Structure and Function of Macromolecules - 1 As we stated in our carbon introduction, the majority of the molecules found in living organisms are based on carbon, (along with nitrogen, oxygen and hydrogen in the functional groups). Their specific chemical properties are, to a large extent, determined by the functional groups attached to the carbon backbones. Many of our molecules are large, and are assembled from smaller molecules that are either identical to each other, or similar to each other. These large molecules are called macromolecules or polymers. The "building blocks" of these polymers are called monomers or subunits, and have a common structure. Because our polymers are large molecules, and based on carbon, we can get a great diversity of them from just a few small monomers, by varying number, sequence and bonding arrangements. Our biological macromolecules are grouped into four categories: proteins, nucleic acids, lipids and carbohydrates. We shall discuss structure and functions of each group. Most of our biological molecules are assembled or broken down using the same type of chemical reaction, one which involves adding or removing water molecules. Macromolecules are formed from their subunits by removing molecules of water (a hydrogen (-H) from one subunit and the hydroxyl (-OH) from the second) to join the subunits together. This is called a dehydration synthesis, or condensation. When larger molecules are broken down, such as in digestion, water molecules are added in to break the macromolecules into their subunits, a process called hydrolysis. The enzymes that facilitate digestion are called hydrolytic enzymes. Let's now look with some detail at the major compounds of living organisms. We shall look at Proteins, Lipids and Carbohydrates, and briefly the fourth: Nucleic Acids. We will deal with the nucleic acids in depth during our unit on molecular genetics.
Structure and Function of Macromolecules - 2 Amino Acids and Proteins Proteins are very large molecules composed of combinations of about 20 different amino acids. The precise physical shape of a protein is very important for its function. A single cell may have 10,000 or more different proteins. This diversity of proteins is essential for the functioning of each cell in a living organism. It has been estimated that there may be over 100,000 different kinds of proteins. As much as half of the non-water component of a typical cell can be protein. Functions of Protein Enzymes • Globular proteins that facilitate chemical reactions. Defense Proteins • Antibodies • Protein toxins Transport Proteins • Plasma membrane proteins carry substances through membranes or form channels or pumps for passage • Oxygen carrier in circulation (hemoglobin) • Mineral protein carriers (iron, zinc) Structural/Support Proteins (Fibrous proteins) • Connective tissue in animals (collagen – the most abundant vertebrate protein) • Webs, cocoon s and other arthropod structures • Hair, nails horns, etc. (keratin) • Fibrins used in blood clotting Contractile Proteins – locomotion and movement • Muscle • Cilia and flagella, • Microtubules, microfilaments and intermediate filaments Regulatory Proteins • Hormones • Gene Regulators • Osmotic regulation Receptor Proteins • Membrane surface receptor proteins • Signal transduction proteins Recognition Proteins • Glycoproteins (carbohydrate-protein hybrids) for identification of "self". Storage Proteins (specialized) • Examples are casein in milk, ferritin for iron storage, calmodulin for calcium and albumin in eggs Energy transfer molecules • Cytochromes
Structure and Function of Macromolecules - 3 Protein Structure Protein structure is critical for its function. Each protein has a unique shape or conformation. However, all proteins are composed exclusively of subunits of amino acids, which join together in long chains called polypeptides that fold or coil into the unique shape of the functional protein. To discuss the formation of a protein we need to first discuss the structure of amino acids. Amino acids • Amino acids contain Carbon, Hydrogen, Oxygen, Nitrogen, and sometimes Sulfur • Amino acids have two function groups NH2 Amino functional group COOH Carboxyl functional group • Both functional groups attach to a specific asymmetric carbon (one in which bonds to four different atoms or molecular fragments) called the alpha (α) carbon, of the carbon chain. The third bonding site of the alpha carbon is typically Hydrogen. The alpha carbon will have at its fourth bonding site a side chain, or R group that gives the amino acid its unique structure and properties. • There are 20+ different amino acids in protein. All have a common structure [see text for structures of the different amino acids] except for the R group.
HO—C=0 = carboxyl (acid) functional group; H—N—H | | H
= amino functional group
Characteristics of amino acids • Some amino acids have R groups that are polar that contain oxygen or sometimes just –H. Polar amino acids are hydrophilic. • Some amino acids have R groups that are nonpolar, typically with -CH2 or -CH3 R groups. nonpolar amino acids are hydrophobic. • Amino acids that ionize have R groups that are acidic (generally with a (-) charge) or basic. • Amino acids with hydrocarbon ring R groups are often aromatic. • Amino acids, when ionized also have the amino functional group positively charged and the carboxyl functional group negatively charged.
