Banana is a major tropical fruit crop distributed in more than 120 countries with an annual production of 102 million tonnes. Banana belongs to the family Musaceae under the order of Zingiberales with two genera, Musa L. and Ensete Bruce . It is highly diversified throughout the world but has been reported to originate from Southeast Asia . The genus Musa consists of around 50 species while Ensete has nine species . On the basis of phenotypic traits and basic chromosome number, Musa has been divided into four sections namely, Eumusa, Rhodochlamys, Austra-limusa and Callimusa. Most edible bananas that have largely evolved from two wild species, M. acuminata Colla and M. balbisiana Colla belong to the section Eumusa [4,5].
Traditionally banana has been used for numerous medicinal purposes. For example, banana can be useful in many forms for the treatment of diarrhea, and gastric ulcers, etc. Banana is an excellent source of potassium and a number of vitamins . Jain et al.  investigated the antibacterial and antioxidant activities of different parts of seeded banana fruits in vitro. Dried peels, pulps and seeds of the fruit were extracted with hexane, ethyl acetate and ethanol. Antibacterial property of the extracts was evaluated against Gram-positive and Gram-negative bacteria using the disc diffusion technique. The antioxidant activities were determined by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging system, ferric reducing ability in plasma (FRAP) assays and total phenolic content (TPC) assays . It was shown that the extracts of local seedy banana fruits possessed significant antibacterial and antioxidant activities. Sampath et al.  also reported that all parts of the banana plant have medicinal applications and at the same time banana lectins exhibit the potential of inhibiting HIV-1 reverse transcriptase activity, suppressing cancer cell proliferation and stimulating macrophage activities. Nevertheless, compared to other plant lectins, very few banana lectins have been structurally characterized or produced in a recombinant form. Therefore the aim of this article is to provide an overview of the structure, functions and exploitable properties of banana lectins.
Lectins are a group of proteins of non-immune origin that recognize and bind to carbohydrates without modifying them . Lectins are known to be important for many biological processes, due to their ability to recognize cell-surface carbohydrates with high specificity. Plant lectins have been model systems for studying protein-carbohydrate recognition, because individually they exhibit high sensitivity and as a group large diversity in recognizing carbohydrate structures. Some of the well-characterized roles of lectins are in cell-cell communication, cancer metastasis, embryogenesis, host-pathogen interactions, and tissue development . A large-scale data acquisition and extensive analysis of sequences and structures of beta-prism-I or jacalin-related lectins (JRLs) have shown that hypervariability in the binding site loops generates carbohydrate recognition diversity .
During an investigation of the human antibody response to various foods, Koshte et al.  observed marked binding of IgG4 to banana (Musa paradisiaca) extract from its fruit. In view of the mucilaginous nature of the banana extract, they considered the possibility of non-specific binding to IgG4 and decided to purify the active principle. They found that this binding activity was largely removed by gel filtration on a Sephadex G-75 column and was recovered by elution with sugar (mannose). This clearly indicated the presence of a lectin. They named it as BanLec-I .
That was how the banana lectin (BanLec or BL) was first isolated from Musa paradisiaca by Koshte and colleagues . They also reported that BanLec is a homodimeric protein that binds mannose and mannose-containing oligosaccharides and functions as a potent T-cell mitogen [9,13]. Later on, Peumans and colleagues cloned and sequenced BanLec (Musa acuminata) and confirmed that it is composed of two identical 15-kDa subunits demonstrating specificity toward mannose. Based on sequence homology and secondary structure predictions, they classified BanLec as a member of JRL .
Peumans et al.  identified one of the predominant proteins in the pulp of ripe bananas (Musa acuminata L.) and plantains (Musa spp.) as a lectin. The identification of these lectins in banana plants and plantains demonstrates that JRLs also occur in monocot species. This point clearly suggests that lectins are actually more widespread among higher plants than is believed.
The banana and plantain lectins are the first documented examples of JRLs, which are present in abundance in the pulp and roots of mature fruits, but are apparently absent from other tissues like peel, leaf and corm. However, Clendennen et al.  demonstrated that after treatment of intact banana plants with methyl jasmonate, BanLec can be induced in leaves. This clearly indicates that lectin might be expressed as a response to abiotic or biotic stress factors.
