---
title: "Succinyl-Coa Synthetase | Biochem II"
description: "Succinyl-Coa synthetase is the citric acid cycle enzyme that converts succinyl-CoA to succinate while making GTP or ATP in Biological Chemistry II."
canonical: "https://fiveable.me/biological-chemistry-ii/key-terms/succinyl-coa-synthetase"
type: "key-term"
subject: "Biological Chemistry II"
unit: "Unit 2"
---

# Succinyl-Coa Synthetase | Biochem II

## Definition

Succinyl-CoA synthetase is a citric acid cycle enzyme that converts succinyl-CoA to succinate and couples that step to substrate-level phosphorylation, making GTP or ATP in Biological Chemistry II.

## What It Is

Succinyl-CoA synthetase is the citric acid cycle enzyme that takes succinyl-CoA and turns it into succinate while directly generating a nucleotide triphosphate. In Biological Chemistry II, this is one of the clearest examples of substrate-level phosphorylation in central metabolism, because the phosphate is transferred through the enzyme itself rather than through the electron transport chain.

The reaction is reversible and usually written as succinyl-CoA + GDP + Pi ⇌ succinate + CoA + GTP, or the ATP-forming version with ADP. Which nucleotide gets made depends on the enzyme isoform and the tissue context. Mammalian cells commonly have a GTP-forming version in the mitochondria, but some forms can work with ADP to make ATP.

What makes this step special is that it sits in the middle of the cycle right after the highly oxidized, high-energy thioester succinyl-CoA is formed by α-ketoglutarate dehydrogenase complex. Breaking that thioester bond provides enough energy to make a nucleoside triphosphate without needing oxygen directly. The enzyme couples the energy of CoA release to phosphorylation through a short enzyme-bound intermediate, so the chemistry is tightly controlled instead of being a free-floating phosphate transfer.

Mechanistically, this step also resets the cycle for what comes next. Once succinyl-CoA becomes succinate, CoA is recycled, and the carbon skeleton can continue through the later oxidation steps toward oxaloacetate. That makes succinyl-CoA synthetase both an energy-producing enzyme and a housekeeping enzyme for mitochondrial metabolism.

A useful way to think about it is that this is the cycle’s direct cash-out step. Most of the citric acid cycle stores energy in NADH and FADH2, but succinyl-CoA synthetase gives you a small, immediate payoff as GTP or ATP. In the lab or on a problem set, if you see succinyl-CoA disappearing and succinate appearing, this is the enzyme doing the work.

## Why It Matters

Succinyl-CoA synthetase matters because it connects the chemistry of the citric acid cycle to actual energy currency. It is one of the few steps in central metabolism that makes GTP or ATP directly, so it is the cleanest example of substrate-level phosphorylation in this pathway.

That makes it a good checkpoint for understanding how the cycle is organized. The cycle is not just a loop of oxidation reactions. It also contains a step where the cell captures the energy stored in a thioester bond and converts it into a usable phosphate bond. If you can explain why this step can make a nucleotide triphosphate, you are already thinking in the way Biochemical Chemistry II expects.

It also helps you follow metabolic flow. Succinyl-CoA comes from α-ketoglutarate dehydrogenase complex, so this enzyme sits right after a major oxidative decarboxylation. Then it feeds into succinate, which continues through the later stages of the citric acid cycle. That makes the step a bridge between carbon rearrangement, energy capture, and coenzyme A recycling.

This term also shows up when a question wants you to compare sources of ATP or GTP. If the prompt contrasts oxidative phosphorylation with substrate-level phosphorylation, succinyl-CoA synthetase is usually the citric acid cycle example you should name.

## Connections

### Citric Acid Cycle

Succinyl-CoA synthetase is one reaction in the cycle, so you should place it between α-ketoglutarate dehydrogenase complex and succinate. It is easy to locate on a pathway diagram because it is the point where the cycle briefly makes a nucleotide triphosphate instead of just NADH or FADH2.