Structure and Function of Macromolecules - 4 The unique properties of the different amino acid R groups will affect the structure of the protein formed so that the number, kind, and bonding sequence of amino acids in a protein is critical. For example: • Cysteine contains sulfur in the R group, so cysteines can form disulfide bonds (disulfide bridges) linking amino acids in the chain together. • Proline causes kinks in the amino acid. • Methionine is the first amino acid in a protein. • Amino acids are joined together by a dehydration synthesis of amino/carboxyl groups forming a peptide bond. How do amino acids join to make a protein? 1. A protein starts as a chain of amino acids, called a polypeptide Amino acids are joined by the peptide bond, via dehydration synthesis t o 2. form the polypeptide that occurs between the carboxyl functional group of one amino acid and the amino functional group of the second. The polypeptide chain is referred to as the primary structure of the protein. 3. The specific amino acids in the polypeptide chain will determine its ultimate 4. conformation, or shape, and hence, its function. Even one amino acid substitution in the bonding sequence of a polypeptide can dramatically alter the final protein's shape and ability to function.
Peptide Bond How 1. 2. 3. 4.
do polypeptides vary? Number of amino acids in the chain: 50—1000 or so Which kind of amino acids are in the chain (of the 20 types) How many of each kind of amino acid The bonding order or sequence of amino acids
Note: The first protein sequenced was insulin, a tiny protein, which was accomplished about 50 years ago by Frederick Sanger's group in Cambridge. Today we use super computers to sequence proteins, but even for the computer, it's a challenge.
Structure and Function of Macromolecules - 5 A Closer Look at Protein shape and structure The polypeptide chain is just the beginning of a protein. Functional proteins undergo further processing to realize a final functional shape or conformation. Some proteins are composed of more than one polypeptide. The surface structure of the protein is critical for its function, such as with hemoglobin where exterior facing R groups must be polar to hold the heme (iron containing) group that binds oxygen molecules. In fact, virtually all proteins have their nonpolar amino acids oriented in the interior of the protein, leaving polar and charged amino acids to face their aqueous environment of the cell. The function of many proteins depends on a specific region of the protein that binds to another molecule. Antibodies, critical to the immune system, function by binding to specific regions of the antigen molecules, to deactivate them. An enzyme binds to the substrate (the reactants) at a specific active site on the enzyme. How do proteins acquire their unique functional shapes? The peptide bonds that form between amino acids result in the primary structure of a protein, the polypeptide. The polypeptide undergoes a number of modifications before assuming its functional shape. Secondary Structure As peptide bonds are formed, aligning the amino acids, hydrogen atoms of amino functional groups are attracted to the double bonded oxygen atom at the peptide bond and form hydrogen bonds. This bonding coils the polypeptide into the secondary structure of the protein, most commonly the alpha (α ) helix, discovered by Linus Pauling. The α-helix coils at every 4t h amino acid. Some regions of the polypeptide have portions that lie parallel to each other (still held by hydrogen bonds) instead of in the alpha helix, in which the amino acids' hydrogen bonds form a pleated structure. Fibrous proteins have significant pleated structures, called the beta ( β ) pleated sheet. Motifs There are common patterns associated with the secondary structure of proteins. These patterns are called motifs. The β α β motif, for example, causes a fold in a protein. Motifs will be discussed in gene regulation, where transcription factors bind to the DNA at specific "binding motifs". One such motif is the " α turn α" or "helix turn helix" motif. A second is the β α β α β motif.
