Cytochrome: Definition & Structure

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  • 0:00 Definition
  • 0:56 Structure
  • 3:13 Roles
  • 4:48 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

In this lesson, you will discover what cytochrome is and get an idea of its structure and information on a few of the different forms it can have. You will also learn some about the roles it plays in cells.


What comes to mind when you see the word chrome? That nice finish on your car or motorcycle? A web browser, perhaps? Or, for the chemistry nut, the element chromium? Did you think of colors? Yes, colors, because the word chrome is derived for the Greek word for color.

Color is how cytochrome was first discovered. A scientist studying the tissues of many organisms noticed different colors of light were being absorbed by something inside cells and proposed the term cytochrome to refer to it. Cyto- means cell, and -chrome means color.

Cytochrome is a protein that can transfer electrons with a chemical group called a heme group. Maybe you've heard of heme before? No? Certainly you know what blood is. Your blood contains hemoglobin, in which there are heme groups. They're what make your blood red when there's oxygen.


The heme groups of cytochrome are similar to those of hemoglobin. Both have the same basic ring structure called a porphyrin ring. You can think of the porphyrin ring structure kind of like a target with rings around a bullseye. In the middle of the ring, the bullseye, is an iron atom (Fe). Four nitrogens surround the iron atom. Many carbons and hydrogens surround and connect to the four nitrogens. There are also several chemical groups that hang off the surrounding carbons and hydrogens. The whole structure forms what's known as a heme group.

You wouldn't shoot an arrow at a dartboard, would you? Just as there are different types of targets, the heme groups of cytochrome have a different job than the heme groups of blood. The job of cytochrome heme groups is to transfer electrons. It does this by altering the state of the iron molecule. Without an electron the iron is Fe3+, whereas when it gains an electron it becomes Fe2+.

Not all dartboards are created equal. Some have the scoring numbers in different places. Some have more numbers, some have less. The heme groups of cytochrome are similar to the different dartboards. They all have the same main function, but vary in appearance. It is the difference in appearance of hemes that categorize the different cytochrome proteins.

There are three main types of cytochromes in animals: A, B, and C. There are also subtypes of the main types. The distinguishing factor between the cytochromes is the variation in four of the chemical groups attached to the main porphyrin ring.

Cytochrome A and B hemes have one attached group that are the same, whereas cytochrome B and C have two that are the same. All other attached chemical groups vary between the three main types.

Cytochrome A has a very long carbon chain attached. Cytochrome B has smaller chains and two double-bonded carbon side chains. Cytochrome C has sulfur attached to two of the side chains.

The heme groups of cytochrome A and B are both attached to proteins within the inner mitochondrial membrane, while cytochrome C heme group moves more freely, being attached to a protein on the outer portion of the inner mitochondrial membrane.

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