Globin

Key takeaways

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

Reference excerpt

The globins are a superfamily of heme-containing globular proteins, involved in binding and/or transporting oxygen. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members include myoglobin and hemoglobin. Both of these proteins reversibly bind oxygen via a heme prosthetic group. They are widely distributed in many organisms.

Structure Globin superfamily members share a common three-dimensional fold. This 'globin fold' typically consists of eight alpha helices, although some proteins have additional helix extensions at their termini. Since the globin fold contains only helices, it is classified as an all-alpha protein fold. The globin fold is found in its namesake globin families as well as in phycocyanins. The globin fold was thus the first protein fold discovered (myoglobin was the first protein whose structure was solved).

Helix packaging The eight helices of the globin fold core share significant nonlocal structure, unlike other structural motifs in which amino acids close to each other in primary sequence are also close in space. The helices pack together at an average angle of about 50 degrees, significantly steeper than other helical packings such as the helix bundle. The exact angle of helix packing depends on the sequence of the protein, because packing is mediated by the sterics and hydrophobic interactions of the amino acid side chains near the helix interfaces.

Evolution Globins evolved from a common ancestor and can be divided into three lineages:

Family M (for myoglobin-like) or F (for FHb-like), which has a typical 3/3 fold. Subfamily FHb, for flavohemoglobins. Chimeric. Subfamily SDgb, for single-domain globins (not to be confused with SSDgb). Family S (for sensor-like), again with a 3/3 fold. Subfamily GCS, for Globin-coupled sensors. Chimeric. Subfamily PGb, for protoglobins. Single-domain. Subfamily SSDgb, for sensor single-domain globins. Family T (for truncated), with a 2/2 fold All subfamilies can be chimeric, single-domain, or tandemly linked. Subfamily TrHb1 (also T1 or N). Subfamily TrHb2 (also T2 or O). Includes 2/2 phytoglobins. Subfamily TrHb3 (also T3 or P). The M/F family of globins is absent in archaea. Eukaryotes lack GCS, Pgb, and T3 subfamily globins. Eight globins are known to occur in vertebrates: androglobin (Adgb), cytoglobin (Cygb), globin E (GbE, from bird eye), globin X (GbX, not found in mammals or birds), globin Y (GbY, from some mammals), hemoglobin (Hb), myoglobin (Mb) and neuroglobin (Ngb). All these types evolved from a single globin gene of F/M family found in basal animals. The single gene has also invented an oxygen-carrying "hemoglobin" multiple times in other groups of animals. Several functionally different hemoglobins can coexist in the same species.

Sequence conservation Although the fold of the globin superfamily is highly evolutionarily conserved, the sequences that form the fold can have as low as 16% sequence identity. While the sequence specificity of the fold is not stringent, the hydrophobic core of the protein must be maintained and hydrophobic patches on the generally hydrophilic solvent-exposed surface must be avoided in order for the structure to remain stable and soluble. The most famous mutation in the globin fold is a change from glutamate to valine in one chain of the hemoglobin molecule. This mutation creates a "hydrophobic patch" on the protein surface that promotes intermolecular aggregation, the molecular event that gives rise to sickle-cell disease.

Subfamilies Leghemoglobin InterPro: IPR001032 Myoglobin InterPro: IPR002335 Erythrocruorin InterPro: IPR002336 Hemoglobin, beta InterPro: IPR002337 Hemoglobin, alpha InterPro: IPR002338 Myoglobin, trematode type InterPro: IPR011406 Globin, nematode InterPro: IPR012085 Globin, lamprey/hagfish type InterPro: IPR013314 Globin, annelid-type InterPro: IPR013316 Hemoglobin, extracellular InterPro: IPR014610

Examples Human genes encoding globin proteins include:

CYGB HBA1, HBA2, HBB, HBD, HBE1, HBG1, HBG2, HBM, HBQ1, HBZ, MB The globins include:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Globin

Start with the simplest possible case. Write down what Globin 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 Globin 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 Globin 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 Globin

In research: Globin 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 Globin 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: Globin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Protein folds, Protein superfamilies, so understanding it makes those chapters shorter.
In everyday life: Look for Globin 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.

How to study Globin in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Globin 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 Globin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Globin in simple terms?

The globins are a superfamily of heme-containing globular proteins, involved in binding and/or transporting oxygen. These proteins all incorporate the globin fold, a series of eight alpha helical segments.

Why does Globin 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 Globin?

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 Globin.