Elemental Composition of Biomolecules

Variation in elemental composition among organisms can be driven by stoichiometric differences at many levels of internal organization. As outlined below, major classes of biomolecules such as lipids, carbohydrates, protein, and nucleic acids contain different concentrations of C, N, P, and other major elements such as H, O, and S (Table 2).

Table 2 Approximate C:N:P:H:O:S stoichiometry (% of mass) of selected macromolecules and other organic compounds

%C

%N

%P

%H

%O

%S

Lipids

Glycerol (triacylglyceride)a

75

0

0

14

12

0

Cholesterol

84

0

0

12

4

0

Phosphatidylcholine (phosphoglyceride)a

67

2

4

11

16

0

Carbohydrates

Starch and cellulose

46

0

0

3

51

0

Lignin

63

0

0

6

31

0

Chitin

44

7

0

7

42

0

Peptidoglycanb

49

12

0

7

30

1.5

Proteins

Proteinc

54

17

0

7

20

2.7

Rubisco (large subunit)

54

17

0

7

21

1.2

Myosin (heavy chain)

53

17

0

7

22

0.8

Collagen

54

18

0

7

21

0.5

Nucleic acids

DNAd

38

17

10

4

31

0

RNAd

36

16

10

3

35

0

Other organic compounds

ATP

24

14

18

2

41

0

Chlorophyll

74

6

0

8

9

0

aFatty acid chains consisting of stearic acid (saturated fatty acid with 18C atoms). ^Oligopeptide side chain with five amino acids, 5% each of all 20 amino acids. hypothetical nonphosphorylated protein consisting of 5% each of all 20 amino acids. ^Assuming 50% G-C base pairs.

As a result, differences in biomolecular mixtures result in stoichiometric variation at higher levels of organization, such as among cellular structures or tissues. Variation in elemental composition among organisms can potentially be explained by stoichiometric differences at any level of organization, provided components at that level (1) have distinctive elemental signatures, (2) vary significantly in concentration among organisms, and (3) contain a sizable fraction of the whole-organism pool of some element. Determining which components explain the variation in whole-organism stoichiometry is a key step in relating elemental composition to biological functions that affect fitness.

What then is known about variation in elemental composition among major biomolecules, and how might this variation be related to biological function?

Lipids

Lipids are a diverse group of chemicals with a variety of biological functions. Triacylglycerols, waxes, and other fats and oils are the principal mechanism for storing energy in most organisms; sterols and phospholipids serve as structural elements in membranes; and other less-abundant lipids act as electron carriers, pigments, and enzyme cofac-tors. Triacylglycerols, waxes, and phospholipids are the primary components of lipid biomass in organisms, and are thus more likely than other lipids to contribute to a distinctive whole-body elemental composition.

Triacylglycerols and waxes consist of only C, H, and O; they contain neither N nor P. Carbon content in these molecules is generally high. For example, a triacylglycerol with three 16-carbon unsaturated fatty acids will contain 76% C; myricyl palmitate, the primary component of beeswax, contains about 82% C. The C content of triacylglycerols is substantially higher than levels in other major biomolecules (Table 2) and in animal bodies as a whole (see Table 1).

Phospholipids contain a glycerol molecule attached to two fatty acids and a functional group. Phospholipids consist almost entirely of C, H, O, and P, which is contained in a phosphodiester linkage between the functional group and the glycerol molecule. The functional group in some phospholipids does contain N, although the N content in phospholipids is generally low. For example, phosphatidylcholine, a common phospholipid, contains 67% C, 1.9% N, and 4.2% P.

Storage lipids are likely the only form of lipids that will contribute significantly to whole-organism stoichio-metry. Storage lipids can make up a substantial fraction of organism biomass. For example, mean lipid concentration in insects has been measured to be about 25%; similar levels have been measured in marine calanoid copepods and albacore tuna. Similarly, triacylglycerols in adipose tissue make up about 21% of the mass of a nonobese 70 kg man. Storage lipid levels can also vary substantially among taxa and among individuals within taxa. In whole tuna, lipid content can vary from 1% to 43% dry mass.

Lipid stores are known to vary with fluctuations in activity demands and resource availability. Because storage lipids are rich in C but contain no N or P, increases in storage lipid levels will increase C:N and C:P ratios in whole organisms, but will have no effect on body N:P ratios. Directional selection on energy storage could thus result in evolutionary increases in whole-body C:N and C:P ratios. Unlike triacylglycerols, phospholipids contribute little to the total biomass of most organisms. For example, in crustacean zooplankton phospholipids make up only about 6% of total body mass; in the mussel Mytilus edulis, phospholipid content ranged from only 0.36% to 0.64% in soft tissue.

Carbohydrates

Carbohydrates generally serve as fuel, energy stores, and building materials, although specific carbohydrate-containing molecules also act in cell-cell recognition and information transmission. Sugars and starches are carbohydrates involved in energy transport and storage; they have a general chemical formula of (CH2O)„. Thus, like storage lipids, carbohydrates with this basic formula contain neither N nor P. However, they contain less C than lipids. For example, glucose contains 40% C and glycogen contains 52% C, while triacylglycerols typically contain more than 70% C. Carbohydrate fuels generally make up only a small fraction of organism biomass: in humans, glycogen is only about 225 g of the mass of a nonobese 70 kg man. They are thus unlikely to contribute substantially to variation in whole-body C:N:P stoichiometry.

