---
title: "Carbamoyl Phosphate Synthetase I | Anatomy"
description: "Carbamoyl Phosphate Synthetase I is the liver mitochondrial enzyme that starts the urea cycle by turning ammonia and bicarbonate into carbamoyl phosphate."
canonical: "https://fiveable.me/anatomy-physiology/key-terms/carbamoyl-phosphate-synthetase"
type: "key-term"
subject: "Anatomy and Physiology I"
unit: "Unit 24"
---

# Carbamoyl Phosphate Synthetase I | Anatomy

## Definition

Carbamoyl Phosphate Synthetase I (CPS I) is a mitochondrial liver enzyme that catalyzes the first committed step of the urea cycle. It converts ammonia and bicarbonate into carbamoyl phosphate to help remove excess nitrogen.

## What It Is

Carbamoyl Phosphate Synthetase I, or CPS I, is the enzyme that starts the urea cycle in liver mitochondria. In Anatomy and Physiology I, you meet it as part of the body’s nitrogen disposal system, the pathway that keeps toxic ammonia from building up after amino acids are broken down.

CPS I takes two raw materials, ammonia and bicarbonate, and uses energy from ATP to build carbamoyl phosphate. That product is not the final waste product yet, but it is the committed entry point into the urea cycle. Once CPS I makes carbamoyl phosphate, the pathway can keep moving toward urea formation, which is the form the body can safely excrete in urine.

The location matters. CPS I works inside the mitochondria of liver cells, not in the cytoplasm, because the urea cycle begins in the mitochondrial matrix and then continues in the cytosol. That compartmental setup is a common A&P theme: enzymes are placed where their substrates and next steps are available, and the cell uses organelles to organize chemistry.

CPS I is also the rate-limiting step of the urea cycle, which means its activity helps set the pace for nitrogen disposal. If there is more amino acid breakdown, more ammonia enters the system, and the liver has to push harder through this enzyme to keep blood ammonia levels safe.

A helpful way to think about it is as a gatekeeper. If CPS I is working normally, nitrogen from protein metabolism gets routed toward urea. If it is deficient or not functioning well, ammonia can accumulate quickly, and that is dangerous because the nervous system is very sensitive to hyperammonemia.

One detail students often mix up is that CPS I is not the same thing as Carbamoyl Phosphate Synthetase II. CPS I belongs to the urea cycle and uses ammonia; CPS II is part of pyrimidine synthesis and uses glutamine. In this course, CPS I is the liver enzyme tied directly to protein metabolism and waste removal.

## Why It Matters

CPS I shows how Anatomy and Physiology connects protein metabolism to homeostasis. When you eat protein or break down your own proteins for energy, nitrogen is removed from amino acids and becomes ammonia, which the body cannot leave floating around in the blood. CPS I is the enzyme that helps funnel that nitrogen into the urea cycle so the liver can turn it into urea.

This term also connects the digestive system, liver, and excretory system. Protein digestion may happen in the stomach and small intestine, but what happens after absorption is just as important. Amino acids travel through the hepatic portal vein to the liver, where excess nitrogen is handled. CPS I is one of the clearest examples of how a liver pathway protects the whole body.

It matters clinically too. If CPS I is missing or defective, ammonia rises and symptoms can become severe fast, especially in newborns with inborn errors of metabolism. That gives you a real reason to care about the pathway instead of memorizing it as isolated vocabulary. In A&P, this is one of those enzymes that links cell biology, metabolism, and disease in a single step.

## Connections

### Urea Cycle

CPS I is the first committed enzyme in the urea cycle, so you need the full pathway to see what happens after carbamoyl phosphate is formed. The cycle turns toxic nitrogen into urea, and each later step depends on CPS I getting the process started in the liver mitochondria.

### Ammonia

Ammonia is the nitrogen-containing waste product CPS I helps remove. In A&P, this is the molecule that becomes dangerous when amino acids are broken down faster than the liver can process them. High ammonia levels can damage the brain, which is why the urea cycle matters so much.

### Bicarbonate

Bicarbonate supplies part of the carbon backbone for carbamoyl phosphate. If you are tracing the reaction, bicarbonate is one of the two starting materials CPS I combines with ammonia. That makes it a good reminder that metabolism often uses simple inorganic molecules to build larger intermediates.

### [Hepatic Portal Vein](/anatomy-physiology/key-terms/hepatic-portal-vein)

Amino acids absorbed from the intestine travel to the liver through the hepatic portal vein before their nitrogen is handled. That route helps explain why CPS I is found in liver cells rather than in muscle or other tissues. The liver acts as the main nitrogen-processing center.

## On the AP Exam

A quiz or lab question may ask you to identify where CPS I acts, what reaction it catalyzes, or what happens if the enzyme fails. You might also be given a case with elevated blood ammonia and asked to trace the problem back to the urea cycle. The move is usually: locate the liver mitochondria, identify ammonia as the toxic starting material, and connect CPS I failure to hyperammonemia.

If your instructor uses process diagrams, you may need to label CPS I as the first step of the urea cycle and show that it makes carbamoyl phosphate. In short-answer work, the strongest answers mention both the reaction and the outcome, not just the word "enzyme."

