---
title: "Ornithine Transcarbamylase | Biochem II"
description: "Ornithine transcarbamylase is the mitochondrial urea-cycle enzyme that turns ornithine and carbamoyl phosphate into citrulline in Biological Chemistry II."
canonical: "https://fiveable.me/biological-chemistry-ii/key-terms/ornithine-transcarbamylase"
type: "key-term"
subject: "Biological Chemistry II"
unit: "Unit 10"
---

# Ornithine Transcarbamylase | Biochem II

## Definition

Ornithine transcarbamylase (OTC) is the mitochondrial urea-cycle enzyme that combines ornithine and carbamoyl phosphate to make citrulline. In Biological Chemistry II, it is one of the steps that moves nitrogen toward urea for excretion.

## What It Is

Ornithine transcarbamylase is the urea-cycle enzyme that transfers a carbamoyl group from carbamoyl phosphate to ornithine, forming citrulline. In Biological Chemistry II, this is the step that connects the first half of the cycle in the mitochondria to the rest of the pathway that finishes in the cytosol.

The reaction happens in liver mitochondria, where nitrogen from amino acid breakdown is being processed for disposal. OTC does not make urea directly. Instead, it creates citrulline, which can leave the mitochondrion and continue through the next reactions of the urea cycle.

That location matters because the urea cycle is split between compartments. Carbamoyl phosphate is made first, then OTC uses it with ornithine. If OTC is working well, ammonia-derived nitrogen keeps moving forward instead of building up in the blood.

A helpful way to picture the step is as a handoff. Carbamoyl phosphate carries activated nitrogen into the reaction, ornithine accepts it, and citrulline is the product that moves the pathway along. If OTC is missing or slowed, the pathway backs up and ammonia can rise quickly.

OTC is also a good example of how metabolic pathways are regulated by substrate flow rather than just on or off switches. When the needed substrates are available, the reaction can proceed; when upstream or downstream intermediates are off balance, the whole cycle is affected. That is why OTC shows up anytime the course is tracing nitrogen excretion, liver metabolism, or the link between protein breakdown and waste removal.

## Why It Matters

OTC matters because it is one of the main checkpoints that keeps nitrogen from becoming toxic. Amino acid metabolism constantly produces nitrogen, and the body has to package that nitrogen into urea before it accumulates as ammonia. OTC sits right in the middle of that job, so it connects protein turnover, liver metabolism, and nitrogen balance.

This term also shows up when you are tracing the logic of the urea cycle step by step. If you know OTC, you can explain why ornithine and carbamoyl phosphate are the starting materials for citrulline, why the mitochondrion is involved, and why the cycle depends on moving intermediates between compartments.

OTC is especially useful in enzyme and disease questions because defects in the pathway have clear biochemical effects. A shortage of OTC can cause hyperammonemia, which is one of the clearest examples of what happens when nitrogen disposal fails. That makes the enzyme a direct bridge between pathway mechanics and clinical chemistry.

## Connections

### Urea Cycle

OTC is one reaction inside the urea cycle, not the whole pathway. If you can place it in sequence, you can explain how nitrogen enters the cycle, where citrulline goes next, and how the liver turns toxic nitrogen into urea. It is easiest to study OTC as the step that follows carbamoyl phosphate formation and precedes the cytosolic reactions.

### Citrulline

Citrulline is the product of the OTC reaction. In problem sets or pathway diagrams, you often identify OTC by spotting the conversion of ornithine to citrulline. If citrulline is missing or low, that can point you to a block early in the urea cycle.

### Carbamoyl Phosphate

Carbamoyl phosphate is the substrate that donates the carbamoyl group in the OTC reaction. It is made upstream, then used by OTC to move nitrogen forward in the cycle. Confusing this step with carbamoyl phosphate synthesis is common, so it helps to separate making carbamoyl phosphate from using it.

### [ammonia toxicity](/biological-chemistry-ii/key-terms/ammonia-toxicity)

OTC protects the body from ammonia toxicity by helping push nitrogen into urea formation. When OTC is deficient, ammonia can rise and affect the brain fast. This connection is why the enzyme matters in metabolic disease questions, not just in pathway memorization.

## On the AP Exam

A quiz question might give you a urea-cycle diagram and ask which enzyme converts ornithine and carbamoyl phosphate into citrulline. That is OTC. In a case question, you may need to explain why a urea-cycle block causes elevated ammonia, then trace the pathway backward to the missing step. If the prompt includes liver mitochondria, ammonia buildup, or X-linked inheritance, those clues can point you to OTC deficiency. On problem sets, you may also be asked to label the mitochondrial versus cytosolic steps and show where citrulline leaves the mitochondrion.

## ornithine transcarbamylase vs carbamoyl phosphate synthetase I

Carbamoyl phosphate synthetase I makes carbamoyl phosphate, while ornithine transcarbamylase uses that carbamoyl phosphate to make citrulline. They are consecutive steps, and both happen in the mitochondria, which is why they get mixed up. If the question asks about the first committed step of the urea cycle, think CPS I; if it asks about ornithine plus carbamoyl phosphate, think OTC.

## Key Takeaways

- Ornithine transcarbamylase is the mitochondrial urea-cycle enzyme that converts ornithine and carbamoyl phosphate into citrulline.
- OTC sits in liver mitochondria, where the body begins turning nitrogen from amino acid breakdown into a form that can be excreted.
- A defect in OTC can cause hyperammonemia because nitrogen cannot move through the urea cycle normally.
- This enzyme is a good checkpoint for tracing how the urea cycle moves from one compartment to another.
- OTC is often tested by asking you to identify the product, the substrates, or the consequence of a block in the pathway.

## FAQs

### What is ornithine transcarbamylase in Biological Chemistry II?

Ornithine transcarbamylase is the urea-cycle enzyme that makes citrulline from ornithine and carbamoyl phosphate. In Biochem II, you study it as part of nitrogen disposal in the liver. It is especially tied to the mitochondrial start of the urea cycle.

### What reaction does OTC catalyze?

OTC catalyzes the transfer of a carbamoyl group from carbamoyl phosphate to ornithine. The product is citrulline, which then continues through the rest of the urea cycle. If you can remember that substrate pair, you can usually identify the enzyme on a pathway diagram.

### How is ornithine transcarbamylase different from carbamoyl phosphate synthetase I?

CPS I makes carbamoyl phosphate, while OTC uses it. Both are mitochondrial urea-cycle enzymes, but they do different jobs in sequence. If a question asks about the step before OTC, that is usually CPS I; if it asks about making citrulline, that is OTC.

### What happens if ornithine transcarbamylase is deficient?

OTC deficiency can cause ammonia to build up in the blood because the urea cycle cannot keep moving nitrogen toward urea. That leads to hyperammonemia, which can damage the nervous system. In genetics questions, this condition is often discussed as X-linked.

## Related Study Guides

- [10.3 Urea cycle and nitrogen excretion in animals](/biological-chemistry-ii/unit-10/urea-cycle-nitrogen-excretion-animals/study-guide/7wQJrWMrtL2boZuX)
- [4.5 Integration of amino acid and protein metabolism](/biological-chemistry-ii/unit-4/integration-amino-acid-protein-metabolism/study-guide/rurC8O9teffw9wXI)

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