The banana and plantain agglutinins (called BanLec and PlanLec, respectively) have been purified in sizeable quantities using a novel isolation procedure, such as by preventing adsorption of the lectins onto insoluble endogenous polysaccharides. Original isolation procedures for BanLec adopted traditional affinity chromatography and acetic acid extraction of overripe bananas, presumably because much of the starch has been degraded and the low pH further reduced starch–lectin interaction. However, Wearne et al. , reported the added disadvantages of this procedure which include a low yield (10 mg/kg) of purified lectin and it may be due to low solubility of the lectin in an acidic medium, and/or to degradation of lectin during the ripening process.
At the other extreme, one-step purification of lectins using sugar-immobilized gold nanoparticles (SGNPs) was developed by Nakamura et al. . With the use of such glycan-coated nanoparticles, a nearly 100-fold greater yield (1 µg/mg) was achieved. They used the 60% ammonium sulfate fraction obtained from the crude plant extract, incubated it with SGNPs overnight at 4 °C to allow to form aggregate of lectin and SGNPs. The lectin was finally dissolved by adding inhibitory sugars. These sugars are similar or identical to the non-reducing sugar moieties on the SGNPs. This procedure reflects the ability of the nanoparticles, coated with a high density of reactive mannosyl/glucosyl groups, to successfully compete with the starch of banana pulp for binding with the lectin [16,17].
Plant lectins have been classified into five structural families  in terms of their polypeptide folds. Of these, the β-prism I fold in lectins was first characterized by Sankaranarayanan et al. in jacalin from jackfruit seeds . The other lectin from jackfruit seeds with different sugar specificity, artocarpin, also exhibits the same fold . Originally, the β-prism I fold was thought to be characteristic of Moraceae lectins. However, lectins from other families were subsequently shown using X-ray analysis to display the same fold: e.g., calsepa from Calystegia sepium (family Convolvulacea) and heltuba from Helianthus tuberosus (family Asteraceae) . On the basis of sequence similarity, Peumans et al., have also disclosed that banana lectin also manifests the same β-prism I fold .
Peumans et al. , showed that both BanLec (banana lectin) and PlanLec (plantain lectin) are dimeric proteins composed of two identical 15-kDa subunits each of which contains 141 amino acid residues. By molecular cloning and molecular modelling studies of the protein they also revealed that BanLec has sequence similarity to previously described lectins of the family of JRLs, possessing the same overall fold and three-dimensional structure . Raval et al. , classified the JRLs family into three categories based on their quaternary structures. The first group contains jacalin and artocarpin, the second group contains calsepa and the third group contains heltuba . Meagher et al. , proposed the fourth structural category of JRLs which contains the BanLec because its quaternary structure shows a unique dimer . Dimeric banana lectin and calsepa, tetrameric artocarpin and octameric heltuba are mannose-specific β-prism I fold lectins with nearly the same tertiary structure .
Khan et al. , proposed the ribbon-shaped structure in BanLec (homodimeric protein), in which each subunit has a single Trp residue at position 10. Also each subunit has twelve β strands arranged in a β-prism-I fold. More importantly, the structure of BanLec obtained by X-ray crystallography revealed the presence of a second sugar binding site. The residues involved in the second site are common to other lectins in this family, potentially signaling a new group of mannose-specific jacalin-related lectins (mJRL) with two sugar binding sites i.e., two primary sugars-binding sites (for the same type of sugar residue) per subunit of banana lectin. Molecular cloning revealed that BanLec has sequence similarity to previously described lectins of the family of JRL, and according to molecular modeling studies it has the same overall fold and three-dimensional structure [14,22].
BanLec is reported to be the only one among the lectins of known structure with β-prism I fold to exhibit more than one primary combining site per subunit . It is known that all forms of monocot plants invariably have the β-prism II fold lectins with three carbohydrate-binding sites in each subunit/domain. Whilst β-prism I fold lectins of known structure are all from dicots and they exhibit one carbohydrate-binding site per subunit/domain. However the BanLec which is from a monocot, has β-prism fold I lectin structure with two very similar carbohydrate-binding sites .
BanLec furnishes a unique example in proteins where subunit association is not a result of primary hydrophobic interactions. It was reported by Gupta et al. , that BanLec is very stable and is denatured only at high concentrations of chaotropic agents. The main reason behind the stability of banana lectin dimer might be attributed to strong hydrogen bonds at the dimeric interface along with the presence of water bridges.
5. BanLec—Properties and Binding Specificity
BanLec, the mannose-specific lectin from banana (Musa acuminata, or polyploidy hybrid cultivars thereof), recognizes terminal α-d-mannosyl/glycosyl units, internal α-1,3-mannosyl/glucosyl units, and reducing terminal β-1,3-glucosyl units such as those occuring in laminarin, the polysaccharide from the brown alga Laminaria digitata. This specificity renders it of potential value in medicinal applications, such as inhibition of HIV replication through interaction with its gp120 coat glycoprotein, as has recently been demonstrated [16,24].
