MCP Flashcards
glycosidic bonds
covalent linkages between monosaccharides that can be cleaved by digestive enzymes and are named according to alpha/beta configuration of the anomeric carbon and the numbers of connecting carbons
major dietary carbohydrates
- amylose: linear, a1-4 linkage
- amylopectin: branched, a1-4 linkage with a1-6 branches
- lactose: disaccharide (galactose + glucose), B1-4 linkage
- sucrose: disaccharide (fructose + glucose), a1-2 linkage, non-reducing sugar
cellulose
cannot be digested by humans since it has B1,4 linkages between glucose residues which is not recognized by any of our enzymes (lactose also has this linkage but is composed to glucose AND galactose)
3 major glycosidases
- endoglycosidases: cleaves internal sugar polymer bonds
- exoglycosidases: cleaves terminal sugar polymer bonds
- disaccharidases: cleaves the glycosidic bonds of disaccharides
*specificity is based on linkage structure, sugars on each side of linkage and position of linkage within polymer
a-amylase
endoglucosidase that cleaves internal a1-4 bonds of starch (amylose and amylopectin)
- salivary a-amylase: cleaves starch polymers into smaller polysaccharides in the moth and is inactivated by stomach acid
- pancreatic a-amylase: secreted into the duodenum and hydrolyzes the products from salivary a-amylase cleavage into smaller fragments (ex: di- and oligosaccharides)
brush border
apical membrane of intestines that has glycosidases synthesized from epithelial cells of the jejunum to digest oligo- and disaccharides into monosaccharides before transportation into the epithelial cells
(there is then transport into the bloodstream by transporters in the basolateral membrane)
glucoamylase
exoglucosidase that cleaves terminal a1-4 linkages between glucoses–> produces glucose + isomaltose
maltase
cleaves a1-4 linkage in maltose and maltotriose–> produces glucose + maltose
isomaltase
cleaves a1-6 linkage in isomaltose and a-dextrins–> produces glucose + glucose polymers
sucrase
cleaves a1-2 linkage in sucrose–> produces glucose + fructose
lactase
cleaves B1-4 linkage in lactose–> produces galactose + glucose
lactase persistence vs. lactase non-persistence
lactase persistence: AD trait that has been positively selected for in which lactase activity continues to be expressed into adulthood
lactase non-persistence: 65% of world’s population has low levels of lactase preventing them from being able to properly digest lactose
lactose intolerance
lactase deficiency in which lactose moves to the colon and is digested by bacteria producing products that cause symptoms such as: diarrhea, nausea, cramps, bloating and/or gas
glucose
the only fuel that can be used by ALL cells and once it enters (passively) the cells of the tissues, it is converted to glucose-6-phosphate which is trapped in the cell and can enter 3 different pathways
3 pathways that glucose-6-phosphate can enter
- glycolysis: oxidation to make ATP and pyruvate (converts to acetyl-CoA to enter citric acid cycle)
- glycogen synthesis: conversion to glycogen which can be stored for later
- pentose phosphate pathway: production of NADPH needed for biosynthetic processes through the oxidation of glucose into a 5-C sugar
citric acid cycle
occurs in cells with a mitochondria and oxygen in which acetyl-CoA is oxidized completely to CO2 and H2O producing high-energy electrons that help produce ATP
glycogenolysis and gluconeogenesis
ways in which the liver can supply glucose to other tissues via the bloodstream when it is lacking from the diet
glycogenolysis: glycogen–> glucose
gluconeogensis: non-carbohydrate sources (ex: AAs) –> glucose
what happens during fasting and the fed state?
fasting: release of glucagon, increased: glycogen breakdown, gluconeogenesis and lipolysis
fed state: release of insulin, increased: glycogen synthesis, fatty acid synthesis and triglyceride synthesis
what converts glucose to glucose-6-phosphate?
hexokinase (phosphorylates glucose by using ATP) with the help of Mg2+
liver glucokinase
has a lower affinity for glucose than hexokinase so that the liver can transport glucose when it is low in tissues
phosphoglucose isomerase (PGI) and its purpose
isomerizes G6P into F6P (fructose-6-phosphate) by moving the carbonyl group from C1 to C2 creating a 5-C ring (aldose–> ketose)
purpose: sets stage for aldol cleavage which will create two equal 3-C fragments after C1 hydroxyl is phosphorylated
phosphofructokinase (PFK) and its role
phosphorylates the C1 hydroxyl group of F6P using a P from ATP creating fructose-1, 6 bisphosphate (FBP)
role: central in glycolysis regulation
aldolase
cleaves FBP into DHAP (ketose) and GAP (aldose)–> both are trioses
triose-P isomerase and why it catalyzes an unfavored reaction
converts DHAP (ketose) from aldolase reaction into GAP (aldose) with an enediol intermediate
*even though DHAP is favored, it is converted to GAP since GAP is being removed