Lect 20. NH3 uptake and aa synthesis

The α-amino and α-keto molecules can be converted to each other if the functional groups on the _____ are exchanged

α-carbons

What doe Transaminases do?

They catalyze the exchange of the functional groups on the α-carbon

An α-keto acid has a____ on the alpha carbon

A keto group (O=)

An α-amino acid has a____ on the alpha carbon

An amino group (H3N+)

Deamination of α-amino and amination of α-keto acids

Transaminase enzymes catalyze the transfer of an amino group from the α-carbon on an amino acid to the α-carbon of an α-keto acid forming an α-amino acid.

The molecule that gains the amino group is ____

Aminated

The molecule that loses the amino group is

Deaminated

The transfer of amino groups for the formation of amino acids almost always uses ___ as the source of the amino group

glutamate

What does this picture show

Formation of Alanine from its α-keto acid counterpart

Explain what is happening here

The amino group on glutamate, (an α-amino acid), can be transferred to an accepting α-keto acid (pyruvate) forming alanine (an α-amino acid) and α-ketoglutarate (an α-keto acid).

How do special bacteria (not plants)keep a high level of glutamate to transfer amino groups into biological pathways

When the amino group on glutamate is used to form an amino acid in a transaminase reaction it regenerates α-ketoglutarate which can now pick up another ammonia thereby regenerating glutamate which can shuttle the amino group to another α-keto acid.

How are glutamate levels kept high to perform both of its NH3 assimilation functions in plants? Because if glutamate is used for incorporating NH3, it cannot be used to donate its amino group to α-keto acids

Reductive amination reaction that is catalyzed by Glutamate Synthase which transfers the amino nitrogen from glutamine to α-ketoglutarate to generate 2 molecules of glutamate

The reductive amination reaction catalyzed by Glutamate Synthase

How does the amination reaction catalyzed by Glutamate Synthase maintain high levels of glutamate for both of its ammonia assimilation functions

1. Glutamate assimilates ammonia to form glutamine.

2. Two glutamate molecules are formed from the reductive amido transfer between α-ketoglutarate and glutamine.

3a. Glutamate (a) can assimilate another ammonia

3b. Glutamate (b) can transfer an amino group to an α-keto acid to form a different amino acid which regenerates α-ketoglutarate.

How many of the amino acids can be derived from 2 TCA cycle intermediates?

The glycolysis intermediate 3-phosphoglycerate can synthesize ___

Serine and glycine

Steps to get 3-phosphoglycerate to serine

1. get rid of the hydroxyl on the α-carbon and replace it with an amino group,

2. remove the phosphoryl on carbon 3.

What does this photo show?

The first step to get from 3-phosphoglycerate to serine

What is the first step to get from 3-phosphoglycerate to serine

oxidize the α-carbon hydroxyl to a ketone.

Once we have the ketone on the α-carbon we have an α- keto acid to which we can add the amino group using a transaminase (phosphoserine transaminase).

The transaminase used to add the amino group to the α-carbon in the first step of Serine and Glycine synthesis

phosphoserine transaminase.

What does this reaction(s) show?

The second step to get from 3-phosphoglycerate to serine and Glycine

What occurs in the second step To get from 3-phosphoglycerate to serine to get from 3-phosphoglycerate to serine

The removal of the phosphate on carbon 3

Which enzyme catalyzes the reaction from 3-phosphoserine to serine and what is released?

Enzyme: Phosphoserine phosphatase

Released: H2O to Pi

How to get from serine to glycine

By replacing the methanol (hydroxymethyl) group on the α-carbon with a proton

Enzyme that catalyzes the conversion of serine to glycine

Serine hydroxymethyl-transferase

Takes folate and releases folate-CH2OH as a result

Which amino acids can be synthesized from oxaloacetate (a TCA intermediate)

Aspartate and Asparagine

Is oxaloacetate an α-keto acid or a α-anmino acid?

It is an α-keto acid

Which amino acid is the α-amino acid counterpart to oxaloacetate?

Aspartate

Which enzyme is used to catalyze the reaction of oxaloacetate to Aspartate

Aspartate transaminase

Uses glutamate (it is the amino donor to the α-keto) which is then turned to α-ketoglutarate as a result

What enzyme is used to convert Aspartate to asparagine?

Asparagine synthetase

-the energy from the hydrolysis of ATP to AMP + 2Pi is used by the enzyme to transfer an amido group from glutamine to Aspartate to form asparagine

The three different scenarios in animals that protein degradation occurs

1. protein-rich diet supplies amino acids in excess of protein synthesis requirements—amino acids are not stored;

2. normal cellular protein turnover—many of the proteins are only required for a brief period of time, therefore once a protein is no longer required, it is identified as requiring breakdown;

3. starvation—protein will be broken down into TCA intermediates to provide energy or precursors for gluconeogenesis. Regardless of the source, amino acids are generally sent to the liver for degradation.

What two pathways should you consider when breaking down amino acids

1. Fate of α-amino groups—they have to be removed and then collected for subsequent degradation and elimination;

2. Fate of carbon skeleton—which are α-keto acids.

The first step of the breakdown of amino acids is to remove ____

the α-amino group from the α-carbon of the amino acid

The two principal stages for the removal of amino groups

1. Transamination reactions

2. Delaminating of glutamate

Transamination reactions

these reactions function to collect amino groups into a pool of glutamate molecules.

Delaminating of glutamate

-functions to funnel the gathered amino groups into the urea cycle

-if the ammonia derived from the removal of amino groups is not required it must be excreted because ammonia is toxic to an organism (unless a specialized bacteria).

Stage 1 transaminase reactions

Transfer amino groups from amino acids to α-ketoglutarate forming a pool of glutamate.

Stage 2: Delaminating of glutamate

Once we have the pool of glutamate molecules in the liver, the amino group can be removed by glutamate dehydrogenase to generate an α-ketoglutarate and ammonia.

Summary of flow of amino groups from amino acids to urea cycle

What happens to carbon skeletons

They are converted into molecules central to metabolism:

1. α-ketoglutarate

2. succinyl CoA

3. fumarate

4. oxaloacetate

5. pyruvate

6. acetyl CoA

The degradation of the majority of amino acids is ____ i.e they can directly supply guconeogenesis pathways

Glucogenic

The amino acid products that are ketogenic can be used to generate

Ketone bodies or acetyl CoA for energy production