Small Organic Molecules and Monosaccharides

• Four main families of small organic molecules form monomeric building blocks for biological macromolecules.
• Small organic building blocks give larger organic molecules of the cell.
• Sugars become polysaccharides, glycogen, or starch.
• Fatty acids become fats and membrane lipids.
• Amino acids become proteins.
• Nucleotides become nucleic acids.
• Fatty acids and sugars provide energy.
• Nucleotides act as energy carriers.

Carbs

• Carbs are formed of monosaccharides as building blocks.
• Their formula is nx CH2O, where n = 3, 4, 5, or 6.
• Carbs contain carbonyl and hydroxyl groups.
• They are polar, attracting other polar molecules, thus they are hydrophilic and water-soluble.

Monosaccharides

• Monosaccharides are aldoses if they contain an aldehyde (end chain).
• They are ketoses if they contain a ketone (within chain).
• Only 1 in 40,000 glucose molecules in solution is linear.
• In aqueous solution, 5 and 6 carbon sugars spontaneously form ring structures.

Isomers

• Isomers have the same molecular formula but different structures.
• Monosaccharides can exist as optical isomers or enantiomers due to chiral centers.
• A chiral center is a carbon atom bonded to four different groups, allowing for two non-superimposable mirror images called enantiomers.
• Most naturally occurring sugars are D-isomers.
• D-glucose (dextrose) can be metabolized by cells in the glycolysis pathway; L-glucose can not.
• If the hydroxyl group is to the right, it is D; if to the left, it is L.
• D-glucose and L-glucose are enantiomers with the same chemical formula but mirror images.
• Stereoisomers share the same chemical formula and connectivity but differ in how their atoms are oriented in three-dimensional space.
• Other isomers of glucose vary in the spatial arrangement of carbonyl and hydroxyl groups.
• Epimers, stereoisomers, differ in configuration at a single asymmetric carbon.
• When glucose is in ring form, the hydroxyl attached to carbon 1 (the aldehyde carbon) has two positions (α and β).
• In cells, glucose is 33% alpha and 66% beta.
• Alpha and beta forms interchange rapidly in solution, and once a bond is set, it is locked in form.
• The body does not possess beta form enzymes
• Glycogen is a polymer of alpha-glucose.
• Alpha holds hands down low (1st carbon); beta up high.

Complex Carbs

• Complex carbs are formed by glycosidic bonds (condensation reaction where water is lost) between monosaccharides.
• The α- or β-configuration is locked when the bond is formed.
• Maltose is produced by amylases on starch: if the first sugar is alpha, the linkage is alpha; if beta, the linkage is beta.
• Disaccharides consist of two monosaccharides linked by a glycosidic bond.
• Polysaccharides act as energy stores.
• Starch (amylose (fibrous) and amylopectin) is found in plants.
• Glycogen is found in animals.
• Glycogen resembles amylopectin but has more branches.

Glycogen bonds

• Linear: α-1,4 linkages.
• Branched: α-1,6 linkages.
• Sugars are modified and linked to lipids or proteins.
• Complex oligosaccharides (a few saccharides) can form recognition molecules on cell surfaces, like blood group determinants.

Lipids and Fats

• Lipids are water-insoluble (hydrophobic, except for the carboxy acid part) but soluble in organic solvents.
• Examples of lipids:
• Triacylglycerols (fats and oils).
• Glycerophospholipids and other membrane lipids.
• Steroids and cholesterol.
• Fatty acids are the monomeric building blocks of triacylglycerols and glycerophospholipids.
• The length and structural formula (saturated or unsaturated) of the fatty acid carbon chain determines its physical properties.
• Carbon chain structure determines:
  • Shape.
  • Melting point.
• Unsaturated fatty acids have one or more double bonds, while saturated ones have no double bonds.
• The double bond in unsaturated fatty acids is rigid, creating a kink in the chain.
• The rest of the chain is free to rotate about the other C-C bonds.
• A double bond in unsaturated fat makes others stay away.
• Saturated fats stick together and can clog arteries.
• Unsaturated fats are liquid at room temperature (e.g., olive oil), while saturated fats are solid (e.g., butter).

Triacylglycerols

• Triacylglycerols (triglyceride fats) are formed by ester linkages between fatty acids and glycerol.
• They serve as crucial energy storage molecules.
• Stored as hydrophobic (insoluble) fat droplets within cells.

