Lecture 16 - Amino Acids as Biosynthetic Precursors

Synthase vs. Synthetase

Synthetases: use ATP in the reaction

Synthases: do not use ATP

What is the goal of the one-carbon and methyl cycle?

To produce methyl groups for the various methylation reactions in the body

One-carbon cycle overview

- serine to glycine

- 5,10-methylene-THF reduction

- Transfer of the methyl group from 5-methyl-THF to homocysteine

Serine to glycine conversion enzyme

SHMT

- transfers hydroxymethyl group from serine

Does SHMT have catalytic activity?

No

What does SHMT rely on?

it relies on its cofactors, PLP and tetrahydrofolate (THF)

Role of PLP in SHMT activity

PLP takes the hydroxymethyl away from the serine forms formaldehyde (short-lived) and glycine

Role of THF in SHMT activity

Stores hydroxymethyl group after PLP has taken it off

Methyl group stored b/w N5 and N10 of THF

Reduction of 5, 10-methylene-THF

- Forms 5-methyl-THF

'Cut' a bond by reducing it and changes the cyclo- group to a methyl group

Reduction of 5,10-methylene THF enzyme

MTHFR

Where do the electrons come from for the reduction of 5,10-methylene THF?

Electrons for this reduction come from NADH that passes it to FADH2 that passes it to MTHFR

FAD is bound to MTHFR

After 5,10-methylene-THF is reduced...?

its methyl groups is transferred to homocysteine

Methyl cycle overview

- Homocysteine to methionine

- Formation of S-Adenosyl Methionine (SAM)

- S-Adenosyl homocysteine recycling

Homocysteine to methionine conversion

5-methyl-THF transfers a methyl group to homocysteine to form methionine.

Once 5-methyl-THF loses its methyl group it becomes..?

it becomes THF

Homocysteine to methionine enzyme

methionine synthase

What coenzyme is required for the conversion of homocysteine to methionine?

Coenzyme B12 (vitamin B12)

Where can we get vitamin B12

from a balanced diet

- Vitamin B12 is mainly found in dairy products, eggs, meat, fish and seafood

S-Adenosyl methionine (SAM) synthesis

Condensation b/w methionine and 5' carbon of a ribose to create SAM

- Frees 3 phosphate groups (pyrophosphate + ppi)

SAM synthesis enzyme

Methionine adenosyl transferase

What is the purpose of SAM?

SAM is the major methyl donor in many transmethylation reactions

What reactions in our body use SAM?

All methylation reactions in our body

S-adenosyl-homocysteine recycling

S-adenosyl-homocysteine is hydrolyzed into homocysteine

Hyperhomocysteinemia (the methyl 'trap')

an accumulation of homocysteine because we can’t convert methionine to homocysteine w/o coenzymes

What causes hyperhomocysteinemia

low levels of:

Vitamin B12

Folate (THF)

Vitamin B6!!!

Risks associated with hyperhomocysteinemia

- Cardiovascular risks

- Neuropsychiatric illness (Alzheimer's schizophrenia, etc.)

Folate and B12 in pregnancy

Canadian women are given folic acid supplements during pregnancy to reduce the risk of neural tube defects (NTDs).

ex: Spina Bifida

MTHFR deficient alleles

Reduced methylation efficiency overall

high conc Serine regulation of pyruvate kinase (PK)

Serine is a positive allosteric regulator of pyruvate kinase

It increases the rate of glycolysis and shutdown the synthesis of serine.

Low conc Serine regulation of pyruvate kinase (PK)

the activity of PK will decrease leading to an increase of serine biosynthesis

How does inhibiting PK, promote serine synthesis?

When you inhibit PK, all the intermediates of glycolysis build up and the increased 3PG can go towards serine biosynthesis.

Serine regulation of PK is...

Completely independent of glucagon and insulin

Serine, glycine and cancer

In many cancers, the allosteric control of serine over PK is lost

Regardless of serine concentration, PK will be inhibited and serine biosynthesis will go on uncontrolled which allows for the uncontrolled proliferation of cancer cells.

Role of methylation

fine tune transcription

Example of methylation

X-chromosome inactivation

Methylation of histone proteins occurs at which amino acid residues?

Occurs at Arg or Lys residues in the protein sequence

How can Arg be methylated?

Arg can be methylated once (monomethylated) or twice (asymmetrically or symmetrically dimethylated)

How can Lys be methylated?

Lys can be methylated once, twice, or three times

Deregulation of DNA methylation was shown to play a role in...?

- Cancer genesis

- Ageing

Amino acids can also be precursors to or are...?

neurotransmitters

Excitatory neurons

cause other neurons to fire

Inhibitory neurons

prevent other neurons from firing

Amino acid neurotransmitters include...?

glycine, aspartate, and glutamate

Neurotransmitters derived from amino acids include...?

dopamine

GABA

Histamine

Serotonine

Which of the amino acid derived NTs are excitatory?

Aspartate

Glutamate

Histamine

Norepinephrine

Epinephrine

Which of the amino acid derived NTs are inhibitory?

GABA

Serotonin

Glycine

Which of the amino acid derived NTs are both excitatory and inhibitory?

L-Dopa

Dopamine

Tryptophan as a precursor of neurotransmitters

produces serotonin

Serotonin functions

Feel good neurotransmitter

regulation of intestine movement

Tyrosine as a precursor of neurotransmitters

produces..

- L-Dopa

- Dopamine

- Norepinephrine

- Epinephrine

Dopamine functions

Low levels of dopamine will affect your mood and energy level

The loss of 80% of dopaminergic neurons in the brain substantia nigra leads to development of Parkinson disease symptoms

What NTs are also hormones?

Epinephrine

Norepinephrine

Epinephrine & norepinephrine as neurotransmitters

Limited functions as NTs

Anxiety

Sleep

Metabolism

Histidine as a precursor of neurotransmitters

produced histamine

Glutamate as a precursor of neurotransmitters

produces

- Glutamate

- GABA

Glycine as a precursor of neurotransmitters

produces glycine

Aspartate as a precursor of neurotransmitters

produces aspartate