---
title: "Malate Dehydrogenase | Biochem II"
description: "Malate dehydrogenase catalyzes malate to oxaloacetate using NAD+ to make NADH, linking the citric acid cycle and mitochondrial shuttles in Biochemical Chemistry II."
canonical: "https://fiveable.me/biological-chemistry-ii/key-terms/malate-dehydrogenase"
type: "key-term"
subject: "Biological Chemistry II"
unit: "Unit 2"
---

# Malate Dehydrogenase | Biochem II

## Definition

Malate dehydrogenase is an enzyme in Biological Chemistry II that reversibly converts malate to oxaloacetate while reducing NAD+ to NADH. It connects citric acid cycle chemistry with the malate-aspartate shuttle.

## What It Is

Malate dehydrogenase is the enzyme that turns malate into oxaloacetate and back again, using the NAD+/NADH pair to move electrons. In Biological Chemistry II, you see it as a reversible redox enzyme, not just a name on a pathway diagram. The same chemistry appears in two places, the mitochondrial matrix and the cytosol, which is why this enzyme matters in both energy production and metabolite shuttling.

In the citric acid cycle, malate dehydrogenase sits at the end of the pathway and catalyzes the oxidation of malate to oxaloacetate. That product, oxaloacetate, is immediately used by citrate synthase to start the next turn of the cycle. So even though the reaction is reversible and has an unfavorable standard free energy, the cell keeps it moving forward by rapidly using oxaloacetate as soon as it is made.

That detail is easy to miss: the enzyme does not act alone in a test tube, it works inside a system with linked reactions, concentration gradients, and compartmentalization. The low cellular concentration of oxaloacetate pulls the reaction forward, which is a good example of how metabolism is controlled by mass action as much as by enzyme identity. If your professor asks why the step can still happen, the answer is not that the enzyme makes it magically favorable, but that the cell couples it to the rest of metabolism.

Malate dehydrogenase also shows up in the malate-aspartate shuttle. In that shuttle, cytosolic oxaloacetate is reduced to malate, which can cross the inner mitochondrial membrane. Once inside, mitochondrial malate dehydrogenase converts malate back to oxaloacetate and produces NADH in the matrix. That is how reducing power from cytosolic NADH gets transferred into mitochondria without NADH itself crossing the membrane.

Because the enzyme exists in both compartments, it helps the cell coordinate redox balance and carbon flow. The mitochondrial form supports energy metabolism, while the cytosolic form helps manage electron transfer between compartments. When you see malate dehydrogenase in a pathway map, think of it as a redox relay point that ties together the citric acid cycle, shuttle systems, and cellular NADH balance.

## Why It Matters

Malate dehydrogenase matters because it sits at the intersection of energy extraction and electron transfer. In the citric acid cycle, it finishes the cycle by regenerating oxaloacetate, which lets acetyl-CoA keep entering the pathway. If this step stalls, the whole cycle slows because citrate synthase no longer has enough oxaloacetate to begin another round.

It also gives you a clean example of how Biological Chemistry II treats metabolism as a network instead of a list of isolated reactions. The enzyme is part of the citric acid cycle, but it also supports the malate-aspartate shuttle, so one protein concept can show up in two different topic areas. That is exactly the kind of connection instructors like to ask about in short-answer questions, pathway tracing, and problem sets.

Malate dehydrogenase is also useful for reasoning about thermodynamics in cells. The reaction is unfavorable under standard conditions, but real cells push it in the needed direction by coupling it to downstream use of oxaloacetate and by controlling metabolite concentrations. That makes it a strong example of why standard free energy does not tell the whole story.

If you understand this enzyme, you can explain why NADH production is linked to mitochondrial electron transport, why shuttles exist at all, and how compartment-specific enzyme forms support metabolism in different parts of the cell.

## Connections

### Citric Acid Cycle

Malate dehydrogenase catalyzes the last step of the citric acid cycle by converting malate to oxaloacetate. That step regenerates the four-carbon acceptor needed for the next turn of the cycle, so it directly affects cycle continuity. When you trace the pathway, this enzyme sits right before citrate synthase uses oxaloacetate again.

### NADH

This enzyme creates NADH when it oxidizes malate to oxaloacetate. That NADH is one of the main energy carriers produced by the citric acid cycle, and it feeds electrons into later ATP-producing steps. In shuttle questions, NADH is also the whole reason the malate-aspartate shuttle exists.

### [Malate-Aspartate Shuttle](/biological-chemistry-ii/key-terms/malate-aspartate-shuttle)

Malate dehydrogenase is one of the core enzymes in this shuttle because it interconverts malate and oxaloacetate across compartments. The shuttle uses that chemistry to move reducing equivalents from the cytosol into the mitochondrial matrix. If you are mapping the shuttle, this is the redox step that makes transfer possible.

### Aspartate Aminotransferase

Aspartate aminotransferase works with malate dehydrogenase in the malate-aspartate shuttle. When oxaloacetate cannot cross the inner mitochondrial membrane, it is converted through a linked transamination step so the carbon skeleton can keep moving. The two enzymes act as a pair to move both electrons and carbon between compartments.

## On the AP Exam

A quiz question might ask you to identify the enzyme that converts malate to oxaloacetate and produces NADH, then explain why the step still runs even though it is unfavorable under standard conditions. In pathway problems, you may need to place malate dehydrogenase at the end of the citric acid cycle or inside the malate-aspartate shuttle and explain what crosses the mitochondrial membrane. If you get a short-answer or case prompt, look for clues about redox balancing, compartmentation, or regenerating oxaloacetate. A good answer usually connects the enzyme to both NADH production and metabolic flux, not just memorization of the reaction name.