Structure and Function of Macromolecules - 6 Tertiary Structure Following the secondary shape, openings for bonding along the side chains (the R groups) of amino acids causes more folding or twisting to obtain a final, threedimensional functional protein, called the tertiary structure. Hydrophobic regions typically form in the tertiary structure among groups of amino acids with non-polar side chains forcing those amino acids to the interior of the protein with a very precise and tight fit. van der Waals interactions occur between these amino acids. Any change in a nonpolar amino acid will affect this precise fit and disrupt the shape of the final protein. • Disulfide bonds (which are strong covalent bonds) between nearby cysteine molecules are important to the tertiary structure as well, as are hydrogen bonds and some ionic bonds between charged R-groups. The final conformation for most proteins is a globular shape. •
Domains The instructions for protein structure are coded in DNA molecules. Within a gene (a region of DNA that codes for polypeptide instructions) are areas called exons. (We will discuss exons molecular genetics unit.) To our point here, however, is that each exon-coded section of a protein folds independently into a unit called the domain. The domains are connected by the rest of the polypeptide. Functionally, domains may perform different functions for a given protein. For example, one domain of an enzyme might be the attachment site for a co-factor and a second domain may function as the active sire of the enzyme. Quaternary Protein Structure If two or more polypeptide chains join in aggregates, they form a quaternary structure, such as in the protein molecule, hemoglobin. Often quaternary proteins are complexed with a different molecule, often a mineral. Hemoglobin contains iron, for example. Collagen, a common protein found in connective tissue, has a collagen helix, produced when three polypeptides coil around each other. Consequently collagen fibers are very strong. Getting the Protein "Folded": Chaparonins Although we may present the sequence of protein structure (primary, secondary and tertiary as an automatic process, it is not spontaneous. Special proteins, called chaparonins, are essential for protein folding. It is now believed that chaparonins function to help proteins that are not folding correctly to unfold and refold correctly. Many work best at higher temperatures when the bonds needed for secondary and tertiary shape are unstable. Science does not yet know how chaparonins really work.
Structure and Function of Macromolecules - 7 One area of study is looking at the role of defective chaparonins in some diseases, such as cystic fibrosis and Alzheimer's. Protein Stability As we have seen, the physical structure, or conformation, of a protein is maintained by weak bonds. Many of these bonds are hydrogen bonds formed from the polarity of the amino acids and their “R” groups. If these weak bonds are broken, the protein structure is destroyed and the molecule can no longer function. This process is called denaturation. Things which can denature protein: Heat (as low as 110 F, many @ 130 F) 1. 2. Heavy metals (e.g., silver, mercury) pH changes 3. Salts 4. 5. Alcohols Ethyl alcohol least toxic 6. Many proteins will denature if placed in a non-polar substance. 7. Other chemicals Enzymes are seriously affected by denaturation – but other proteins of the body can also be denatured. Although in most cases, a denatured protein loses its function permanently, in some cases, re-naturation can occur if the substance that promotes the denaturation is removed from the protein. This is more true of chemical denaturants and particularly in experimental environments.
Structure and Function of Macromolecules - 8 Nucleotides and Nucleic Acids Nucleic acids are our information carrying compounds -- our genetic molecules. As with many of our other compounds, the nucleic acids are composed of subunits called nucleotides. Nucleotides also have independent functions. Functions of Nucleotides • Components of nucleic acids (which are long chains of nucleotides) • Energy carrier molecules (ATP) • Energy transport coenzymes (NAD+, NADP+, FAD+ ) •
Chemical intracellular messengers (e.g., Cyclic AMP, a cyclic nucleotide that carries messages from the cell membrane to molecules within the cell, to stimulate essential reactions)
ATP
Energy Transfer Nucleotides
NAD
Functions of Nucleic Acids Storage of genetic information (DNA) Transmit genetic information from generation to generation (DNA) Transmit genetic information for cell use (RNA) DNA self-replication Most of the information on nucleotides and nucleic acids will be discussed when we discuss genetics and energy relationships of cells. For now we shall just present the basic structure of the nucleotides and nucleic acids. Nucleotide Structure 1. 5–carbon sugar component Ribose Deoxyribose
Structure and Function of Macromolecules - 9 2.
Phosphate group Attached to the sugar's 5' carbon with a phosphodiester bond
3.
Nitrogen Base component attached to the sugar's 1'carbon. There are two types of nitrogen bases: • Single six-sided ring pyrimidines Cytosine Thymine Uracil
•
Double ring purines (six- and five-sided) Adenine Guanine
Arrangement of a Nucleotide:
Nucleic acids (polynucleotides) are formed when covalent phosphodiester linkages form between one nucleotide's sugar's 3' carbon and the phosphate of the next nucleotide to form long chains. In DNA, a double chain is formed when 2 nitrogen bases hydrogen bond between the sugar-phosphate backbones. RNA molecules are single chains.
