chemistry

Leghemoglobin

Leghemoglobin 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 Leghemoglobin rather than just read about it. In short: Leghemoglobin (also leghaemoglobin or legoglobin) is an oxygen-carrying phytoglobin found in the nitrogen-fixing root nodules of leguminous plants. It is produced by these plants in response to the roots being colonized by nitrogen-fixing bacteria, termed rhizobia, as part of the symbiotic interaction between plant and bacterium: roots not colonized by Rhizobium do not synthesise leghemoglobin.

Key takeaways

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

Reference excerpt

Leghemoglobin (also leghaemoglobin or legoglobin) is an oxygen-carrying phytoglobin found in the nitrogen-fixing root nodules of leguminous plants. It is produced by these plants in response to the roots being colonized by nitrogen-fixing bacteria, termed rhizobia, as part of the symbiotic interaction between plant and bacterium: roots not colonized by Rhizobium do not synthesise leghemoglobin. Leghemoglobin has close chemical and structural similarities to hemoglobin, and, like hemoglobin, is red in colour. It was originally thought that the heme prosthetic group for plant leghemoglobin was provided by the bacterial symbiont within symbiotic root nodules. However, subsequent work shows that the plant host strongly expresses heme biosynthesis genes within nodules, and that activation of those genes correlates with leghemoglobin gene expression in developing nodules. In plants colonised by Rhizobium, such as alfalfa or soybeans, the presence of oxygen in the root nodules would reduce the activity of the oxygen-sensitive nitrogenase, which is an enzyme responsible for the fixation of atmospheric nitrogen. Leghemoglobin is shown to buffer the concentration of free oxygen in the cytoplasm of infected plant cells to ensure the proper function of root nodules. That being said, nitrogen fixation is an extremely energetically costly process, so aerobic respiration, which necessitates high oxygen concentration, is necessary in the cells of the root nodule. Leghemoglobin maintains a free oxygen concentration that is low enough to allow nitrogenase to function, but a high enough total oxygen concentration (free and bound to leghemoglobin) for aerobic respiration. Leghemoglobin falls into the class of symbiotic globins, which also include the root nodules globins of actinorhizal plants such as Casuarina. The Casuarina symbiotic globin is intermediate between leghemoglobin and nonsymbiotic phytoglobin-2.

Structure Leghemoglobins are monomeric proteins with a mass around 16 kDa, and are structurally similar to myoglobin. One leghemoglobin protein consists of a heme bound to an iron, and one polypeptide chain (the globin). Similar to myoglobin and hemoglobin, the iron of heme is found in its ferrous state in vivo, and is the moiety that binds oxygen. Despite similarities in the mechanism of oxygen binding between leghemoglobin and animal hemoglobin, and the fact that leghemoglobin and animal hemoglobin evolved from a common ancestor, there is dissimilarity in amino acid sequence between these proteins at about 80% of positions.

Oxygen binding affinities of leghemoglobins are between 11 and 24 times higher than oxygen binding affinities of sperm whale myoglobin. Differences in the affinities are due to differential rates of association between the two types of proteins. One explanation of this phenomenon is that in myoglobin, a bound water molecule is stabilized in a pocket surrounding the heme group. This water group must be displaced in order for oxygen to bind. No such water is bound in the analogous pocket of leghemoglobin, so it is easier for an oxygen molecule to approach the leghemoglobin heme. Leghemoglobin has a slow oxygen dissociation rate, similar to myoglobin. Like myoglobin and hemoglobin, leghemoglobin has a high affinity for carbon monoxide. In the primary structure of Leghemoglobin A in soybeans, a valine(F7) is found in place where a serine(F7) is in Myoglobin. Without a hydrogen bond fixing the orientation of the proximal histidine side chain the imidazole ring can occupy a staggered conformation between pyrrole nitrogen atoms and can readily move upward to the heme plane. This greatly increases the reactivity of the iron atom and oxygen affinity. In Leghemoglobin A the distal histidine side chain is also rotated away from the bound ligand by formation of a hydrogen bond with Tyrosine. Heme groups are the same in all known leghemoglobins, but the amino acid sequence of the globin differs slightly depending on bacterial strain and legume species. Even within one leguminous plant, multiple isoforms of leghemoglobins can exist. These often differ in oxygen affinity, and help meet the needs of a cell in a particular environment within the nodule.

Debate on principal function Results of a 1995 study suggested that the low free oxygen concentration in root nodule cells is actually due to the low oxygen permeability of root nodule cells. It follows that the main purpose of leghemoglobin is to scavenge the limited free oxygen in the cell and deliver it to mitochondria for respiration. But, scientists of a later 2005 article suggest that leghemoglobin is responsible both for buffering oxygen concentration, and for delivery of oxygen to mitochondria. Their leghemoglobin knockout studies showed that leghemoglobin actually does significantly decrease the free oxygen concentration in root nodule cells, and that nitrogenase expression was eliminated in leghemoglobin knockout mutants, assumably due to the degradation of nitrogenase with high free oxygen concentration. Their study also showed a higher ATP/ADP ratio in wild-type root nodule cells with active leghemoglobin, suggesting that leghemoglobin also assists with delivery of oxygen for respiration. Plants contain both symbiotic and nonsymbiotic hemoglobins. Symbiotic hemoglobins are thought to be important for symbiotic nitrogen fixation (SNF). In legume, SNF takes place in specialized organs called nodules which contain bacteroids, or nitrogen fixing rhizobia. The induction of nodule-specific plant genes, which include those that encode for symbiotic leghemoglobins (Lb), accompany nodule development. Leghemoglobins accumulate to millimolar concentrations in the cytoplasm of infected plant cells prior to nitrogen fixation to buffer free oxygen in the nanomolar range, which can avoid inactivation of oxygen-labile nitrogenase while keeping a high enough oxygen flux for respiration in the cell. The leghemoglobins are required for SNF but are not required for plant growth and development in the presence of an external source of fixed nitrogen. Leghemoglobins make the essential contribution of establishing low free-oxygen concentrations while keep a high energy status in cells. These are the conditions necessary for effective SNF.

Other plant hemoglobins

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Leghemoglobin

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

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

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

Frequently asked questions

What is Leghemoglobin in simple terms?

Why does Leghemoglobin 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 Leghemoglobin?

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 Leghemoglobin.