Lecture 36

Identify the important roles of Glutamate (Glu) and Glutamine (Gln) as nitrogen (N) donors in biosynthetic reactions.
Understand the biosynthesis pathways of the amino acids: Alanine (Ala), Aspartate (Asp), Asparagine (Asn), Serine (Ser), Glycine (Gly), Cysteine (Cys), Methionine (Met), and S-adenosyl-Methionine (SAM).
Explain the role of tetrahydrofolate (THF) in carrying one-carbon (C1) units for biosynthetic reactions.
Explain the function of S-adenosyl methionine (SAM) in methylation reactions, including how homocysteine is recycled back into methionine and the two possible fates of homocysteine.
Identify the coenzymes utilized by methionine synthase and the source of the one-carbon unit transferred in this enzymatic reaction.
Understand the defect in congenital hyperhomocystinuria.
Know the biosynthesis pathway of aromatic amino acids, specifically chorismate.
Explain the biochemical basis for phenylketonuria (PKU).
Identify the amino acids that serve as precursors for the synthesis of neurotransmitters: histamine, serotonin, and catecholamines.

Glutamate:
- The α-amino group of Glutamate serves as the source of the α-amino group in most amino acids.
- This reaction requires the action of aminotransferases, which utilize pyridoxal phosphate (PLP) as a cofactor.
Glutamine:
- Donates its side-chain nitrogen (amide nitrogen) in the biosynthesis of a variety of compounds.
- Specific reactions are catalyzed by glutamine amidotransferases.

Mechanism involves two domains of the enzyme:
1. Hydrolysis of the amide bond in the side-chain of glutamine, releasing ammonia (NH3).
2. Ammonia is then channeled through a conduit to prevent loss before it reacts with the acceptor substrate.

Most bacteria and plants can synthesize all 20 common amino acids.
Mammals are capable of synthesizing about half of these amino acids; the remainder must be obtained from dietary sources.
Notably, Threonine was the last of the common amino acids to be discovered, at the University of Illinois by Prof. William Rose and colleagues.

Intermediates from various pathways play significant roles in amino acid biosynthesis:
- Glycolysis: Key intermediates include 3-phosphoglycerate, phosphoenolpyruvate (PEP), and pyruvate.
- Pentose Phosphate Pathway: Important intermediates are erythrose-4-phosphate and ribose-5-phosphate.
- TCA Cycle: Notable intermediates include oxaloacetate and α-ketoglutarate.

Alanine Synthesis:
- Reaction: [
  ext{Pyruvate} + ext{Glutamate} \xrightarrow{PLP, \text{aminotransferase}} ext{Alanine} + \text{α-Ketoglutarate}
  ]
Aspartate Synthesis:
- Involves transamination using Glutamate as an amino group donor, similar to alanine synthesis.
Asparagine Synthesis:
- Formed by the amidation of Aspartate, catalyzed by Asparagine synthetase, with ATP used in the reaction.

Serine from 3-Phosphoglycerate:
- Involves dehydrogenation, transamination and subsequent phosphorylation.
Glycine from Serine:[
ext{Serine} \xrightarrow{\text{serine hydroxymethyl transferase}} ext{Glycine} + ext{THF}
]
Enzyme requires cofactor pyridoxal phosphate (PLP).

THF carries one-carbon units in various oxidation states, crucial for biosynthetic pathways:
- N5,N10-methylene THF: Carries groups in methionine metabolism and nucleotide synthesis.
- N10-formyl THF: Involved in the conversion of Uridine to Thymidine and in purine biosynthesis.

Glycine can also be synthesized from CO2 and NH4+ through action of glycine synthase.
The methyl group donor in this case is N5,N10-methylenetetrahydrofolate.

Homocysteine:
- Originates from methionine metabolism, which is an essential amino acid.
Enzymatic reactions are similar to those of cysteine synthesis and also involve PLP as a cofactor.

SAM: Serves as a methyl donor in numerous significant methylation reactions such as:
- Methylation of DNA, RNA, phospholipids, and proteins.

Methionine synthase is among the two human enzymes that use coenzyme B12 (from vitamin B12).
The active form of coenzyme B12 utilized is methylcobalamin.

Methionine (Met) is essential and participates in various biosynthetic processes.
Efficient recycling of S-adenosylhomocysteine (SAH) back to methionine is vital for metabolic homeostasis.
Deficiency in the enzymes of this pathway or in coenzyme B12 can lead to hereditary diseases in humans.

Homocystinuria and Hyperhomocysteinemia:
- Result from deficiencies in critical boxed enzymes.
- Can also result from vitamin deficiencies (B6, folate, B12).
Associated health problems include accelerated atherosclerosis and thrombosis.

Three non-essential and six essential amino acids are derived from oxaloacetate and pyruvate.

Chorismate:
- A key intermediate in the synthesis of aromatic amino acids in bacteria and plants, derived from:
  - 2 molecules of PEP and 1 molecule of erythrose-4-P.

Requires Glutamine’s amido-NH (source for N in Tryptophan’s indole ring) and 2-carbons from PRPP along with Serine contributions.

Involves a series of enzyme-catalyzed reactions:
1. Chorismate mutase: Converts chorismate to prephenate.
2. Prephenate dehydrogenase: Converts prephenate to 4-hydroxyphenylpyruvate.
3. Prephenate dehydratase: Converts prephenate to phenylpyruvate.
Ultimately leads to the synthesis of Tyrosine and Phenylalanine, where:
- Tyrosine is conditionally essential as it depends on phenylalanine intake.