Section 3: part 5 - other carbo biosynthetic pathways | Quizlet

what is the primary responsibility of the liver

maintaining a constant glucose level

how can glucose be produced

gluconeogenesis

glycogen breakdown

Describe lactose

disacc (both sacc. units derived from glucose)

synthesized for later use as a metabolic fuel

produced in mammary gland by lactose synthase (LS)

what are the 2 subunits of LS

galactosyltransferase

α-lactalbumin

describe the galactosyltransferase subunit of LS

catalytic subunit

catalyzes the reaction of UDP-galactose and acetylglucosamine

yields N-acetyllactosamine

- constituent of many complex oligosaccs

describe the α-lactalbumin subunit of LS

mammary gland protein

no catalytic activity

alters specificity of galactosyltransferase

- uses glucose as acceptor

- forms lactose instead of acetyllactosamine

synth. triggered by hormonal changes at parturition

- promotes lactose synth for milk production

Describe lactose intolerance in infants

lactose is hydrolyzed by β-D-galactosidase

- intestinal enzyme

allows for absorption into blood

galactose is enzymatically converted to glucose

Describe lactose intolerance in adult mammals

low levels of β-D-galactosidase

ingested lactose moves to colon

bacterial fermentation generates large quantities of CO2, H2, and irritating organic acids

these result in painful digestive upset

Describe how β-D-galactosidase levels fluctuate by age

β-D-galactosidase levels decrease only mildly with age

What do glycosidic bonds do

link sugars to each other and other molcs.

how much energy is needed to form a glycosidic bond

ΔG° =1 16 kJ/mol

how do glycosidic bonds get their required energy

acquired though synth of nucleotide sugar from monosacc and a nucleoside triphosphate

nucleotide sugar synth -> release of PPi

Describe the glycosylation of proteins

almost all secreted and mem.-associated proteins of euks. are glycosylated

oligosaccs. are covalently attached to proteins by either O- or N- glycosidic bonds

Describe O linked oligosaccs

most common O-glycosidic attachment involves the disascc core β-galactosyl-(1→3)-α-N-acetylgalactosamine linked to the OH of either Ser or Thr

Where does O-linked oligosacc synthesis occur

in golgi apparatus

by serial additions of monosacc units to competed polypeptide

required nucleoside sugars as donors

what are the 2 steps of O-linked oligosacc synthesis

step 1: transfer of N-acetyl-galactosamine (GalNAc)

- from UDP-GalNAc

- to a Ser or Thr residue on the polypep by GalNAc

transferase

Step 2: stepwise addition of sugars

- like galactose, sialic acid, -acetylglucosamine, fructose

Describe N-acetyl-glucosamine (GlcNAc)

invariably β-linked to the amide N from asparagine (Asn) residue

Asn-X-Ser or Asn-X-Thr

-X never pro, rarely Asp, Glu, Leu, or Trp

Describe N-glycosylation

occurs cotranslationally

proteins containing N-linked oligosaccs typically glycosylated then processed

initial glycosylation reaction takes place at ER mem.

what are sugar residues added to in the early stages of N-linked oligosacc synth

sequentially added to the lipid carrier Dolichol pyrophosphate

Describe dolichol and what it does

long chain polyisoprenol containing 17-21 isoprene units

"activates" a sugar residue for subsequent transfer

what is first formed on the dolichol carrier

(N-acetylglucosamine)2 (mannose)9 (glucose)3

12 steps to create this

What is step 1 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

addition of N-acetylglucosamine-1-P and N-acetylglucosamine to dolichol-P

what is step 2 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

addition of 5 mannosyl residues from GDP-mannose

-catalyzed by 5 diff. mannostyltransferases

what is step 3 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

mem. translocation of dolichol-PP-(N-acetylglucosamine)2(mannose)5 to ER lumen

what is step 4 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

cytosolic synth of dolichol-P-mannose from GDP mannose and dolichol-P

what is step 5 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

mem translocation of dolichol-P-mannose to lumen of ER

what is step 6 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

addition of 4 mannosyl residues from dolichol-P-mannose in reaction is

- by 4 diff. mannosyltransferases

what is step 7 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

cytosolic synth of dolichol-P-glucose

what is step 8 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

mem translocation of dolichol-P-glucose

what is step 9 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

addition of 3 glucosyl residues from dolichol-P-glucose

what is step 10 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

transfer of oligosacc from dolichol-PP to the polypep chain of Asn residue

what is step 11 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

translocation of dolichol-PP to cytoplasmic surface of ER mem

what is step 12 of the (N-acetylglucosamine)2(mannose)9(glucose)3 synthesis process

hydrolysis of dolichol-PP to dolichol-P

complete the sentence: One the 14-residue oligosacc is linked to __________ and ___________ starts trimming

Asn glucosidases, mannosidases

(mannose)3(N-acetylglucosamine)2 core is found where

in ALL N-linked oligosaccs