Glycerophospholipids

• Glycerophospholipids are based on glycerol, but one fatty acid (of 3) is replaced by a phosphate group.
• The phosphate group is linked to a hydrophilic head group, an amino lipid.
• Are amphipathic (have a hydrophilic "head" and hydrophobic "tails").
• In aqueous environments, they aggregate, with head groups in the water.
• The phospholipid bilayer forms the cell membrane.

Steroids and Cholesterol

• The steroid template (fused alkyl rings) forms the basis for steroid hormones and the sterol lipid, cholesterol.
• Cholesterol is a necessary component of animal cell membranes.
• Its rigid structure inserts between glycerophospholipids, which modulates membrane fluidity to keep it at an ideal level.
• Ribonucleic acid and deoxyribonucleic acid (RNA and DNA) serve as information molecules.

Nucleic Acids

• Nucleotides are the building blocks of RNA and DNA.
• Nucleotides consist of:
  • Pentose sugar.
  • Nitrogenous base.
  • Phosphate.
• ATP carries chemical energy in phosphoanhydride bonds.

Recognition

• Adenine (A) pairs with Thymine (T) through 2 hydrogen bonds.
• Guanine (G) pairs with Cytosine (C) through 3 hydrogen bonds.

DNA

• More stable (due to lacking 2'-OH group)
• Long-term genetic information storage

RNA

• Less stable (presence of 2'-OH makes it more reactive)

• Involved in protein synthesis, regulation, and sometimes genetic info (in some viruses)

• Nucleotides join via phosphodiester bonds (pentose-phosphate backbone).

• The base-pair sequence of DNA forms the genetic code.

• DNA is a double helix with strands anti-parallel.

• It goes from 5 prime to 3 prime and has bases connected on the 1 prime carbon.

• Proteins have mechanical, structural, and transport functions in the body.

• They act as enzymes (biological catalysts).

• Different proteins display a wide variety of shapes and sizes.

• Each protein structure is unique, determined by the DNA sequence of the gene.

• Proteins spontaneously fold up from linear chains of amino acids.

Amino Acids

• Amino acids are the building blocks of proteins.
• 20 amino acids are common to all living organisms.
• Amino acids are water-soluble and electrically charged at physiological pH.
• Each amino acid has unique properties of its side chain (R group or residue).

Polypeptides

• In polypeptides (proteins), amino acids link via peptide bonds.
• Side chains (R groups) are not involved in peptide bonding.

Amino Acid Groups.

• Non-polar: Hydrophobic, e.g., alanine (CH3 side chain).
• Polar: Hydrophilic, e.g., serine (-OH group).
• Acidic: Negative charge at physiological pH, e.g., glutamate.
• Basic: Positive charge at physiological pH, e.g., lysine.
• Properties of side chains determine protein folding.
• Carbohydrates, composed of monosaccharides, are polar and hydrophilic with the formula Cn(H2O)n, possessing carbonyl (C=O) and hydroxyl (–OH) groups.
• Lipids are hydrophobic molecules like fats, phospholipids, and steroids used for energy storage and cell membranes.
• Proteins are made from amino acids linked by peptide bonds, and their structure (folding) determines their function.
• Nucleic acids, DNA and RNA, are composed of nucleotides that store genetic information and energy (e.g., ATP).
• Monomers build larger molecules; sugars form polysaccharides, fatty acids form fats and membrane lipids, amino acids form proteins, and nucleotides form nucleic acids.
• Aldose sugars have an aldehyde group (e.g., glucose) at the end of the carbon chain, while ketose sugars have a ketone group (e.g., fructose) within the carbon chain.
• In water, monosaccharides (5/6 carbons) form ring structures.
• Enantiomers, isomers, D- and L-glucose are mirror images.
• Anomers: When glucose forms a ring, the hydroxyl group on carbon 1 can point down (a) or up (β).
• Glycosidic bonds link monosaccharides, while alpha-glycosidic bonds are first sugar's anomeric carbon points down (a) and beta-glycosidic bondis first sugar's anomeric carbon points up.
• A peptide bond forms between the carboxyl group of one amino acid and the amino group of another, releasing water in a condensation reaction.
• Amino acids are grouped by their side chains (R-groups), include side chains may be Non-polar, Polar, Acidic, or Basic.