Write all the steps in the mechanism of the transamination reaction of PMP with α-ketoglutarate

Write all the steps in the mechanism of the transamination...

Key Concepts

PLP and PMP Cofactors

Pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP) are essential coenzymes in amino acid metabolism. They participate in transamination reactions by forming reversible covalent bonds with amino acid substrates. The aldehyde group of PLP and the amino group of PMP allow these coenzymes to act as carriers of amino groups, facilitating the transfer process by stabilizing reaction intermediates through resonance and electron delocalization.

Transamination Reaction Mechanism

Transamination involves the transfer of an amino group from an amino donor to an acceptor, typically a keto acid, resulting in the interconversion of amino acids and keto acids. This reaction is central to amino acid metabolism and is catalyzed by PLP-dependent enzymes. The mechanism features a series of nucleophilic attacks, proton transfers, and rearrangements that ensure the amino group is efficiently transferred, preserving the overall configuration and reactivity of the coenzyme.

Schiff Base (Imine) Formation

A key step in the catalytic cycle of PLP-dependent enzymes is the formation of a Schiff base, an imine linkage that forms between the aldehyde group of PLP and an amino group, either from the substrate or from an active site lysine. This reversible imine formation plays a critical role in positioning the substrate for subsequent steps, facilitating electron delocalization, and ultimately leading to the proper alignment for the transfer of the amino group during the reaction process.

Quinonoid Intermediate and Proton Transfer

During the transamination process, formation of a quinonoid intermediate occurs immediately after the initial imine formation and subsequent deprotonation step. This intermediate is characterized by a delocalized negative charge that stabilizes the structure and primes it for further reaction. Critical proton transfer events then convert the quinonoid intermediate into subsequent species, ultimately leading to the regeneration of the reactive coenzyme form and ensuring optimal electronic configurations for product release.

Role of the Enzyme Lysine Residue

The lysine residue in the active site of PLP-dependent enzymes provides a critical nucleophilic amino group that initially forms an internal aldimine with the PLP coenzyme. In the transamination mechanism, this lysine-carried imine bond is broken and reformed, facilitating the transfer of the amino group from PMP to PLP or vice versa. The reversible interaction between the enzyme’s lysine and the coenzyme is essential for properly positioning the coenzyme during catalysis and for promoting subsequent reaction steps such as substrate binding and product release.

Transcript

00:01 Okay, great.

00:03 So we have our plp on the left here, and then we have our alpha -quita gluterate on the right.

00:10 So the first thing we're going to do is remove water from these compounds, and then that's going to be able to join them together.

00:19 So the water is coming from this oxygen here, combining with these two hydrogens here.

00:26 And by doing that, we're going to form a double bond between this nitrogen and this carbon.

00:32 So as you can see, this is the structure that is formed.

00:38 I always like to count my carbons to make sure that i know geometrically which way the compounds have shifted.

00:48 So, for instance, we've shifted this a little bit.

00:50 So i always just like to count one, two, three, four, five.

00:54 Okay, one, two, three, four, five.

00:57 So i can prove to myself that it is the same compound.

01:01 Great.

01:02 So from there, we're just going to shift this double bond around.

01:06 So we're going to use these electrons from the hydrogen.

01:09 They are going to form a double bond between this carbon and the nitrogen.

01:13 And then these electrons are going to be pushed back onto this carbon, which is going to form the following structure.

01:20 So as you can see, we've linked plp with the alpha ketogliterate.

01:24 At this point, the enzyme is going to come in.

01:28 So we'll just say enzyme.

01:33 With the lysine on it.

01:35 So we're going to have an nh2 and a pair of electrons on the nitrogen...

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