Structure and Function of Macromolecules - 1 0 Genes (specific regions of DNA molecules) contain the hereditary information of an organism. The linear sequence of nitrogen bases of the nucleotides determines the amino acid sequence for proteins in the cells and tissues. As with all of biology, the processes of evolution are validated in DNA information. Organisms more closely related evolutionarily, have more similar DNA. When the sequence of the beta chain of human hemoglobin is compared to other animals, other primates have virtually no differences, and the number of differences increases when less closely related animals are checked.
Structure and Function of Macromolecules - 1 1 Lipids Many of our common substances are lipids, which include fats, oils (triglycerides), phospholipids, steroids (or sterols), prostaglandins, waxes and terpenes. Lipids generally are not polymers, although some are reasonably large molecules. Lipids are grouped together because they are (mostly) hydrophobic and not soluble in water. Most lipids have a large proportion of C-H bonds that are nonpolar. Lipid Functions • Fuel reserve molecules (Lipids are energy rich, having 9 calories/gram.) Humans store fat reserves in adipose tissue. Adipose cells have a remarkable ability to swell and shrink depending on the amount of fat reserves they contain. • Structure of cell membranes (the function of phospholipids) • Protective surface coatings and insulation • Many hormones (regulatory chemicals) Lipid Characteristics • Most lipids are strictly nonpolar and hydrophobic, so they dissolve in nonpolar substances, but not in water. Many lipids in water tend to take on a conformation that will expose any polar portion to the water and protect the nonpolar regions. This is critical to phospholipids and cell membrane structure. • Most lipids feel "greasy" • Lipids contain large regions of just carbon and hydrogen, as carbon-carbon bonds and carbon-hydrogen bonds and small amounts of oxygen relative to the carbonhydrogen atoms. • The most common lipids are the triglycerides (fats and oils) Fats and Oils (the Triglycerides or Triacylglycerols) The terms fats and oils are terms of convention Fats are "hard" or solid at room temperature Oils are liquids at room temperature Triglycerides are composed of two subunits: • One molecule of the alcohol, glycerol • Three fatty acids [which determine the characteristics or properties of the fat] Fatty acids are chains of hydrocarbons 4—22 carbons long with the carboxyl functional (acid) group at the "head".
Structure and Function of Macromolecules - 1 2 The fatty acid hydrocarbon tails are strictly non-polar, so that triglycerides are hydrophobic molecules. Each carbon within the chain has 2 spots for bonds with hydrogen •
If each carbon has 2 hydrogens the fatty acid is saturated H H H H H H H O=C–C–C–C–C–C–C–C–H HO H H H H H H H
•
If two carbon atoms are double bonded, so that there is less hydrogen in the fatty acid, it is monounsaturated H H H H H H H H H O=C–C–C–C=C–C–C-C-C-C-C–C–H HO H H H H H H H H H H H
•
If more than 2 carbon atoms are unsaturated, the fatty acid is polyunsaturated H H H H H H H O=C–C–C–C=C–C–C-C-C-C=C–C–H HO H H H H H H H H H H H
•
A trans-fatty
acid might look like this: H H H H H H H H H H O=C–C–C–C=C–C–C-C-C-C-C–C–H HO H H H H H H H H H H
Ester bonds attach the glycerol (by a dehydration reaction) to each of the 3 fatty acids, by removing the H from the glycerol's hydroxyl function group and the hydroxyl functional group from the carboxyl head of each fatty acid. O H H H ||
glycerol
|
|
|
H-C-O---C-C-C-C |
|
H
H-H-H
|
fatty acid tail
|
Ester Bond
Structure and Function of Macromolecules - 1 3 Ways that fatty acids are different: 1. Length of chain in fatty acid • Usually an even number of 4 – 26 carbons long (most are 14 - 18) Short chains are more soluble Short chains are more easily broken down Short chains oxidize more easily (process by which fats become "rancid") 2.
Degree of saturation Saturated Monounsaturated Polyunsaturated • Most plant fats tend to be unsaturated, but fats from tropical plants tend to be very saturated • Fish oils tend to be unsaturated (from cold water and salt water fish). Other animal fats tend to be saturated
3.