The crystallographic, modeling, and simulation studies conducted by Sharma et al. , provide fresh insights into the geometrical details of banana lectin–carbohydrate interactions. The lectin associates at the primary binding site with disaccharides involving mannose or glucose residues on α-1,3 linkage towards the non-reducing end. The reducing end occupies the site when the linkage is β-1,3. Both arrangements appear to be allowed to nearly the same extent when the linkage is α-1,2. Interestingly α-1,3-linked disaccharides cannot bind to galactose-specific β-prism I fold lectins, with the non-reducing end at the primary site on account of steric clashes with an aromatic residue that is present only when the lectin is galactose-specific. BanLec is unique in its specificity for internal β-1,3 linkages as well as β-1,3 linkages at the reducing termini . BanLec also explicitly binds to terminal nonreducing α-d-glucosyl/mannosyl units of oligo-/polysaccharide chain ends [21,27,28]. Mo et al. , performed extensive experiments in order to find out the exact specificity of BanLec (Musa acuminata) and lectin from the closely related plantain (Musa spp.) by using the techniques of quantitative precipitation, hapten inhibition of precipitation, and isothermal titration calorimetry and showed that they are mannose/glucose binding proteins with a preference for the α-anomeric form of these sugars. Both generate precipitin curves with branched chain α-mannans (yeast mannans) and α-glucans (glycogens, dextrans, and starches), but not with linear α-glucans containing only α-1,4- and α-1,6- glucosidic bonds (isolichenan and pullulan). Another novel observation was also made that banana and plantain lectins recognize their specific binding sites that occur in the linear polysaccharides of elsinan and nigeran, which is unlike that of other mannose/glucose binding lectins (concanavalin A and lectins from pea and lentil).
Again Goldstein et al. , further showed that this lectin also binds to the reducing glucosyl groups of β-1,3-linked glucosyl oligosaccharides (e.g., laminaribiose oligomers). Additionally, banana lectin also recognizes β-1,6-linked glucosyl end groups (gentiobiosyl groups) which occur in many fungal β-1,3/1,6-linked polysaccharides. This behavior clearly distinguishes the banana lectin from other mannose/glucose binding lectins, such as concanavalin A and the pea, lentil and Calystegia sepium lectins. Therefore BanLec is unique in its recognition of internal glucosyl/mannosyl residues linked α-1,3-, but not β-1,3-, and especially in its recognition of the reducing sugar unit of laminaribiose (Glcβ1,3Glc) and its higher homologues [27,28,29].
An X-ray analysis of BanLec was undertaken by Singh et al. [18,30], owing to its interesting sugar-binding properties and its similarity to and differences from other JRLs. It was subsequently shown that BanLec is mannose/glucose-specific, with a preference for α-anomeric forms. In addition to α-1-3-linked glucosyl residues, BanLec recognizes reducing glucosyl groups of β-1-3-linked glucosyl oligosaccharides and β-1-6-linked glucosyl end-groups [18,27,29]. Cheung et al. , reported another banana lectin from Del Monte bananas (Musa acuminata) having fructose as the most potent inhibitory sugar.
6. Biological Properties
BanLec is a mannose-specific lectin which readily agglutinates rabbit erythrocytes. BanLec exhibits a strong mitogenic activity towards murine T-cells. The relevance of the mitogenicity of the banana lectin in terms of their physiological role and the impact on food safety was also discussed by Peuman et al. , Singh et al.  and Sankaranarayanan et al. . BanLec is also reported to act as a potent inhibitor of HIV replication [21,32]. This lectin binds to high mannose carbohydrate structures, including those found on viruses containing glycosylated envelope proteins such as human immunodeficiency virus type-1 (HIV-1). Therefore, Swanson et al. , hypothesized that BanLec might inhibit HIV-1 through binding of the glycosylated HIV-1 envelope protein, gp120. They also found that BanLec inhibits primary and laboratory-adapted HIV-1 isolates of different tropisms and subtypes with IC50 values in the low nanomolar to picomolar range. The mechanism of BanLec-mediated antiviral activity was explained by their data indicating that BanLec inhibits HIV-1 infection by binding to the glycosylated viral envelope and blocking cellular entry. They also compared the relative favorable anti-HIV activity of BanLec towards other anti-HIV lectins, such as snowdrop lectin and griffithsin, and to anti-HIV drugs currently in clinical use (T-20 and maraviroc). Based on these results, they concluded that BanLec is a potential anti-viral microbicide that could be used to prevent sexual transmission of HIV-1.
