The Macromolecules of the Cell

• 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

• 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

• 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

• 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