## Carbamoyl Phosphate Synthetase I vs Carbamoyl Phosphate Synthetase II

These enzymes sound almost identical, but they do different jobs. CPS I works in the urea cycle in liver mitochondria and uses ammonia to help dispose of nitrogen. CPS II works in pyrimidine synthesis and uses glutamine instead. If you see a question about ammonia or urea, you want CPS I.

## Key Takeaways

- Carbamoyl Phosphate Synthetase I is the mitochondrial liver enzyme that starts the urea cycle.
- It converts ammonia and bicarbonate into carbamoyl phosphate, using ATP to drive the reaction.
- CPS I matters because it helps prevent toxic ammonia from building up in the blood.
- If CPS I does not work well, hyperammonemia can develop and affect the brain very quickly.
- A&P questions often connect CPS I to protein metabolism, the liver, and nitrogen disposal.

## FAQs

### What is Carbamoyl Phosphate Synthetase I in Anatomy and Physiology I?

CPS I is the enzyme in liver mitochondria that begins the urea cycle. It combines ammonia and bicarbonate to make carbamoyl phosphate, which moves the body toward making urea. In A&P, it shows up when you study how the liver handles nitrogen from protein breakdown.

### Where is Carbamoyl Phosphate Synthetase I located?

It is located in the mitochondria of liver cells. That location matters because the first steps of the urea cycle happen in the mitochondrial matrix before the pathway continues in the cytosol. If you are tracing a diagram, think "liver mitochondrion" first.

### What happens if Carbamoyl Phosphate Synthetase I is deficient?

A deficiency can cause ammonia to build up in the blood, which is called hyperammonemia. Because ammonia is toxic to the nervous system, this can become a serious medical problem. A&P questions often link the enzyme defect to confusion, lethargy, or other signs of impaired nitrogen disposal.

### How is CPS I different from CPS II?

CPS I is part of the urea cycle and helps remove nitrogen in the liver. CPS II is part of pyrimidine synthesis, which is used to make nucleotides. The easiest way to tell them apart is to look for the context: ammonia and urea point to CPS I, while nucleotide building points to CPS II.

## Related Study Guides

- [24.4 Protein Metabolism ](/anatomy-physiology/unit-24/4-protein-metabolism/study-guide/1ueYphikYYwQeOOM)

## 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/anatomy-physiology/key-terms/carbamoyl-phosphate-synthetase#resource","name":"Carbamoyl Phosphate Synthetase I | Anatomy","url":"https://fiveable.me/anatomy-physiology/key-terms/carbamoyl-phosphate-synthetase","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/anatomy-physiology/key-terms/carbamoyl-phosphate-synthetase#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:20:36.800Z","isPartOf":{"@type":"Collection","name":"Anatomy and Physiology I Key Terms","url":"https://fiveable.me/anatomy-physiology/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/anatomy-physiology/key-terms/carbamoyl-phosphate-synthetase#term","name":"Carbamoyl Phosphate Synthetase I","description":"Carbamoyl Phosphate Synthetase I (CPS I) is a mitochondrial liver enzyme that catalyzes the first committed step of the urea cycle. It converts ammonia and bicarbonate into carbamoyl phosphate to help remove excess nitrogen.","url":"https://fiveable.me/anatomy-physiology/key-terms/carbamoyl-phosphate-synthetase","inDefinedTermSet":{"@type":"DefinedTermSet","name":"Anatomy and Physiology I Key Terms","url":"https://fiveable.me/anatomy-physiology/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is Carbamoyl Phosphate Synthetase I in Anatomy and Physiology I?","acceptedAnswer":{"@type":"Answer","text":"CPS I is the enzyme in liver mitochondria that begins the urea cycle. It combines ammonia and bicarbonate to make carbamoyl phosphate, which moves the body toward making urea. In A&P, it shows up when you study how the liver handles nitrogen from protein breakdown."}},{"@type":"Question","name":"Where is Carbamoyl Phosphate Synthetase I located?","acceptedAnswer":{"@type":"Answer","text":"It is located in the mitochondria of liver cells. That location matters because the first steps of the urea cycle happen in the mitochondrial matrix before the pathway continues in the cytosol. If you are tracing a diagram, think \"liver mitochondrion\" first."}},{"@type":"Question","name":"What happens if Carbamoyl Phosphate Synthetase I is deficient?","acceptedAnswer":{"@type":"Answer","text":"A deficiency can cause ammonia to build up in the blood, which is called hyperammonemia. Because ammonia is toxic to the nervous system, this can become a serious medical problem. A&P questions often link the enzyme defect to confusion, lethargy, or other signs of impaired nitrogen disposal."}},{"@type":"Question","name":"How is CPS I different from CPS II?","acceptedAnswer":{"@type":"Answer","text":"CPS I is part of the urea cycle and helps remove nitrogen in the liver. CPS II is part of pyrimidine synthesis, which is used to make nucleotides. The easiest way to tell them apart is to look for the context: ammonia and urea point to CPS I, while nucleotide building points to CPS II."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Anatomy and Physiology I","item":"https://fiveable.me/anatomy-physiology"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/anatomy-physiology/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 24","item":"https://fiveable.me/anatomy-physiology/unit-24"},{"@type":"ListItem","position":4,"name":"Carbamoyl Phosphate Synthetase I"}]}]}
```