## Key Takeaways

- Malate dehydrogenase converts malate to oxaloacetate and uses the NAD+/NADH pair as part of a reversible redox reaction.
- In the citric acid cycle, it regenerates oxaloacetate so the pathway can keep turning.
- In the malate-aspartate shuttle, it helps move reducing equivalents into the mitochondrial matrix without moving NADH directly.
- The reaction is unfavorable under standard conditions, but cells drive it forward by using oxaloacetate quickly and keeping metabolite levels balanced.
- Because it exists in mitochondrial and cytosolic forms, this enzyme links compartment-specific metabolism to overall cellular energy flow.

## FAQs

### What is malate dehydrogenase in Biological Chemistry II?

Malate dehydrogenase is the enzyme that interconverts malate and oxaloacetate while coupling the reaction to NAD+/NADH. In Biological Chemistry II, it shows up in the citric acid cycle and the malate-aspartate shuttle. It is a good example of how one enzyme can support both energy production and redox transfer.

### Why is the malate dehydrogenase reaction unfavorable?

The malate to oxaloacetate conversion has an unfavorable standard free energy, so it does not look like an easy forward reaction on paper. Cells still make it proceed by keeping oxaloacetate levels very low, since oxaloacetate is quickly used in the next step of the citric acid cycle or in shuttle reactions. That coupling is the real reason the step works.

### How does malate dehydrogenase work in the malate-aspartate shuttle?

The shuttle uses malate dehydrogenase to convert oxaloacetate to malate in one compartment and then back to oxaloacetate in another. That lets reducing equivalents move across the mitochondrial membrane indirectly, since NADH itself cannot cross. The enzyme is part of the redox transfer system, not just a standalone metabolic step.

### How is malate dehydrogenase different from aspartate aminotransferase?

Malate dehydrogenase performs a redox reaction, moving electrons between malate and oxaloacetate with NAD+/NADH. Aspartate aminotransferase does a transamination reaction, moving an amino group between amino acids and keto acids. They work together in the malate-aspartate shuttle, but they are doing different chemistry.

## Related Study Guides

- [2.3 Citric acid cycle: reactions and regulation](/biological-chemistry-ii/unit-2/citric-acid-cycle-reactions-regulation/study-guide/DMxQp2F02A0OlreC)
- [6.5 Mitochondrial transport and shuttles](/biological-chemistry-ii/unit-6/mitochondrial-transport-shuttles/study-guide/wNzLelorCsh0ZSDb)

## About This Document

Canonical Fiveable pages are available as Markdown at the same path plus `.md`.

- [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns
- [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing
- [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`)
- [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`)

## Structured Data

```json
{"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/biological-chemistry-ii/key-terms/malate-dehydrogenase#resource","name":"Malate Dehydrogenase | Biochem II","url":"https://fiveable.me/biological-chemistry-ii/key-terms/malate-dehydrogenase","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/biological-chemistry-ii/key-terms/malate-dehydrogenase#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:20:55.189Z","isPartOf":{"@type":"Collection","name":"Biological Chemistry II Key Terms","url":"https://fiveable.me/biological-chemistry-ii/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/biological-chemistry-ii/key-terms/malate-dehydrogenase#term","name":"Malate dehydrogenase","description":"Malate dehydrogenase is an enzyme in Biological Chemistry II that reversibly converts malate to oxaloacetate while reducing NAD+ to NADH. It connects citric acid cycle chemistry with the malate-aspartate shuttle.","url":"https://fiveable.me/biological-chemistry-ii/key-terms/malate-dehydrogenase","inDefinedTermSet":{"@type":"DefinedTermSet","name":"Biological Chemistry II Key Terms","url":"https://fiveable.me/biological-chemistry-ii/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is malate dehydrogenase in Biological Chemistry II?","acceptedAnswer":{"@type":"Answer","text":"Malate dehydrogenase is the enzyme that interconverts malate and oxaloacetate while coupling the reaction to NAD+/NADH. In Biological Chemistry II, it shows up in the citric acid cycle and the malate-aspartate shuttle. It is a good example of how one enzyme can support both energy production and redox transfer."}},{"@type":"Question","name":"Why is the malate dehydrogenase reaction unfavorable?","acceptedAnswer":{"@type":"Answer","text":"The malate to oxaloacetate conversion has an unfavorable standard free energy, so it does not look like an easy forward reaction on paper. Cells still make it proceed by keeping oxaloacetate levels very low, since oxaloacetate is quickly used in the next step of the citric acid cycle or in shuttle reactions. That coupling is the real reason the step works."}},{"@type":"Question","name":"How does malate dehydrogenase work in the malate-aspartate shuttle?","acceptedAnswer":{"@type":"Answer","text":"The shuttle uses malate dehydrogenase to convert oxaloacetate to malate in one compartment and then back to oxaloacetate in another. That lets reducing equivalents move across the mitochondrial membrane indirectly, since NADH itself cannot cross. The enzyme is part of the redox transfer system, not just a standalone metabolic step."}},{"@type":"Question","name":"How is malate dehydrogenase different from aspartate aminotransferase?","acceptedAnswer":{"@type":"Answer","text":"Malate dehydrogenase performs a redox reaction, moving electrons between malate and oxaloacetate with NAD+/NADH. Aspartate aminotransferase does a transamination reaction, moving an amino group between amino acids and keto acids. They work together in the malate-aspartate shuttle, but they are doing different chemistry."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Biological Chemistry II","item":"https://fiveable.me/biological-chemistry-ii"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/biological-chemistry-ii/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 2","item":"https://fiveable.me/biological-chemistry-ii/unit-2"},{"@type":"ListItem","position":4,"name":"Malate dehydrogenase"}]}]}
```