The Macromolecules of the Cell
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Proteins
Nucleic Acids
Polysaccharides
Lipids
Proteins
• The monomers are amino acids
• The polymers are polypeptides and proteins
• Several kinds of bonds and interactions are
important in protein folding and stability
• Protein structure depends on amino acid
sequence and interaction
Several kinds of bonds and interactions are
important in protein folding and stability
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Disulfide bond
Hydrogen bond
Ionic bonds
Van der Waals interactions (force)
Hydrophobic interaction
Primary (1°) structure
• Specific linear sequence of amino acids in chain;
all levels of structure are ultimately determined by
the primary level
– Amino acid sequence contains mostinformation
needed to specify protein 3D shape & thus its
function; changes in sequence resulting from
mutation may not be readily tolerated
– first was protein hormone insulin determined
by Sanger & coworkers, Cambridge, early
1950s
Secondary (2°) structure
• Form hydrogen bond between imino group (-NH-：
亞硝酸胺) and carbonyl group (-CO-)
• α-helix - backbone assumes form of cylindrical,
twisting spiral; backbone inside helix, R groups
project outwards
– Seen in X-ray diffraction patterns of actual
proteins in 1950s
• β-pleated sheet- consists of several polypeptide
segments lying side-by-side; the backbone of each
segment of polypeptide adopts a folded or pleated
conformation
Tertiary Structure
• Tertiary (3°) structure is the conformation of entire
protein; results from (intramolecular) noncovalent
interactions between R groups
– X-ray crystallography
– NMR spectroscopy– 3D structure of small
proteins (<30 kDa)
– Most proteins have both α-helix & β-pleated
sheet; triosephosphate isomerase mostly β -sheet
• Fibrous proteins
– Collagens & elastins of connective tissue,
keratins (hair, skin, fingernails), silk
• Globular proteins– most proteins in cell
– Myoglobin - storage site for O2 in muscle tissue
Quaternary Structure
• Quaternary (4°) structure is the linking of polypeptide
chains to form multisubunit functional protein via
intermolecular R group interactions
• May be linked by disulfide bonds, but more often
noncovalent bonds (hydrophobic, H bonds, etc.) like
hydrophobic patches on complementary surfaces of
neighboring polypeptides
• Chains may be identical or nonidentical
– Protein composed of 2 identical subunits homodimer
– Protein composed of 2 nonidentical subunits heterodimer
The Macromolecules of the Cell
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 •
•
•
Proteins
Nucleic Acids
Polysaccharides
Lipids
The Macromolecules of the Cell
• Proteins
• Nucleic Acids
 • Polysaccharides
(Carbohydrates)
• Lipids
Carbohydrates
• Carbohydrates comprise a group of substances,
including simple sugars (monosaccharides) & larger
molecules made from them
– Serve primarily as chemical energy storehouse &
durable building material for biological
construction
– Most have general formula (CH2O)n
• Important ones in cell metabolism have from 3
to 7 carbons (n = 3 - 7)
• Trioses, tetroses, pentoses, hexoses, & heptoses
- 3, 4, 5, 6, & 7 carbons, respectively
The Macromolecules of the Cell
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•
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Proteins
Nucleic Acids
Polysaccharides
Lipids
Lipids
• Composed principally of C, H & O - not macromolecules,
but aggregate to form large complexes
• Triglyceride (neutral lipid, fats, triacylglycerol) - serves
as lipid storage form for fuel (stored in adipocytes)
• Fatty acids - long, unbranched hydrocarbon chains with
single carboxyl group at one end
• Sterols and steroids – complex & characteristic 4 ringed
hydrocarbon structures (4 joined rings differ in numbers
& positions of double bonds & functional groups)
• Phospholipids (phosphoglyceride, diacylglycerol) glycerol + 2 fatty acids + phosphate group on third
hydroxyl (often an amino group as well); highly charged
at physiological pH; amphipathic
Lipids
• Fatty acids are the building blocks of
several classes of lipids
– Saturated and unsaturated fatty acids
• Triacylglycerols are storage lipids
• Phospholipids are important in membrane
structure
• Glycolipids are specialized membrane
components
• Steroids are lipids with variety of functions