Biochem Exam 4B - Amino Acid Metabolism

Transamination mechanism

-First phase of the reaction begins with pyridoxal form of enzyme and alpha-amino substrate. It ends with formation of the alpha-keto product and the pyridoxamine form of the enzyme.  -In the second phase, the pyridoxamine form of the enzyme reacts with the alpha-keto substrate to generate the new alpha amino acid and return enzyme to pyridoxal form.

How would you dispose of the 'waste' ammonia? (Ammonia itself is a neurotoxin.)

-Urea Cycle -Free ammonia gets converted to urea and secreted

Synthesis of Glutamate, Alanine, and Aspartate are ___-step conversions

1

Amino acid biosynthetic families

1. Glutamate family 2. Aspartate family 3. Aromatic family 4. Serine family 5. Pyruvate family 6. Histidine family -Amino acids can be arranged in families based on the origin of the carbon backbone

Transamination mechanism is divided into ___ stages.

2

Synthesis of serine is a ___-step conversion

3

Keto-acid Amino-acid "couples"

Acceptor Keto Acid Donor Amino Acid *Figure shows 1-step conversions

Generic transamination scheme

Acceptor α-keto acid + Donor α-amino acid <--> New α-amino acid + new α-keto acid

Donor amino acid of acceptor keto acid, pyruvate

Alanine

Amino acid is divided into origin of ___ and origin of ___.

Amino acid is divided into origin of amino group and origin of carbon backbone (aka skeleton).

Donor amino acid of acceptor keto acid, oxaloacetate

Aspartate

What is the origin of the carbon backbone in amino acids?

Comes out of glucose metabolism

What enters and exits the urea cycle?

Enters: Carbamoyl phosphate & aspartate (carries in the two amino groups) Exits: Urea (main player) & fumurate (other major metabolite) *Figure: Urea-TCA bicycle (deriving energy from excess amino acids and disposing of waste ammonia)

Cannot make these amino acids, so need to eat them

Essential amino acids -Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine *Whenever making proteins, need all 20 amino acids. Essential refers to nutritionally.

What is the origin of the amino group in amino acids?

Free ammonia (glutamate and glutamine) or Transamination (all the rest)

Synthesis of glutamine

Free ammonia + glutamate + ATP <-glutamine synthetase-> Glutamine + water + ADP + Pi -ATP-dependent reaction -Glutamate becomes glutamine with ATP-dependent addition of ammonia by glutamine synthetase.

Synthesis of glutamate (aka glutamic acid)

Free ammonia + α-ketoglutarate + NADPH + H+ <-glutamate dehydrogenase-> glutamate + water + NADP+ -Reductive amination reaction -α-keto group --> α-amino acid -NADPH (or NADH, depending on source of enzyme) drives reaction -α-ketoglutarate is the substrate which accepts the ammonium ion to make glutamate in a reaction catalyzed by glutamate dehydrogenase.

Origin of amino group in glutamate and glutamine

Free ammonia, NH4+

Donor amino acid of acceptor keto acid, α-ketoglutarate

Glutamate

Transamination

Glutamate and glutamine are the primary nitrogen donor molecules, transferring the α-amino group of glutamate and the side chain amino group of glutamine to other molecules in a transamination reaction. -movement of the amino group from one amino acid to another

In general, people (non-vegetarians) take in more amino acids from protein than needed to make new proteins. What happens to the excess?

It is metabolized

We can make these amino acids, so do not need to eat them

Non-essential amino acids -Alanine, Arginine, Asparagine, Aspartate, Cysteine, Glutamate, Glutamine, Glycine, Histidine, Proline, Serine, Tyrosine *Whenever making proteins, need all 20 amino acids. Non-essential refers to nutritionally.

How would you derive energy from those excess amino acids?

Origin of carbon backbone goes back to respective glucose metabolism where it generates energy -Figure: With α-amino group removed from the amino acid, the carbon backbone can enter metabolism, directly or indirectly 'feeding' the TCA cycle to generate energy

Precursor for α-amino acid is always a/an ___.

Precursor for α-amino acid is always an α-keto acid.

Transamination of pyruvate family

Pyruvate + Glutamate <-> Alanine + α-ketoglutarate

Players in transamination of pyruvate family

Pyruvate = Acceptor keto acid Glutamate = Donor amino acid Alanine = New Amino acid α-ketoglutarate = New keto acid product

Synthesis of Serine

Requires 3 steps from a glycolytic intermediate 1. glycerate 3-phosphate + NAD+ --dehydrogenase--> 3-phosphohydroxypyruvate + NADH + H+ 2. 3-phosphohydroxypyruvate + Glutamate --aminotransferase--> 3-phosphoserine + α-ketoglutarate 3. 3-phosphoserine + H2) --phosphatase--> Serine + Pi

The intermediate in glucose metabolism that is the starting point for Histidine family

Ribose-5-phosphate (from pentose phosphate pathway)

Why does serine synthesis require more than one step?

Step 1. Need to generate ketone to accept amino group Step 2. Transamination requires to make amino acid (not yet an amino acid at this step) Step 3. Complete synthesis by hydrolyzing phosphate

How is the amino group removed from the amino acid?

The glutamate dehydrogenase reaction (reductive amination) is fully reversible and thus can generate free ammonia from oxidation of glutamate.

What happens after a large amount of protein consumption?

The proteins have been broken down by the digestive system into amino acids that were absorbed by the small intestine and transported to the blood.

Why are amino acids least important when generating energy?

Whatever energy we do get from an amino acid, some of it has to be reinvested immediately to eliminate ammonia.

Specific enzyme required for transamination

aminotransferase or transaminases (old nomenclature)

Enzyme in synthesis of glutamate

glutamate dehydrogenase *Reductive amination reaction

Enzyme in synthesis of glutamine

glutamine synthetase *ATP-dependent reaction

The intermediate in glucose metabolism that is the starting point for Serine family

glycerate-3-phosphate (from 2nd phase of glycolysis)

Acceptor keto acid of donor amino acid, aspartate

oxaloacetate

The intermediate in glucose metabolism that is the starting point for Aspartate family

oxaloacetate (from TCA cycle)

The intermediate in glucose metabolism that is the starting point for Aromatic family

phosphoenolpyruvate (from 2nd phase of glycolysis)

Coenzyme to amino transferases

pyridoxal-5'-phosphate (vitamin B6)

Acceptor keto acid of donor amino acid, alanine

pyruvate

The intermediate in glucose metabolism that is the starting point for Pyruvate family

pyruvate (from glucose - glycolysis)

Acceptor keto acid of donor amino acid, glutamate

α-ketoglutarate

The intermediate in glucose metabolism that is the starting point for Glutamate family

α-ketoglutarate (from TCA cycle)