Lecture 13 Flashcards
Cyclization of monosaccharides
- former carbonyl carbon becomes new chiral center –> anomeric carbon
- carbonyl oxygen becomes hydroxyl group
- position of hydroxyl determines whether a/b
- if trans to CH2OH of the ring = a (alternative sides)
- if cis to CH2OH of the ring = b (same side)
a arrangement of sugar
OH is trans to CH2OH
a = alternative
b arrangement of sugar
oh on same side as CH2OH
Oligosaccharides
2-10 monosacharides linked by glycosidic bonds
- often covalently bonded to proteins and lipids on cell surfaces and act as recognition signals
- human blood groups get specificity from oligosaccharide chains
polysaccharides
- consist of more than ten (sometimes hundreds of) monosaccharides joined through dehydration sunthesis
–> starch, glycogen, cellulose are polymers of glucose that are covalently bonded differently
glucose structires
a –> starch and glycogen, twists and packs tightly, for energy
b –> cellulose, linear strands, good for structure
What are the 4 main fates fo glucose?
- synthesis of complex polysaccharides
- storage as glucose polymers
- oxidation to ribose 5-phosphate via the pentose phosphate pathway
- oxidation to pyruvate via glycolysis
Preparatory stage of glycolysis
- need to thermodynamically destabilize the ring twice
- converts it from a 5 carbon to a 4 carbon structure
- 2 ATP invested
Energy conserving stage of glycolysis
- 2 G3P molecules are oxidized to 2 pyruvic acids
Net 2 ATP produced
(4 total - 2 used in preparatory)
Importance of hexokinase
- adds Pi to glucose so it cannot pass back trhough plasma membrane
- substrate-induced conformation
phosphohexose isomerase
step 2
makes 5 carbon ring into a 4 carbon ring
phosphofructokinase
setp 3
major allosteric enzyme that sets the pace
Aldolase
- seems unfavorable but mass action allows it
- concentrations are not that of standard conditions
triose phosphate isomerase
a and b structure, very simple
uses Hist and Glu
Glyceraldehyde 3 phosphate dehydrogenase
coupled reaction of oxidation and acyl-phosphate formation
huge Ae between steps 1 and 2
thioester intermediate bridges