Janet Macaulay; Week 1 MED 1011; Biochemistry
Macromolecules are formed by covalent bonds between monomers and include polysaccharides, proteins, nucleic acids. Monomers are bound by condensation reactions and split in hydrolytic reactions. Different functional groups give macromolecules specific properties.
Proteins are macromolecules of amino acids. They function to provide support and protection, catalysis, transport, defence, regulation and movement. They may require a prosthetic group.
20 amino acids are found in proteins, which consist of an amino group, a carboxyl group and a hydrogen, and a variable (Z) group/side chain. Side chains may be charged, polar or hydrophobic. Disulfide bridges can be formed. Amino acids are covalently linked together by peptide linkages to form proteins; peptide linkages: amino-hydroxyl groups.
Primary structure of a protein is the sequence of amino acids bound by peptide linkages. Secondary structures can be maintained by hydrogen bonds between atoms of the amino acid residues. They form alpha-helices or beta-sheets. Tertiary structure is bending and folding of the polypeptide chain. Quartenary structure is arrangement of polypeptides into a single functional unit with more than one polypeptide subunit.
Proteins denatured by heat, acid or chemicals lose tertiary and secondary structure and biological function.
Chaperonins assist protein folding by preventing binding to inappropriate ligands (denatured protein enters, lid closes on chaperonin, protein folds to appropriate shape and is released).
Carbohydrates are carbon bound to H and OH groups. Hexoses are monosaccharides that contain 6 carbon atoms and are the most common monosaccharide; and pentoses have 5 carbons and are found in nucleic acids. Glycosidic linkages between monosaccharides can have either an alpha or beta orientation. They link monosaccharides into larger units.
Starch, glycogen and cellulose have bonds between 1-4. The amount of branching through glycosidic bond formaton and the type of bond (a or b) distinguishes these. Starch and glycogen have alpha 1-4 bonds, cellulose has beta 1-4. The human body cannot break down 1-4 linkages.
- Cellulose has a linear formation, starch is branched, glycogen is highly branched
Chemically modified monosaccharides include sugar phosphates and amino sugars.
Functions of carbohydrates include energy, protein sparing, providing bulk in diet, aiding in synthesis of other molecules (glycolipids, glycoproteins, non-essential amino acids). Glycoproteins are aggregates of lipids and proteins, and play a major role in lipid transport around the body.
Formation of triacylglycerides requires condensation of 3 molecules to join 3 fatty acid molecules to 1 molecule of glycerol. Saturated fatty acids are bound by all single bonds, unsaturated fatty acids have at least one double bond, making packing more difficult. Functions of lipids include energy source, storage of energy (85% of body's energy), structural component of membrane, signalling molecules, enzyme cofactors, synthesis of bile acids and vitamins, vitamin transport, insulation and padding, can enhance satiety and palatability of food.
Phospholipids have a hydrophobic fatty tail and a hydrophilic phosphate head. Sphingomyelin is the only phospholipid that does not contain a glycerol. Cholesterol is another lipid but does not have fatty acid chains (4 carbon ring structure), is synthesised by the liver, has a role in cell membranes and can be used to synthesise bile acids, steroids and vitamins.
In cells, DNA is hereditary material, and DNA + RNA play roles in protein formation. Nucleic acids are polymers of nucleotides with a phosphate group, a sugar and a nitrogen containing base (without phosphate is a nucleoside). The DNA bases are adenine, cytosine, thymine and guanine. In RNA, uracil substitutes for thymine.
- RNA has ribose sugar, DNA has deoxyribose
RNA is single stranded, DNA is double stranded helix with complementary base pairings between adenine and thymine; and guanine and cytosine.