Structural carbohydrates are more significant contributors to organismal stoichiometry, especially in plants. Insoluble carbohydrate polymers serve structural and protective roles in the cell walls of plants and bacteria and in animal connective tissue. Examples of common structural polysaccharides include cellulose and lignin, which are tough, fibrous polymers in plant cell walls; chitin, which strengthens insect exoskeleton; and hyalur-onate, which provides viscosity and lubrication in vertebrate joints. Cellulose (C6H10O5)„ contains 46% C and no N or P. It makes up most of the mass of wood, and will thus largely determine the elemental composition of large terrestrial plants with extensive support structure. Selection favoring investment in cellulose will increase C:N and C:P ratios in whole plants but will not affect plant N:P ratios. Some other structural polysaccharides do contain N. For example, chitin has an acetylated amino group (-NH) in place of the hydroxyl group (-OH) in cellulose. As a result, it contains 6.9% N, which is within the range (6-12% N) found in the bodies of insects. Chitin generally makes up <10% of insect biomass, but chitin investment can differ among species. Because chitin itself contains no P, variation in chitin investment could explain some of the variation in insect C:P and N:P ratios.

Protein

Proteins are versatile nitrogenous biomolecules that serve structural, signaling, and catalytic roles in cells. They occur in thousands of varieties ranging in size from small peptides to large polymers with molecular weights in the millions. The 20 amino acids that are the monomers or building blocks of proteins all have a carboxyl group and an amino group bonded to a carbon atom, but they have different side chains which determine their distinct chemical properties. Differences in side-chain composition are also what determine differences in the elemental composition among amino acids. On average, the 20 common amino acids contain 53% C, 17% N, and no P; greater investment in protein will thus increase levels of N and reduce P content in organisms. Proteins constitute the largest fraction of the biomass in most cells. For example, proteins make up 50% of the dry mass in the bacteria Escherichia coli, 30-50% of the dry mass in crustacean zooplankton, 30% of plant leaf biomass, and about 40% of the dry mass in a nonobese human male. Protein investment thus is a major determinant of whole-body elemental composition.

There is also considerable variability in elemental composition among amino acids. C content ranges from less than 30% in cysteine to 65.5% in phenylalanine, and N content ranges from 7.7% in tyrosine to 32.1% in arginine. This variation has some relationship to amino acid function. For example, amino acids with aromatic side chains (phenylalanine, tyrosine, tryptophan) are relatively nonpolar and participate in hydrophobic interactions; they also have high C (mean = 63.2%) and low N (mean = 10%). In contrast, amino acids classified by their positively charged side chains (lysine, arginine, histidine) contain low C (mean = 45.7%) and high N (mean = 26.2%). These amino acids are important for enzyme catalysis and are often involved in weak electrostatic interactions with negatively charged biomolecules like nucleic acids. Selection on traits biasing the overall amino acid composition of protein could thus be manifested in the elemental composition of whole organisms.

Sulfur is not present in most macromolecules in organisms, but does occur in protein (Table 2). It is present in two amino acids: methionine and cysteine. The average S content of the 20 commonly occurring amino acids is 2.7%, but protein S content is generally lower (on average 1.3%). This can at least partly be explained by the relatively low S content in some of the most commonly occurring proteins, that is, rubisco (ribulose bisphosphate carboxylase, the enzyme catalyzing C fixation in plants, and thus probably the most common protein in the world), myosin (a major protein in muscle tissue), and collagen (a major protein in the extracellular matrix of metazoan animals).

Proteins not only make up a substantial fraction of intracellular mass, they are also key components of important extracellular excretions such as hair, nails, skin, feathers, horns, spider webs, poisons, and venoms. Most of the dry mass of hair, claws, hooves, tortoise shells, and horns consists of a-keratin, a strong fibrous protein. Fibroin, the protein of silk, is a polypeptide rich in N-rich alanine and glycine. These excretions clearly have important functional consequences, and thus provide a rich set of opportunities for determining how elemental composition relates to the evolution of traits.

Nucleotides

Nucleotides are the constituents of nucleic acids (DNA, RNA), which store and transmit genetic information. They also act as carriers of chemical energy in cells, as enzyme cofactors, and as secondary messengers. Nucleotides consist of a nitrogenous base, a five-carbon sugar, and a phosphate group. In DNA and RNA, nucleo-tides are covalently linked by the phosphate group; the negative charge of the phosphate group at neutral pH is essential for stabilizing nucleotides against hydrolysis and for retaining them within a lipid membrane. Differences in the nitrogenous base determine the elemental and functional variation among nucleotides. Nucleotides, as they appear in RNA, contain on average 36.2% C, 16% N, and 9.6% P; the elemental composition of nucleotides in DNA is very similar. Thus, nucleotides in nucleic acids have N levels that are similar to those found in protein (16% vs. 17%), which are in turn much higher than levels in most organisms. Most notably however, nucleotides contain very high levels of P and low C:P and N:P ratios relative to other major biomolecules. The P content of an average nucleotide is also an order of magnitude higher than the P content of most insects, marine invertebrates, and plants.

