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chemistry

Heme

Heme is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Heme rather than just read about it. In short: Heme (American English), or haem (Commonwealth English, both pronounced /hi:m/ HEEM), is a ring-shaped iron-containing molecule that serves as a ligand of various proteins, especially as a component of hemoglobin, which carries oxygen in the bloodstream. It is composed of four pyrrole rings with two vinyl and two propionic acid side chains.

Heme — main illustration
Heme — illustration

Key takeaways

  • Heme belongs to chemistry; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Heme to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Heme from memory before moving on to harder problems.

Reference excerpt

Heme (American English), or haem (Commonwealth English, both pronounced /hi:m/ HEEM), is a ring-shaped iron-containing molecule that serves as a ligand of various proteins, especially as a component of hemoglobin, which carries oxygen in the bloodstream. It is composed of four pyrrole rings with two vinyl and two propionic acid side chains. Heme is biosynthesized in both the bone marrow and the liver. Heme plays a critical role in several redox reactions in mammals, due to its ability to carry the oxygen molecule. Reactions include oxidative metabolism (cytochrome c oxidase, succinate dehydrogenase), xenobiotic detoxification via cytochrome P450 pathways (including metabolism of some drugs), gas sensing (guanyl cyclases, nitric oxide synthase), and microRNA processing (DGCR8). Heme is a coordination complex, consisting of an iron ion coordinated to a tetrapyrrole acting as a tetradentate ligand, and to one or two axial ligands". The definition is loose, and many depictions omit the axial ligands. Among the metalloporphyrins deployed by metalloproteins as prosthetic groups, heme is one of the most widely used and defines a family of proteins known as hemoproteins. Hemes are most commonly recognized as components of hemoglobin, the red pigment in blood, but are also found in a number of other biologically important hemoproteins such as myoglobin, cytochromes, catalases, heme peroxidase, and endothelial nitric oxide synthase. The word haem is derived, from Ancient Greek, αἷμα, (Latinized: haima; Ancient Greek pronunciation: [/hâi̯.ma/]), meaning 'blood'.

Function

Hemoproteins have diverse biological functions including the transportation of diatomic gases, chemical catalysis, diatomic gas detection, and electron transfer. The heme iron serves as a source or sink of electrons during electron transfer or redox chemistry. In peroxidase reactions, the porphyrin molecule also serves as an electron source, being able to delocalize radical electrons in the conjugated ring. In the transportation or detection of diatomic gases, the gas binds to the heme iron. During the detection of diatomic gases, the binding of the gas ligand to the heme iron induces conformational changes in the surrounding protein. In general, diatomic gases only bind to the reduced heme, as ferrous Fe(II) while most peroxidases cycle between Fe(III) and Fe(IV) and hemeproteins involved in mitochondrial redox, oxidation-reduction, cycle between Fe(II) and Fe(III). It has been speculated that the original evolutionary function of hemoproteins was electron transfer in primitive sulfur-based photosynthesis pathways in ancestral cyanobacteria-like organisms before the appearance of molecular oxygen. Hemoproteins achieve their remarkable functional diversity by modifying the environment of the heme macrocycle within the protein matrix. For example, the ability of hemoglobin to effectively deliver oxygen to tissues is due to specific amino acid residues located near the heme molecule. Hemoglobin reversibly binds to oxygen in the lungs when the pH is high, and the carbon dioxide concentration is low. When the situation is reversed (low pH and high carbon dioxide concentrations), hemoglobin will release oxygen into the tissues. This phenomenon, which states that hemoglobin's oxygen binding affinity is inversely proportional to both acidity and concentration of carbon dioxide, is known as the Bohr effect. The molecular mechanism behind this effect is the steric organization of the globin chain; a histidine residue, located adjacent to the heme group, becomes positively charged under acidic conditions (which are caused by dissolved CO2 in working muscles, etc.), releasing oxygen from the heme group.

Types

Major hemes There are several biologically important kinds of heme:

The most common type is heme B; other important types include heme A and heme C. Isolated hemes are commonly designated by capital letters while hemes bound to proteins are designated by lower case letters. Cytochrome a refers to the heme A in specific combination with membrane protein forming a portion of cytochrome c oxidase.

Other hemes The following carbon numbering system of porphyrins is an older numbering used by biochemists and not the 1–24 numbering system recommended by IUPAC, which is shown in the table above.

… excerpt ends here. Continue reading the full article.

Illustrations

Heme: Binding of oxygen to a heme prosthetic group
Binding of oxygen to a heme prosthetic group
Heme: Space-filling model of the Fe-protoporphyrin IX subunit of heme B. Axial ligands omitted. Color scheme: grey=iron, blue=nitrogen, black=carbon, white=hydrogen, red=oxygen
Space-filling model of the Fe-protoporphyrin IX subunit of heme B. Axial ligands omitted. Color scheme: grey=iron, blue=nitrogen, black=carbon, white=hydrogen, red=oxygen
Heme: The heme group of succinate dehydrogenase bound to histidine, an electron carrier in the mitochondrial electron transfer chain. The large semi-transparent sphere indicates the location of the iron ion. From PDB: 1YQ3​.
The heme group of succinate dehydrogenase bound to histidine, an electron carrier in the mitochondrial electron transfer chain. The large semi-transparent sphere indicates the location of the iron ion. From PDB: 1YQ3​.
Heme illustration
Heme: Structure of Fe-porphyrin subunit of heme B.
Structure of Fe-porphyrin subunit of heme B.

Worked examples

Example 1 — a first encounter with Heme

Start with the simplest possible case. Write down what Heme claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Heme before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Heme ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Heme

In research
Heme appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Heme in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Heme is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biomolecules, Cofactors, Iron(II) compounds, so understanding it makes those chapters shorter.
In everyday life
Look for Heme outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Heme in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Heme means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Heme out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Heme in simple terms?

Heme (American English), or haem (Commonwealth English, both pronounced /hi:m/ HEEM), is a ring-shaped iron-containing molecule that serves as a ligand of various proteins, especially as a component of hemoglobin, which carries oxygen in the bloodstream. It is composed of four pyrrole rings with tw…

Why does Heme matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Heme?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Heme.

Tags

  • Biomolecules
  • Cofactors
  • Iron(II) compounds
  • Iron complexes
  • Porphyrins

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