Liquid vs solid • Short chains and unsaturated chains are liquid at room temperature (molecules are smaller and less dense (the double bonds distort the molecules so they don't fit close together) • Saturated chains are solid (denser) because chains fit together better
Synthetic Fats Olestra is a synthetic fat, marketed under the trade name of Olean. It mimics the texture and properties of triglycerides, is fat soluble, but not digestible or absorbed into the body, so all Olestra consumed passes through the digestive tract. Hence, it is considered to be calorie-free. Olestra is a sucrose polyester, composed of fatty acids attached to sucrose rather than glycerol. Six to eight fatty acids are attached to the sucrose molecule so the lipase digestive enzymes can't function to hydrolyze the ester bonds. Olestra is synthesized from cottonseed or soybean oil, heated in a base-catalyzed reaction with methanol to detach the fatty acids as methyl esters. The glycerol settles out and is drawn off, and the fatty acid methyl esters are distilled. Sucrose and another base catalyst are then added to the fatty acid methyl esters, with emulsifiers. Under high temperature, sucrose polyesters form and methanol is removed. Further processing removes leftover fatty acid esters and emulsifiers. The processing is completed with bleaching and deodorizing. (This information about the structure and processing of Olestra was taken from C&E News 4 / 2 1 / 9 7 . )
Simplesse is a fat substitute that mimics the texture of fat in the oral cavity. It is synthesized from egg and milk proteins. The shape of the simplesse molecule is spherical, resembling miniature marbles, so the product has the slick texture of fat. Simplesse is not heat stable, and cannot substitute for fats in frying or baking.
Structure and Function of Macromolecules - 1 4 Waxes Waxes are similar to fats, although the alcohol component contains more carbon, and more fatty acids are bonded. Waxes are solids. Phospholipids • Structural molecules – major component of all membranes of cells • Phospholipids are composed of a glycerol molecule with two fatty acids attached by ester bonds and a polar phosphate-containing compound attached to the third carbon.
•
• • •
•
The value of phospholipid structure is that the phosphate region of the molecule is charged, ideal for the cell membrane structure, and the fatty acid portions are strictly non-polar. Molecules that have these properties are said to be amphipathic. Hydrophilic portion in the phosphate region (which is negatively charged) Hydrophobic portion in the fatty acid tails Often phospholipids in solution will form micelles, droplets in which the tails point inward and the hydrophilic heads to the outer circumference. Cell membranes are structured from a phospholipid bilayer -- with the heads pointed to the external and the internal environments. The most common phospholipid is lecithin
Structure and Function of Macromolecules - 1 5 Sterols (Steroids) Steroids are composed of hydrocarbon chains with four interconnected rings. Different steroids have different functional groups and are used for variety of purposes: Vitamins A & D Hormones (adrenal cortex & sex hormones) Cholesterol Precursor to most steroid hormones and vitamin D Necessary for structure of nerve system cells Component of animal cell membranes – not found in plants apart from trace amounts Cholesterol is made in the liver from digested fatty acids Although rarely found in plants, certain plant steroids, such as the soy flavinoids, are similar in structure to the estrogen hormones of animals.
Structure and Function of Macromolecules - 1 6 Terpenes Terpenes are found in plants, and include some important pigments such as the carotenoid pigments that are responsible for the orange, red and yellow colors of many plants. There are over 22,000 different terpenes in plants. Many of the aromatic oils found in plants are terpenes. Taxol, an extract from yew, is used to treat ovarian cancer, and digitalin is a cardiac medicine. Two plant hormones, abscissic acid and the gibberllins, are also terpenes, as are two of the electron transfer molecules (ubiquinone and plastoquinone). Economically, rubber is an important terpene. Terpenes are lipid soluble and hydrophobic. Terpenes are composed of isoprene units (C5H 8).
(β-carotene)
Eicosanoids Eicosanoids are modified fatty acids that are important chemical messengers in vertebrates. They include prostaglandins, thromboxanes and leukotrienes. They are synthesized from the omega-3 and omega-6 long-chain essential fatty acids. They are important in immune responses and in regulating body functions such as blood pressure, blood-clotting and immune system responses. Eicosanoids often have antagonistic response. One will cause vasodilatation and a second will cause vasoconstriction of blood vessels, for example.
Structure and Function of Macromolecules - 1 7 Carbohydrates The word carbohydrate is one of convention, derived from carbon and water, the component elements of the carbohydrate monomers (subunits). Carbohydrates include the simple sugars (properly called monosaccharides and disaccharides) and the large polymers or polysaccharides. There are a few oligosaccharides as well. Carbohydrate Functions • Basic energy source (fuel) for virtually all living organisms • Structural molecules, especially of plants, most fungi and arthropods (e.g., cellulose, chitin) • Fuel reserve molecules (e.g., starch, glycogen) All carbohydrates are composed of one or more monosaccharides. The simple sugars are formed from one or two monosaccharides, and the complex carbohydrates (polymers) are formed from long chains of monosaccharides, formed by dehydration synthesis reactions. Structure of the monosaccharide Chemically, monosaccharides contain Carbon, Hydrogen and Oxygen The ratio of atoms in a monosaccharide is: (CH2O) e.g.