RNA can comprise a large, but variable fraction of organism biomass. For example, RNA content can range from 12% to 30% of cell dry mass in E. coli, from 0.1% to 14% dry mass in invertebrates, and from 0.02% to 9% dry mass in birds, mammals, and fish. This substantial variation coupled with the high-P, low-N content of RNA make this molecule a major source of variation in organismal C:N:P ratios.

DNA levels are known to be much lower than RNA levels in most organisms. Genome size varies by up to five orders of magnitude among taxa, but this variation is accompanied by corresponding changes in cell size. As a result, DNA levels as a fraction of cellular biomass appear to be quite consistent across organisms and thus likely explain very little of the variation in C:N:P stoichiometry among taxa.

ATP is a nucleoside that is widely used for transporting energy in cells. An ATP molecule contains 24% C,

14% N, and 18% P; it is thus even more P-rich than the nucleotides in nucleic acid. However, ATP generally makes up only a small fraction of the total biomass of most organisms. For example, ATP levels range from 0.3% to 1.8% dry mass in marine copepods, and from only 0.02% to 2% dry mass in insects. As a result, variation in ATP content is unlikely to explain much of the variation among organisms in C:N:P stoichiometry.

Biominerals

Biominerals are inorganic solids produced by a wide variety of organisms to harden and stiffen tissues. Hard tissues containing biominerals are used for support, protection, and resource acquisition. The physical properties of these tissues depend on the identity of biominerals and the degree of biomineralization; thus, there is often a clear link between tissue elemental composition and function. Biominerals also make up a large fraction of the biomass in some organisms.

The three principal classes of skeletal biominerals are calcium carbonates, silica, and calcium phosphates. Calcium carbonate is the most abundant and widespread biogenic mineral. It makes up a large component of mollusk shells. Bird egg shells also consist mainly of calcium carbonate; hen egg shells contain about 95% calcium carbonate. Investment in thick, calcium carbonate-rich egg shells is thought to be an adaptation to hard nesting substrates or other conditions where egg damage is likely. Silica-based scales and skeletons are common in several ameba groups, but are found in only a few animals (sponges, a few copepods, some brachiopod larvae). Diatoms contain uniquely high concentrations of silica (Si:N ratios in diatoms are typically about 1), which serves as a main constituent of diatom cell walls. Silica is also an important component of some grasses and sedges. For example, plants in the genus Equisetum (horsetails) use silicic acid to maintain stem erectness; levels of silica in Equisetum palustre have been reported to be as high as 7.4% of dry mass. Calcium phosphates serve as skeletal material in vertebrates and a few brachiopods. Bone consists mainly of a calcium phosphate known as hydroxyapatite [Ca10(PO4)6], which is also prominent in teeth and antlers. Although calcium-, silica-, and phosphorus-containing molecules are the most common biominerals, other elements such as iron and zinc form the basis of structural molecules in some organisms. For example, the marine bloodworm Glycera dibranchiate contains a copper-based biomineral (atacamite, Cu2(OH)3Cl) in their jaws, which are extremely resistant to abrasion.

In summary, storage lipids, structural carbohydrates, protein, RNA, and biominerals are the biomolecular constituents of organisms that will likely account for most of the variation in elemental composition among organisms. Greater investment in lipids, for instance, will increase

Figure 1 Stoichiometric diagram illustrating how altering storage lipid allocation proportionally affects %N and %P, leaving N:P constant but changing C:P and C:P (arrow A); how altering protein allocation increases %N while lowering %P, increasing N:P (arrow B); and how altering nucleic acid allocation disproportionately affects % P, lowering N:P (arrow C). Dashed lines indicate particular N:P ratios. Sterner RW and Elser JJ; Ecological Stoichiometry. © 2002 Princeton University Press. Reprinted by permission of Princeton University Press.

Figure 1 Stoichiometric diagram illustrating how altering storage lipid allocation proportionally affects %N and %P, leaving N:P constant but changing C:P and C:P (arrow A); how altering protein allocation increases %N while lowering %P, increasing N:P (arrow B); and how altering nucleic acid allocation disproportionately affects % P, lowering N:P (arrow C). Dashed lines indicate particular N:P ratios. Sterner RW and Elser JJ; Ecological Stoichiometry. © 2002 Princeton University Press. Reprinted by permission of Princeton University Press.

body C but lower N and P contents, higher protein levels will increase body N but lower body P content, and greater investment in RNA will substantially increase body P levels (Figure 1). These differences show how selection for functions met by particular biomolecular mixtures can influence the evolution of body composition. Conversely, an organism's elemental composition should reflect its ability to respond to specific adaptive challenges.

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    What are the chemical composition of biomolecules?
    10 months ago

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