MicroPhys - Ch.7 Part 1
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What must happen to polysaccharides before they can be transported into the cell? What enzymes are important in this process?
Polysaccharides such as starch, glycogen and cellulose as well as proteins need to hydrolyzed into oligomers or monomers prior to transport into the cell.
An endoglucanse such as a-amylase can chop the chains in monomers, dimers and oligomers (glucose, maltose and dextrin.
B-amylase is exoglucanase that removes the disaccharide maltose and glucoamylase removes glucose units from the amylose non-reducing end.
What enzymes are capable of hydrolyzing the a-1,6-hydrolytic bonds of amylopectin?
Pullalanase and isoamylase
Why is cellulose so much more difficult to hydrolyze than starch?
It is a straight chain with B-1,4 linkages that forms extensive hydrogen bonding. The structure is very stable.
What is this?
How is cellulose hydrolyzed. What enzymes are important for this?
Endo-B-glucanases are too large to infiltrate the structure to hydrolyze the B1,4 bond. One hypothesis is that cellulose is attacked in its amorphous regions first by the endo-B-glucanases generating cellobiose which is degraded by exo-B-glucanases and B-glucosidase. May also be degraded by a C-1 cellulase that primarily loosens the crystalline structure to form amorphous regions.
How is pectin degraded? Name an organism that utilizes this pathway.
Degraded first by pectin esterase (which removes a methyl group as methanol) and then pectinase that breaks down the a-1,4 polygalacturonate to monomers of galacturonate.
Erwinia carotovora attacks fruits and vegetables dues to its ability to breakdown pectin.
What is the structure of chitin and why is it difficult to hydrolyze?
Chitin is poly-B-1,4-N-acetylglusoamine and is second most abundant polymer on earth next to cellulose.
Chitin also forms hydrogen bonds like cellulose. In addition it may be modified with proteinaceous matrix for extra strength or calcium carbonate to produce a tougher shell.
How is chitin hydrolyzed. Name an organism that utilizes this pathway.
Chitin deacetylase removes the acetyl group producing chitosan, which hydrolyzed to glucosamine by chitinase. Chitinase breaks the B-1,4 linkage producing monomers of N-acetylglucosamine.
How are sugars utilized?
Hydrolysis of polysaccharides by secreted enzymes generates various hexoses and pentoses and their derivatives. The sugars are transported into the cell.
What enzymes transfer sugars into cells without the use of ATP?
What is happening here?
Fructose is phosphorylated fructose-1-Ph by group transport or to fructose-6-Ph by hexose kinase after active transport.
Fructose-1-Ph is converted to Fructose 1,6 diphosphate by 1-phosphofructokinase.
Mannose is actively transported into the cell hexose kinase converts it into Mannose-6-Ph, then an isomerase converts it fructose-6-Ph.
Galactose is eventually converted to glucose-1-Ph. Glucose-1-Ph is converted to Glucose-6-Ph by phosphoglucomutase
How are the pentose sugars xylose, ribose and arabinose utilized?
Ribose is phosphorylated. Xylose and arabinose are converted by isomerases and metabolized in hexose monophosphate pathway.
XXXXX are oxidized to YYYYYY in B-oxidation
X = Fatty acids, Y = acetyl CoA
What are the steps in B-oxidation?
Fatty acid must first be activated or charged catalyzed by acyl-CoA synthetase.
- The next 4 reactions are know as B-Oxidation:
- 1) Fatty acyl-CoA dehydrogenase-oxidation; FADH2 formed
- 2) 3-hydroxyacyl-CoA hydrolyase - Water added
- 3) L-3-hydroxyacyl-CoA dehydrogenase - Oxidation; NADH + H+ formed
- 4) acyl-CoA acyltransferase - Acetyl-CoA split off and CoA added to acyl portion
How can B-oxidation function anaerobically?
This pathway is aerobic but may operate anaerobically if an alternate electron acceptor is available.
In E.coli it will operate if nitrate is available.
Aerobic condition help regenerate FAD and NAD. In E. coli under anaerobic conditions B-oxidations will continue if nitrate is available (for anaerobic respiration).
What are the products of B-oxidation in odd and even fatty acids?
- Even: acetyl CoA
- Odd: acetyl CoA and one propionyl coA
What happens to propionyl CoA?
1) It is converted to pyruvate through the acryl-CoA pathway Once pyruvate is formed, pyruvate will be converted to acetyl CoA via pyruvate dehydrogenase.
2) E. coli uses an alternate anaerobic pathway and form pyruvate from propionate via methylcitrate
What is this pathway and what metabolic pathway is it similar to?
This is the methyl citrate pathway to form pyruvate from Propionyl CoA. This pathway shares similarity with glyoxylate pathway. Instead of Isocitrate being split (glyoxylate pathway) it is 2-methylisocitrate forming pyruvate rather than glyoxylate.
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