BanLec is analogous to receptors present in the plasma and blood cells of crayfish and insect species which recognize and bind to β1,3-glucans . A mannose-specific plant lectins from the Amaryllidaceae family also showed microbicidal property for preventing HIV infection . Balzarini et al. , showed that the α-(1-3)-d-mannose- and α-(1-6)-d-mannose-specific agglutinins (lectins) from Galanthus nivalis, Hippeastrum hybrid, Narcissus pseudonarcissus, and Listera ovata inhibited infection of MT-4 cells by human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) and simian immunodeficiency virus. They also suggested that these plant lectins interfere with an event in the HIV replicative cycle (fusion process). It explains why BanLec possesses anti-HIV property as it also has specificity towards similar internal α-1-3-linked mannosyl/glucosyl residues.
Cheung et al. , reported a lectin from Del Monte banana (Musa acuminata) with cytokine-inducing activity in murine splenocytes. It was able to induce the expression of cytokines such as interferon-gamma, tumor necrosis factor-alpha, and interleukin-2. The lectin also inhibited proliferation of leukemia (L1210) cells and hepatoma (HepG2) cells and the activity of HIV-1 reverse transcriptase. Wong et al. , also isolated another banana lectin from Musa basjoo cv. (Emperor Banana) demonstrating stimulatory effects on uptake of [3H-methyl]-thymidine by mouse splenocytes and nitric oxide production by mouse macrophages. The lectin inhibited proliferation of leukemia cells (L1210) and the activity of HIV-1 reverse transcriptase. Hinge et al. , and Bajaj et al. , showed that with respect to gene expression as well as the signaling mechanisms involved, BanLec from Musa paradisiaca produced an adipogenic action on mesenchymal cells like to that mediated by insulin. They also suggested that this banana lectin may serve as micronutrients for therapeutic purposes in hematological deficiencies. In another paper, Bajaj et al. , showed that BanLec interacted with insulin receptors on M210B4 cells and induced a mitogen-activated protein kinase (MEK)-dependent ERK signaling in these cells. Biological properties of various reported BanLec are summarized in Table 1.
Table 1. Biological properties of various reported banana lectins.
Your class has been writing a few argumentative essays here and there, and you have to admit … you’re getting pretty good at it. But now your instructor says that you need to take it a step further and write a synthesis essay.
The name might be a little intimidating, but don’t worry—I’ll be here to give you example topics and walk you through the steps to writing a great synthesis.
First … What Is a Synthesis Essay?
Before we jump right into generating ideas and writing your synthesis, it would be pretty useful to know what a synthesis essay actually is, right?
When you think about a synthesis essay, you can think of it as being kind of like an argumentative essay.
There is one key difference, though—your instructor provides you with the sources you are going to use to substantiate your argument.
This may sound a little bit easier than an argumentative essay. But it’s a different kind of thinking and writing that takes some time to get used to. Synthesis essays are all about presenting a strong position and identifying the relationships between your sources.
Don’t fall into the trap of simply summarizing the sources. Instead, make your point, and back it up with the evidence found in those sources. (I’ll explain this in more detail when we talk about the writing process.)
Many of your sources will probably have information that could support both sides of an argument. So it’s important to read over them carefully and put them in the perspective of your argument.
If there’s information that goes against your main points, don’t ignore it. Instead, acknowledge it. Then show how your argument is stronger.
If this all seems a little too theoretical, don’t worry—it’ll all get sorted out. I have a concrete example that takes a page from the Slytherins’ book (yes, of Harry Potter fame) and uses cunning resourcefulness when analyzing sources.
Great and Not-So-Great Topics for Your Synthesis Essay
A great topic for a synthesis essay is one that encourages you to choose a position on a debatable topic. Synthesis topics should not be something that’s general knowledge, such as whether vegetables are good for you. Most everyone would agree that vegetables are healthy, and there are many sources to support that.
Bad synthesis topics can come in a variety of forms. Sometimes, the topic won’t be clear enough. In these situations, the topic is too broad to allow for you to form a proper argument. Here are a few example bad synthesis essay topics:
Synthesis on gender
Write about education
Form an argument about obesity
Other not-so-great examples are topics that clearly have only one correct side of the argument. What you need is a topic that has several sources that can support more than one position.
Now that you know what a bad topic looks like, it’s time to talk about what a good topic looks like.
