline synthesised in the body?

Answer 9

False. Proline in a nutritionally non essential amino acid. It is synthesised from glutamate:

1. glutamate is phosphorylated at its y-COOH group, giving glutamate-y-phosphate
2. the phosphate group leaves, and NADPH is oxidised, giving NADP+ and (L)-glutamate semialdehyde
3. the glutamate undergoes cyclisation spontaneously and is reduced, giving (L-) proline

Question 10

From what amino acid is cysteine synthesised from? How?

Answer 10

Cysteine is synthesised from methionine. Its mechanism is as follows:

• homocysteine reacts with serine, forming cystathionine
• the cystathionine is hydrolysed to form cysteine and homoserine