biology

Methemoglobin

Methemoglobin is a biology 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 Methemoglobin rather than just read about it. In short: Methemoglobin (British: methaemoglobin, shortened MetHb) (pronounced "met-hemoglobin") is a hemoglobin in the form of metalloprotein, in which the iron in the heme group is in the Fe3+ (ferric) state, not the Fe2+ (ferrous) of normal hemoglobin. Sometimes, it is also referred to as ferrihemoglobin.

Key takeaways

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

Reference excerpt

Methemoglobin (British: methaemoglobin, shortened MetHb) (pronounced "met-hemoglobin") is a hemoglobin in the form of metalloprotein, in which the iron in the heme group is in the Fe3+ (ferric) state, not the Fe2+ (ferrous) of normal hemoglobin. Sometimes, it is also referred to as ferrihemoglobin. Methemoglobin cannot bind oxygen, which means it cannot carry oxygen to tissues. It is bluish chocolate-brown in color. In human blood a trace amount of methemoglobin is normally produced spontaneously, but when present in excess the blood becomes abnormally dark bluish brown. The NADH-dependent enzyme methemoglobin reductase (a type of diaphorase) is responsible for converting methemoglobin back to hemoglobin. Normally one to two percent of a person's hemoglobin is methemoglobin; a higher percentage than this can be genetic or caused by exposure to various chemicals and depending on the level can cause health problems known as methemoglobinemia. A higher level of methemoglobin will tend to cause a pulse oximeter to read closer to 85% regardless of the true level of oxygen saturation.

Etymology The word methemoglobin derives from the Ancient Greek prefix μετα- (meta-: behind, later, subsequent) and the word hemoglobin. The name hemoglobin is itself derived from the words heme and globin, each subunit of hemoglobin being a globular protein with an embedded heme group.

Common causes of elevated methemoglobin Reduced cellular defense mechanisms Children younger than 4 months exposed to various environmental agents Pregnant women are considered vulnerable to exposure of high levels of nitrates in drinking water Cytochrome b5 reductase deficiency G6PD deficiency Hemoglobin M disease Pyruvate kinase deficiency Various pharmaceutical compounds Local anesthetic agents, especially prilocaine and benzocaine. Amyl nitrite, chloroquine, dapsone, nitrates, nitrites, nitroglycerin, nitroprusside, phenacetin, phenazopyridine, primaquine, quinones and sulfonamides Environmental agents Aromatic amines (e.g. p-nitroaniline) Arsine Chlorobenzene Chromates Nitrates/nitrites Umbellulone Inherited disorders Some family members of the Fugate family in Kentucky, due to a recessive gene, had blue skin from an excess of methemoglobin. In cats Ingestion of paracetamol (i.e. acetaminophen, tylenol)

Therapeutic uses Amyl nitrite is administered to treat cyanide poisoning. It works by converting hemoglobin to methemoglobin, which allows for the binding of cyanide (CN–) anions by ferric (Fe3+) cations and the formation of cyanomethemoglobin. The immediate goal of forming this cyanide adduct is to prevent the binding of free cyanide to the cytochrome a3 group in cytochrome c oxidase.

Methemoglobin saturation

Methemoglobin is expressed as a concentration or a percentage. Percentage of methemoglobin is calculated by dividing the concentration of methemoglobin by the concentration of total hemoglobin. Percentage of methemoglobin is likely a better indicator of illness severity than overall concentration, as underlying medical conditions play an important role. For example, a methemoglobin concentration of 1.5 g/dL may represent a percentage of 10% in an otherwise healthy patient with a baseline hemoglobin of 15 mg/dL, whereas the presence of the same concentration of 1.5 g/dL of methemoglobin in an anemic patient with a baseline hemoglobin of 8 g/dL would represent a percentage of 18.75%. The former patient will be left with a functional hemoglobin concentration of 13.5 g/dL and potentially remain asymptomatic while the latter patient with a functional hemoglobin concentration 6.5 g/dL may be severely symptomatic with a methemoglobin of less than 20%.

1–2% Normal Less than 10% metHb - No symptoms 10–20% metHb - Skin discoloration only (most notably on mucous membranes) 20–30% metHb - Anxiety, headache, dyspnea on exertion 30–50% metHb - Fatigue, confusion, dizziness, tachypnea, palpitations 50–70% metHb - Coma, seizures, arrhythmias, acidosis Greater than 70% metHb - High risk of death This may be further compounded by the "functional hemoglobin's" decreased ability to release oxygen in the presence of methemoglobin. Anemia, congestive heart failure, chronic obstructive pulmonary disease, and essentially any pathology that impairs the ability to deliver oxygen may worsen the symptoms of methemoglobinemia.

Blood stains Increased levels of methemoglobin are found in blood stains. Upon exiting the body, bloodstains transit from bright red to dark brown, which is attributed to oxidation of oxy-hemoglobin (HbO2) to methemoglobin (met-Hb) and hemichrome (HC).

See also Blue baby syndrome Carboxyhemoglobin Methemoglobinemia Blue Fugates

References

External links Methemoglobin at the U.S. National Library of Medicine Medical Subject Headings (MeSH) MetHb Formation The Blue people of Troublesome Creek

Illustrations

Worked examples

Example 1 — a first encounter with Methemoglobin

Start with the simplest possible case. Write down what Methemoglobin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, 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 Methemoglobin 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 Methemoglobin 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 Methemoglobin

In research: Methemoglobin appears in biology 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 Methemoglobin 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: Methemoglobin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cellular respiration, Hemoglobins, Hemoproteins, so understanding it makes those chapters shorter.
In everyday life: Look for Methemoglobin 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 Methemoglobin in 20 minutes

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

Frequently asked questions

What is Methemoglobin in simple terms?

Why does Methemoglobin matter?

Because it connects several biology 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 Methemoglobin?

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 Methemoglobin.