Download Macromolecules of the Cell

Macromolecules of the Cell
Four different types of macromolecules are found in cells. These are a. proteins b. carbohydrates
c. nucleic acids d. lipids.
Proteins
These are the most ubiquitous macromolecules in all organisms. They occur nearly in everywhere
in the cell. According to function, proteins are classified into the following classes.
1. Enzymes
2. Structural proteins
3. Motility proteins
4. Regulatory proteins
5. Transport proteins
6. Hormonal proteins
7. Defensive proteins
8. Storage proteins
9. Receptor proteins
The monomers are amino acids
Proteins are linear polymers of amino acids. 20 kinds of amino acids are used in protein synthesis.
Additional amino acids in the cell are due to modifications that occur after protein synthesis.
9 of these amino acids are hydrophobic and 11 are hydrophilic.
The polymers are polypeptides and proteins
The formation of a polypeptide depends on the removal of water molecule in a process called
dehydration or condensation reaction. The bond that is formed between these amino acids is
called the peptide bond.
Proteins that are composed of a single polypeptide chain are called a monomeric protein, whereas
a protein that is composed of more than one subunit is called a multimeric protein.
Bonds and interactions that are responsible for protein stability and folding
a. Disulfide bonds which are a covalent bond that takes place between two cysteine amino
acid residues.
b. Hydrogen bonds
c. Ionic bonds
d. Van der Waals interactions
e. Hydrophobic interactions
All these types of bonds are responsible for the stability of proteins and therefore their functions.
Primary Structure
This depends mainly on amino acid sequence that appears from one end of the molecule to the
other end.
Secondary structure
It involves repetitive patterns of local structure. These patterns result from H-bonding between
atoms in the peptide bonds along the polypeptide backbone. Such interactions result in the
formation of two structural patterns, referred to as a-helix and B-sheet conformations.
Alpha-helix pattern: Every peptide bond in the helix is H-bonded through its CO group to the
peptide bond immediately below it in the spiral and through its NH group to the peptide bond just
above it.
B-sheet pattern: this also depends on the CO and NH interactions.
Certain combinations of a-helices and B-sheets have been identified, these are called motifs. The
a-helices and /or B-sheets in each motif are connected by loops of varying length. Among the
most common motifs are B-a-B motif and the helix-turn-helix motifs
Tertiary structure
This depends mainly on the interactions that take place between the R-groups of the amino acids.
This structure reflects nonrepetitive and unique aspect of each polypeptide because it depends on
its R group. Different types of bonds are used for the folding, coiling and twisting of the protein
into its native conformation.
According to the distribution of these patterns in the proteins, it has been found that in general
proteins can be divided into two categories: fibrous proteins and globular proteins.
Fibrous proteins: have extensive secondary structure throughout the molecule, giving them highly
ordered repetitive structure. The final shape of the protein will be filamentous.
Examples: fibroin protein of silk, keratin of wool and hair, and collagen in tendons.
Globular proteins: they are involved in cellular structure. They are folded into compact form.
The formation of a-helix or B-sheets depends on the type of amino acids present in the
polypeptide. Leucine, methionine, and glutamate are strong a-helix formers, whereas isoleucine,
valine, and phenylalanine are strong B-sheet formers. Proline and glycine are helix breakers.
Many globular proteins consist of a number of segments called domains, which are discrete,
locally folded units of the tertiary structure that usually has a specific function.
Quaternary structure
It is the level of organizationconcerned with subunit interactions and assembly. This is applied for
the multimeric proteins.
Example: multiprotein complex ( pyruvate dehydrogenase complex).
Nucleic Acids
Nucleic acids:
They are very important macromolecules for the cell. They have important role in the storage,
transmission and expression of genetic information. They are linear polymers of nucleotides. The
two types of nucleic acids are DNA and RNA molecules. DNA molecules serve as the repository
of genetic information, whereas RNA has many functions including the expression of that
information.
Nucleotides as the monomers
They are nonidentical monomer units. There are 4 types of them in each nucleic acid. These are
the purines (A and G), and the pyrimidines (T, U and C). Each nucleotide is composed of ribose
sugar, phosphate group and nitrogen base. A nucleoside is composed only of nitrogen base and
sugar without the phosphate group. Two nucleotides are joined together by phosphodiester bond.
The addition of nucleotides occur only at the 3-end of the polymer.
DNA molecule
It is composed of two strands (double helix), arranged in an antiparallel manner. Both strands are
complementary to each other. There are different types of DNA in the nucleus; these are B-DNA,
A-DNA and the Z-DNA.
RNA molecule
It is characterized by being composed of one strand of nucleotides. It depends on DNA for its
synthesis by a process called transcription. The pyrimidine bases are composed of C and U
instead of T. there are different kinds of RNA in the cell such as mRNA, tRNA, rRNA, snRNA
and siRNA.
Polysaccharides
These are long chain polymers of sugars and sugar derivatives. The basic structural unit is the
sugar (monomer). Polysaccharides are composed of sugar units bound together by glycosidic
bonds. Short polysaccharides are called oligosaccharides, when attached to proteins on cell
surface they play important role in cellular recognition of external signals. Polysaccharides act
either as structural (cellulose) or as storage molecules (starch and glycogen). Each of these
polymers contains the six carbon sugar glucose; they differ in the nature of the bond between
successive glucose units as well as in the presence and extent of side branches on the chains.
