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# Glycine Decarboxylase Reaction Mechanism

BIOCHM-JWB4QN

The enzyme glycine decarboxylase (GDC) is found in both plant and animal mitochondria as a complex of 4 different protein subunits (P, H, T, and L). The enzyme is a major mitochondrial protein in many ${ C }_{ 3 }$ plants due to its key role in the photorespiratory carbon salvage pathway.

During photorespiration, the GDC complex catalyzes the oxidative decarboxylation of glycine as shown in Reaction 1 below.

Reaction #1: ${ \text{Glycine}+\;H }_{ 4 }{ \text{folate} \;+\;NAD }^{ + \;}{ \longleftrightarrow 5,10-\text{methylene}-H }_{ 4 }{ \text{folate} \;+\;CO }_{ 2 }{\; +\;NH }_{ 3 }{\; +\;NADH\;+\;H }^{ + }$

Serine hydroxymethyltransferase then catalyzes the transfer of the 1-C unit on tetrahydrofolate (${ H }_{ 4 }{\text{ folate} }$) to a second glycine, producing serine as shown in Reaction 2. The Cs in serine are eventually salvaged and re-enter the Calvin cycle.

Reaction #2: ${ \;2\; \text{Glycine}+NAD }^{ + }{ +\; H }_{ 2 }{ O\longrightarrow \text{Serine} \; +\; }{ CO }_{ 2 }{ \; +\; NH }_{ 3 }{ \; +\; H }_{ 2 }{ O\; +\; NADH\; +\; H }^{ + }$

Consider the reactions above as well as the reaction mechanism schematic below illustrating the role of each of the 4 GDC protein subunits in catalyzing the oxidative decarboxylation of glycine. The 5 steps of the reaction mechanism are numbered 1-5 and the 4 GDC subunits are labeled P, H, T, and L.

Which of the following statements is NOT supported by this scheme and your understanding of photorespiration?

A

A Schiff base forms between pyridoxal phosphate (PLP) on subunit P and glycine in Step 1.

B

Glycine is oxidatively decarboxylated in Step 2 releasing ${ CO }_{ 2 }$. The remaining methylamine group is transferred to reduced lipoic acid on subunit H.

C

In Step 3, the cofactor tetrahydrofolate (${ H }_{ 4 \;}{ \text{folate} }$) is used to release ammonia from the methylamine group. The remaining single C fragment is transferred to ${ H }_{ 4 \;}{ \text{folate} }$.

D

The 1-C fragment associated with ${ H }_{ 4 \;}{ \text{folate} }$ is transferred to a second glycine producing serine (see Reaction #2 above). The subsequent return of the serine Cs to the Calvin cycle represents a salvaging of 25% of the photorespiratory-C generated by RuBisCO fixing 2 ${ O }_{ 2 }$.

E

The 2 sulfhydryl groups of lipoic acid are oxidized in Steps 4 and 5 with the flow of electrons generating ${ NADH }$.