### GTP

In many mitochondria, succinyl-CoA synthetase makes GTP rather than ATP. That matters because GTP can be used directly in some biosynthetic reactions or converted to ATP by nucleoside diphosphate kinase, so the reaction still feeds the cell’s energy budget.

### Coenzyme A

Coenzyme A is attached to succinyl-CoA before the reaction and released when succinate is formed. Releasing CoA helps drive the reaction forward, and recycling it keeps the cycle moving without wasting the coenzyme.

### [α-ketoglutarate dehydrogenase complex](/biological-chemistry-ii/key-terms/a-ketoglutarate-dehydrogenase-complex)

This complex makes succinyl-CoA, which is the substrate for succinyl-CoA synthetase. The two steps are paired in the cycle, one building the high-energy thioester and the next using that energy to make GTP or ATP.

## On the AP Exam

A quiz question may give you a citric acid cycle diagram and ask which enzyme makes GTP or ATP at the substrate-level phosphorylation step. Your move is to identify succinyl-CoA synthetase and connect it to the conversion of succinyl-CoA to succinate. If the question asks why the step matters, say that it captures energy from a thioester bond and recycles CoA.

In a problem set, you may also need to track where GTP comes from in metabolism and distinguish it from ATP made by oxidative phosphorylation. If a case asks about a mutation in a citric acid cycle enzyme, this is a good place to explain how lowering succinyl-CoA synthetase activity would reduce direct nucleotide triphosphate production and slow cycle throughput. On pathway maps, look for the step where the cycle briefly generates a high-energy phosphate product.

## succinyl-coa synthetase vs substrate availability

Students sometimes mix up the enzyme with the conditions that affect it. Succinyl-CoA synthetase is the catalyst for the succinyl-CoA to succinate step, while substrate availability refers to how much succinyl-CoA or succinate is around to push the reaction. One is the machine, the other is the supply of fuel.

## Key Takeaways

- Succinyl-CoA synthetase converts succinyl-CoA to succinate in the citric acid cycle and directly makes GTP or ATP.
- This is substrate-level phosphorylation, not oxidative phosphorylation, so the phosphate transfer happens through the enzyme reaction itself.
- The step releases CoA and uses the energy in a thioester bond to drive nucleotide triphosphate formation.
- In many mitochondria, the product is GTP, but some forms of the enzyme can make ATP instead.
- If you can place this enzyme between α-ketoglutarate dehydrogenase complex and succinate, you have the pathway logic right.

## FAQs

### What is succinyl-CoA synthetase in Biological Chemistry II?

It is the citric acid cycle enzyme that converts succinyl-CoA into succinate and couples that reaction to the formation of GTP or ATP. This makes it the cycle’s substrate-level phosphorylation step. You usually encounter it when tracing how the cycle captures energy after α-ketoglutarate is converted to succinyl-CoA.

### Does succinyl-CoA synthetase make ATP or GTP?

It can do either, depending on the enzyme isoform. Many mammalian tissues use a GTP-forming version, while some forms can use ADP to make ATP. The bigger idea is that the enzyme makes a nucleoside triphosphate directly from the energy released by succinyl-CoA breakdown.

### How is succinyl-CoA synthetase different from oxidative phosphorylation?

Succinyl-CoA synthetase makes GTP or ATP by substrate-level phosphorylation, which means the phosphate is transferred directly during the enzyme reaction. Oxidative phosphorylation uses the electron transport chain and ATP synthase to make ATP from the proton gradient. They both produce energy currency, but by very different mechanisms.

### Why does succinyl-CoA synthetase matter in the citric acid cycle?

It is the step where the cycle makes a direct energy payoff and recycles CoA. That means it links carbon flow, coenzyme A chemistry, and nucleotide triphosphate production in one reaction. If a diagram or question asks where the cycle makes GTP, this is the enzyme to name.

## Related Study Guides

- [2.3 Citric acid cycle: reactions and regulation](/biological-chemistry-ii/unit-2/citric-acid-cycle-reactions-regulation/study-guide/DMxQp2F02A0OlreC)

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