The Monomers are monosaccharides
Monosaccharides are of two types. These are aldosugars , with a terminal carbonyl group and the
ketosugars, with an internal carbonyl group. Most sugars have between 3-7 carbon atoms. The
single most common monosaccharide is the aldohexose D-glucose (C6H12O6).
Glycosidic bonds
These are of two types according to the location of the hydroxyl group on C atom 1. a(1-4)
glycosidic bond and B(1-4) glycosidic bond.
The polymers are storage and structural polysaccharides.
Starch and glycogen are storage polysaccharides. The type of bond found in each is the a(1-4)
glycosidic bond and a(1-6) glycosidic bond along the backbone, giving rise to side chains.
Storage polysaccharides can be branched or unbranched depending on the presence or absence of
a(1-6) linkages.
Glycogen is highly branched polysaccharide. It is found in muscles and liver tissue.it is also
found in Bacteria as a glucose reserve.
Starch is found in plants. It occurs either as branched (amylopectin) or unbranched (amylase).
Starch deposits are 10-30% amylase and 70-90% amylopectin. Starch is stored as starch grains
within plastids-either chloroplasts (sites for sugar synthesis) or within amyloplasts (specialized
for starch storage).
Cellulose is a structural polysaccharide found in plant cell walls. It is a polymer of repeated units
of B-glucose and the linkage is the B(1-4). Humans cannot digest cellulose because they lack the
enzyme responsible for its digestion.
Cellulose is also found in fungal cell walls B(1-4) or B(1-3) linkages.
Cell wall of bacteria is composed of N-acetylglucosamine and N-acetylmuramic acid and the
bond is B(1-4) linkage.
Chitin is another structural polysaccharide found in insect exoskeletons, crustacean shells, and
fungal cell walls. It is compossed of N-acetylglucosamine and the bond is B(1-4).
Lipids
These are not considered as polymers but macromolecules because of their high molecular weight
and their presence in important cellular structures (membranes). Their synthesis takes place by
the usual condensation reactions. The distinguishing feature of lipids is their hydrophobic nature.
They are soluble only in nonpolar solvents such as chloroform and ether. Some lipids are
amphipathic, having both polar and nonpolar ends.
Functions
1. They serve as forms of energy storage.
2. Involved in cellular membrane structure.
3. some are involved in special biological functions such as transmission of chemical signals
within and into cells.
Lipid classes
1. Fatty acids
2. Triacylglycerols
3.
4.
5.
6.
Phospholipids
Glycolipids
Steroids
Terpenes
Fatty acids
They are the building blocks for several kinds of lipids. These are found as saturated or
unsaturated fatty acids. This depends on the presence or absence of double bonds between the C
atoms in the hydrocarbon chain.
Triacylglycerols
They are called triglycerides; consist of glycerol molecule and three fatty acids linked together by
ester linkage.
The main function is to store energy. They provide insulation layer for certain animals against
low temperature. The presence of saturated fatty acids in these molecules gives them the solid
appearance (fats). The presence of unsaturated fatty acids gives these molecules the liquid
appearance at room temperature (oil).
Phospholipids
They are similar in structure to triglycerides. They are important in membrane structure due to
their amphipathic nature. They are classified as phosphoglycerides or sphingolipids.
Phosphoglycerides are present in most membranes. They consist of fatty acids esterified to a
glycerol molecule. The basic component is phosphatidic acid, which has two fatty acids and one
phosphate group attached to the glycerol backbone.
An alcohol is attached to the phosphate group of the phosphatidic acid. The alcohol is usually
serine, ethanolamine, choline, or inositol groups that contribute to the polar nature of the
phospholipid head group.
Sphingolipids
These lipids are based on amine alcohol sphingosine and not glycerol. Sphingisine can form an
amide bond to a long chain fatty acid at its amino group. The resulting molecule is called a
ceramide and consists of a polar region flanked by two long, nonpolar tails.
Glycolipids
They are derivatives of sphingosine (or sometimes glycerol) that contain carbohydrate group
instead of a phosphate group. Those containing sphingosine are called glycosphingolipids. These
are amphipathic molecules and are specialized constituents of some membranes (some plant cells
and nerve cells).
Steroids
They are derivatives of a four-ringed hydrocarbon skeleton, which makes them structurally
distinct from other classes of lipids.they are relatively nonpolar and hydrophobic. They are found
in eukaryotic cells. Cholesterol is the common steroids in animal cells. Cholesterol is an
amphipathic molecule. It is insoluble and found primarily in membranes. It is found in most
membranes except the inner membranes of mitochondria and chloroplasts.
Similar membrane sterols are found in other cells, including stigmasterol and sitosterol in plant
cells, ergosterol in fungal cells.
Cholesterol is the basic unit for the synthesis of steroid hormones (sex hormones,
mineralocorticoids, and glucocorticoids).
Terpenes
They are synthesized from the five-carbon compound isoprene and are therefore called
isoprenoids. These include vitamin A, carotinoid pigments, dolichols, coenzyme Q and
plastoquinone.