Carbs

• Carbohydrates are sugars, polyhydroxy aldehydes/ketones, and the most abundant biological molecules produced from photosynthesis
• Carbohydrates are heterogeneous in size and composition
• Biological roles of carbohydrates include being an energy source and storage, a structural component of cell walls and exoskeletons, and informational molecules in cell-cell signaling
• Examples of milk oligosaccharides and their functions:

  • Lacto-N-tetraose: prebiotic oligosaccharides providing a growth advantage to bifidobacteria
  • Lacto-N-fucopentaose-1: acts as a pathogen decoy, preventing bad bacteria from attaching to infants; pathogens fail to bind to intestinal cells
• Reason we cannot metabolize the raffinose series of plant oligosaccharides:

  • Lack of α-galactosidase enzyme needed to hydrolyze the α-1,6 glycosidic bond
• Examples of structural and storage polysaccharides:

  • Structural elements include cellulose (plant cell walls) and chitin (animal exoskeleton)
  • Storage forms used for fuel include starch in plants and glycogen in animals
• Reason we can't digest cellulose:

  • Lack of the enzyme, cellulase, required to hydrolyze the β1 —> 4 linkage between the glucose molecules in cellulose
• Reason for storing glucose as glycogen (or starch in plants) instead of in the monomeric form:

  • Easier to store as it greatly reduces the osmotic pressure that would result from its storage in monomeric form
• Glycoproteins are macromolecules consisting mostly of protein, some transported to the plasma membrane for insertion or secretion, while others are targeted to cellular organelles such as mitochondria and peroxisomes
• 1. linked glycoproteins:

  • Carbohydrate linkage occurs through the amide N atom in the side chain of Asn
  • N-glycosylating enzymes attach the sugar to asparagine residues contained within the consensus sequence Asn-X-Ser/Thr (X can be any amino acid except Pro)
• 1. linked glycoproteins:

  • Carbohydrate linkage occurs through the O atom in the side chain of Ser or Thr residues
  • No preferred consensus sequence
  • O-linked glycoproteins are called mucins and protect epithelial cells in the intestinal, urinary, and respiratory tracts
• Lectins are glycan binding proteins that recognize glycan groups on glycoconjugates, playing an important role in cell signaling and immunity
• Blood group determination:

  • Expression of α-1,3-N-acetylgalactosaminyltransferase (GTA) and α-1,3-galactosyltransferase (GTB) enzymes determines blood type (O, A, B, or AB)
• Proteoglycans are macromolecules where the bulk consists of carbohydrates; found in the extracellular matrix in humans and in the cell wall in bacteria
• Peptidoglycan is a component of the bacterial cell wall composed of linear chains of MurNAc and GlcNAc tethered together by linkages between short peptides
• Lysozyme and penicillin target peptidoglycan by inhibiting transpeptidase, with penicillin forming a suicide inhibitor complex with the enzyme; bacteria become penicillin resistant by expressing β-lactamase enzyme
• Methicillin is effective on penicillin-resistant bacteria as it blocks transpeptidase activity without being a substrate for β-lactamase; MRSA bacteria acquired resistance to methicillin by expressing a variant form of the transpeptidase enzyme
• Pentose Phosphate Pathway (PPP) role:

  • Oxidation of glucose-6-phosphate to produce NADPH; occurs in the cytosol; biosynthesis
• Outcomes of oxidative and non-oxidative phases of PPP:

  • Oxidative phase: convert glucose-6-P to ribulose-5-P producing 2 NADPH and 1 CO2
  • Non-oxidative phase: carbon shuffle reactions converting ribulose-5-P into F6P and GAP
• G6PDH deficiency leads to cells being prone to oxidative stress, causing cellular damage and lipid peroxidation; G6PDH deficiency provides resistance to malaria due to increased oxidative stress in red blood cells negatively influencing the malaria parasite
• Non-oxidative phase of PPP consists of carbon shuffle reactions with the outcome of converting ribulose-5-P into glucose-6-P
• G6PDH is a regulated enzyme in the PPP pathway, and its deficiency can lead to clinical manifestations under certain environmental factors
• Effects of G6PDH deficiency include cellular damage, breakdown of cell membrane, oxidation of proteins and DNA, and susceptibility to oxidative stress; resistance to malaria due to increased oxidative stress in red blood cells
• MRSA bacteria acquired resistance to methicillin by expressing a variant form of the transpeptidase enzyme that is not inhibited by the antibiotic