HINT2

Key takeaways

HINT2 belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect HINT2 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of HINT2 from memory before moving on to harder problems.

Reference excerpt

Histidine triad nucleotide binding protein 2 (HINT2) is a mitochondrial protein that in humans is encoded by the HINT2 gene on chromosome 9. This protein is an AMP-lysine hydrolase and phosphoamidase and may contribute to tumor suppression.

Structure As a member of the histidine triad nucleotide-binding (Hint) protein family, which is a subfamily of the histidine triad (HIT) family, HINT2 contains a conserved histidine and HIT sequence motif (His-X-His-X-His-X-X), and the latter two histidines contribute to a catalytic triad. The 163-amino acid protein encoded by this gene forms a 17-kDa homodimer. Compared to other members of the Hint family, HINT2 has a 61% sequence homology to HINT1 and 28% sequence homology to HINT3. When compared with HINT1, the 35–amino acid extension at the HINT2 N-terminal corresponds to a predicted mitochondria import signal.

Function HINT2 is a member of the HIT superfamily and Hint subfamily, which are characterized as nucleotide hydrolases and transferases that act on the alpha-phosphate of ribonucleotides. The Hint family is the oldest within the HIT superfamily and thus, its members are highly conserved among eukaryotes and archaebacteria. The Hint proteins function as AMP-lysine hydrolases and phosphoramidases. In mammals, HINT2 is expressed in the liver, adrenal cortex, and pancreas and localizes to the mitochondria within their cells. Specifically, the protein is located in the inner mitochondrial membrane, facing the mitochondrial matrix. This positioning likely facilitates the transport of cholesterol from the cytosol to the matrix, which is necessary for steroidogenesis, by providing a contact site for the hydrophobic molecule and allowing it to cross the mitochondrial intermembrane space. HINT2 regulates steroidogenesis through calcium-dependent and calcium-independent signalling pathways that may serve to maintain a favorable mitochondrial potential. Its role in calcium homeostasis may also contribute to its proapoptotic function in hepatocytes and other non-steroidogenic cells, though the exact mechanism remains unclear.

Clinical significance Hint2, one of the three members of the Hint family of proteins, is localized to mitochondria of various cell types. In human adrenocarcinoma cells, Hint2 modulates Ca2+ handling by mitochondria. In all living organisms, intracellular calcium controls a wide variety of physiological processes. Extracellular stimuli generate temporally organized Ca2+ signals, which most of the time occur as repetitive spikes. The frequency of these oscillations controls the nature and the extent of the cellular response. Ca2+ oscillations originate from the repetitive opening of the inositol 1,4,5-trisphosphate (InsP3) receptors that are Ca2+ channels embedded in the membrane of the endoplasmic reticulum (ER). Opening of these channels is initiated by the stimulus-induced rise in InsP3; because their activity is biphasically regulated by the level of cytoplasmic Ca2+, oscillations can occur. Mitochondria also affect cytoplasmic Ca2+ signals. They can both buffer cytosolic Ca2+ changes (7 and 8) and release Ca2+. At rest, intramitochondrial ([Ca2+]m) and cytosolic Ca2+ concentration ([Ca2+]i) are similar, of the order of 100 nM (9). The Hint family has been implicated in tumor suppression. Int2, a member of the superfamily of histidine triad proteins, has been localized exclusively in mitochondria, near the contact sites of the inner membrane. This enzyme is highly expressed in the liver, where it has been shown to stimulate mitochondrial lipid metabolism, respiration, and glucose homeostasis. Hint2 modulates cytoplasmic and mitochondrial Ca2+ dynamics by stimulating the activity of the mitochondrial respiratory chain. It appears that the absence of Hint2 leads to a premature opening of the mitochondrial permeability transition pore (mPTP) in mitochondrial suspensions. As such, HINT2 plays a prominent role in mitochondrial cell death signaling (e.g. apoptosis) and in ischemia-reperfusion injury (for instance during heart attacks) through calcium homeostasis. In particular, HINT2 is also observed to be upregulated in breast, pancreatic, and colon cancer cells, while it is downregulated in hepatocellular carcinoma and endometrial cancer. Its exact role in tumor suppression remains unknown, though studies suggest it may promote apoptosis in hepatocellular carcinoma and endometrial cancer. In double knockout Hint2 mice, higher acylation and morphological alterations were observed in the mitochondria, suggesting that Hint2 may regulate glucose and lipid metabolism.

Interactions Currently, HINT2 has no known protein-protein interaction partners.

See also Histidine triad nucleotide-binding protein 1 (HINT1)

References

Illustrations

Worked examples

Example 1 — a first encounter with HINT2

Start with the simplest possible case. Write down what HINT2 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 HINT2 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 HINT2 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 HINT2

In research: HINT2 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 HINT2 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: HINT2 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Genes on human chromosome 9, Proteins, so understanding it makes those chapters shorter.
In everyday life: Look for HINT2 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 HINT2 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what HINT2 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 HINT2 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is HINT2 in simple terms?

Histidine triad nucleotide binding protein 2 (HINT2) is a mitochondrial protein that in humans is encoded by the HINT2 gene on chromosome 9. This protein is an AMP-lysine hydrolase and phosphoamidase and may contribute to tumor suppression.

Why does HINT2 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 HINT2?

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 HINT2.