Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate.

To identify the step in the tricarboxylic acid (TCA) cycle (also known as the Krebs cycle or citric acid cycle) that does not involve the oxidation of a substrate, let us analyze the chemical transformations and energy-coupling events occurring in each of the given options:

1. Malic acid → Oxaloacetic acid:
In this step, malate is oxidized to oxaloacetate by the enzyme malate dehydrogenase. This reaction involves the transfer of electrons to ${\text{NAD}}^{+}$, reducing it to $\text{NADH}+{\text{H}}^{+}$. Since the substrate (malic acid) loses hydrogen/electrons, it undergoes oxidation.

2. Succinic acid → Malic acid:
This transformation occurs via two steps: first, succinate is oxidized to fumarate by succinate dehydrogenase (where $\text{FAD}$ is reduced to ${\text{FADH}}_2$), followed by hydration of fumarate to malate by fumarase. Because the conversion from succinic acid to malic acid includes a major oxidation step, it involves substrate oxidation.

3. Succinyl-CoA → Succinic acid:
In this reaction, the high-energy thioester bond of succinyl-CoA is cleaved by succinyl-CoA synthetase (also known as succinate thiokinase) to form succinate. This reaction is coupled with the phosphorylation of GDP to GTP (or ADP to ATP) via substrate-level phosphorylation. There is no transfer of electrons or loss of hydrogen atoms from the carbon skeleton of the substrate to an electron carrier (like ${\text{NAD}}^{+}$ or $\text{FAD}$). Thus, this step is a simple cleavage/hydrolysis coupled to phosphorylation, and it does not involve the oxidation of the substrate.

4. Isocitrate → α-ketoglutaric acid:
In this step, isocitrate undergoes oxidative decarboxylation catalyzed by isocitrate dehydrogenase to form α-ketoglutarate. The reaction involves the reduction of ${\text{NAD}}^{+}$ to $\text{NADH}+{\text{H}}^{+}$, which means the substrate is oxidized.

Therefore, the conversion of Succinyl-CoA to Succinic acid is the only step among the options that does not involve oxidation of the substrate.