Many great synthesis essay topics are concentrated around social issues. There’s a lot of gray area and general debate on those issues—which is what makes them great topics for your synthesis. Here are a few topics you could write about:
Do video games promote violence?
Is the death penalty an effective way to deter crime?
Should young children be allowed to have cell phones?
Do children benefit more from homeschooling or public school?
The list of good topics goes on and on. When looking at your topic, be sure to present a strong opinion for one side or the other. Straddling the fence makes your synthesis essay look much weaker.
Now that you have an idea of what kinds of topics you can expect to see, let’s get down to how to actually write your synthesis essay. To make this a little more interesting, I’m going to pick the following example topic:
Are Slytherin House members more evil than members of other houses?
Steps to Writing an Impressive Synthesis Essay
As with any good essay, organization is critical. With these five simple steps, writing a surprisingly good synthesis essay is surprisingly easy.
Step 1: Read your sources.
Even before you decide on your position, be sure to thoroughly read your sources. Look for common information among them, and start making connections in your mind as you read.
For the purposes of my Slytherin synthesis example, let’s say I have four different sources.
- Source A is a data table that lists the houses of all members of the Death Eaters.
- Source B is a complete history of the Slytherin House, including the life and views of Salazar Slytherin.
- Source C is a document containing the names of students who were sorted into a different house than what the Sorting Hat had originally assigned to them.
- Source D is a history of the Battle of Hogwarts.
Step 2: Decide what your position is.
After you work through your sources, decide what position you are going to take. You don’t actually have to believe your position—what’s more important is being able to support your argument as effectively as possible.
Also, remember that once you pick a position, stick with it. You want your argument and your synthesis to be as strong as possible. Sticking to your position is the best way to achieve that.
Back to our example … after reading through my documents, I decide that the students and alumni of the Slytherin House are not more evil than students in the other houses.
Step 3: Write an awesome thesis statement.
Once you’ve decided on a position, you need to express it in your thesis statement. This is critical since you will be backing up your thesis statement throughout your synthesis essay.
In my example, my thesis statement would read something like this:
Students and alumni from Slytherin are not more evil than students in the other houses because they fill the whole spectrum of morality, evil wizards are found in all houses, and their house traits of cunning, resourcefulness, and ambition do not equate to an evil nature.
Step 4: Draft a killer outline.
Now that you have your argument down in words, you need to figure out how you want to organize and support that argument. A great way to do this is to create an outline.
When you write your outline, write your thesis statement at the top. Then, list each of your sub-arguments. Under each sub-argument, list your support. Part of my outline would look like this:
Thesis statement: Students and alumni from Slytherin are not more evil than students in the other houses because they fill the whole spectrum of morality, evil wizards are found in all houses, and their house traits of cunning, resourcefulness, and ambition do not equate to an evil nature.
I. Evil wizards are found in all houses.
A. Source A: Examples of Death Eaters from other houses
B. Source D: Examples of what Death Eaters from other houses did at the Battle of Hogwarts
In my outline, I used my sources as the second level of my outline to give the names of the sources and, from each, concrete evidence of how evil non-Slytherin wizards can be.
This is only an example of one paragraph in my outline. You’ll want to do this for each paragraph/sub-argument you plan on writing.
Step 5: Use your sources wisely.
When thinking about how to use your sources as support for your argument, you should avoid a couple mistakes—and do a couple of things instead.
Don’t summarize the sources. For example, this would be summarizing your source: “Source A indicates which houses the Death Eaters belong to. It shows that evil wizards come from all houses.”
Do analyze the sources. Instead, write something like this: “Although many Death Eaters are from Slytherin, there are still a large number of dark wizards, such as Quirinus Quirrell and Peter Pettigrew, from other houses (Source A).”
Don’t structure your paragraphs around your sources. Using one source per paragraph may seem like the most logical way to get things done (especially if you’re only using three or four sources). But that runs the risk of summarizing instead of drawing relationships between the sources.
Do structure your paragraphs around your arguments. Formulate various points of your argument. Use two or more sources per paragraph to support those arguments.
Step 6: Get to writing.
Once you have a comprehensive outline, all you have to do is fill in the information and make it sound pretty. You’ve done all the hard work already. The writing process should just be about clearly expressing your ideas. As you write, always keep your thesis statement in mind, so your synthesis essay has a clear sense of direction.
Now that you know what a synthesis essay is and have a pretty good idea how to write one, it doesn’t seem so intimidating anymore, does it?
If your synthesis essay still isn’t coming together quite as well as you had hoped, you can trust the Kibin editors to make the edits and suggestions that will push it to greatness.
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