Cn(H2O)n C6 H 12O6
C3H6O3 The functional groups of monosaccharides are: –OH Hydroxyl =O Carbonyl A monosaccharide will have one carbonyl functional group, and the remaining oxygen atoms will all be in hydroxyl functional groups. Isomers are common. The common monosaccharides of living organisms are: C6 H 12O6 (glucose, galactose, fructose) Some 5 carbon (ribose, ribulose, xylose) Some 3 carbon (glyceraldehyde)
Structure and Function of Macromolecules - 1 8 Note:
Although we show monosaccharides and other carbohydrates in the chain structure, the equilibrium for carbohydrates in living organisms favors a ring shape. Your text has some good illustrations of the ring forms of some common sugars. In glucose, for example, the ring is formed when the number 1 carbon joins to the number 5 carbon.
Formation of Disaccharides and polysaccharides Disaccharides Disaccharides are 2 monosaccharides joined by a dehydration synthesis, or condensation, which is the removal of a water molecule. The "H" is taken from one monosaccharide and the "OH" from the second. The two molecules are then joined by a C—O—C bond, called a glycosidic bond. Examples of common disaccharides are sucrose, lactose, and maltose. • Lactose is the sugar found in milk, consisting of a glucose bonded to a galactose. • Maltose is most commonly a breakdown product of starch, and consists of two glucose monomers. • Sucrose is a common disaccharide of plants, and is formed from glucose and fructose. (Sucrose is also the only "sugar" which legally must be called "sugar". A food product can be called "sugar-free" if it contains no sucrose, even though it may contain large amounts of other mono- or disaccharides.)
Structure and Function of Macromolecules - 1 9 Polysaccharides Polysaccharides are formed by joining several monosaccharides, each to the next by a dehydration synthesis, forming glycosidic bonds. The position of the bonds is important to the structure and function of the polysaccharides. The common polysaccharides are: Starch (alpha 1–4 glucose linkages) (boat) (hydroxyl groups on the top side)
Glycogen • Both starch and glycogen are polysaccharides of glucose. Starch is a very long coiled, unbranched (amylose) or branching chain (amylopectin), with about 1000 glucose molecules in any branch. Glycogen branches frequently (about every 10 or so glucose units) and is more easily broken down. • Starch and glycogen are important fuel storage molecules. Starch is the most important storage molecule in plants and is stored in amyloplasts, one of the plant organelles. Cells that are specialized for nutrient storage in plants are full of amyloplasts. (A botanist who specializes in such things, can identify amyloplasts from different types of plants. Amyloplasts are also called starch grains (but not by biology students).
Structure and Function of Macromolecules - 2 0 Cellulose (beta 1–4 linkage) (chair) (alternating top/bottom hydroxyl groups) • Long chains of glucose forming linear, flat (pleated) molecules • Cellulose is for most living organisms, non-digestible. Few organisms have the enzyme needed to break down cellulose. Cellulose and related compounds form most of what we call fiber. • It is estimated that cellulose if the most abundant organic compound on earth. • The beta 1-4 linkage means that -OH groups of one cellulose molecule can hydrogen bond to adjacent cellulose molecules forming the microfibrils we see in cell walls.
Chitin • •
Long modified glucose chains, in which a nitrogen-containing functional group replaces one of the hydroxyl groups on each glucose subunit. Chitin forms the exoskeleton of many invertebrate animals (mostly arthropods) and certain groups of fungi whose cell walls are composed of chitin. Many exoskeletons also have calcium carbonate crystals impregnated into the chitin.
Oligosaccharides Oligosaccharides are composed of a few monosaccharides bonded together. Legumes contain some oligosacchaides, most of which can not be digested by humans. Bacteria in our intestines can digest these sugars, and their respiratory by-products often cause intestinal "discomfort" and social embarrassment. In spite of this "reputation", legumes are nutrient dense food items and should be an important part of one's diet. PS: Although your text speculates about the sweetness and non-digestibility of the L-stereoisomers o f our common sugars, they are not a part of our current vast array of sweeteners on the market.
