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compare and contrast GLUT2/GLUT4: important tissues

GLUT2: liver and pancreas
GLUT4: adipose tissue and muscle


compare and contrast GLUT2/GLUT4: Km

GLUT2: high Km (~15 mM)
GLUT4: low Km (~5 mM)


compare and contrast GLUT2/GLUT4: saturated at normal glucose levels?

GLUT2: no; cannot be saturated at normal physiological conditions
GLUT4: yes; saturated when glucose levels are only slightly above 5 mM


compare and contrast GLUT2/GLUT4: response to insulin?

GLUT2: No, but serves as glucose sensor to cause release of insulin in pancreatic B-cells
GLUT4: Yes


How does insulin promote glucose entry into cells?

GLUT4 is saturated when glucose levels are only slightly above 5 mM, so glucose entry can only be increased by increasing the number of transporters. Insulin promotes the fusion of vesicles containing preformed GLUT4 with the cell membrane


Hexokinase: function/regulation/reversible?

phosphorylates glucose to form glucose-6-phosphate, "trapping" glucose in the cell. inhibited by glucose-6-phosphate. irreversible.


Glucokinase: function/regulation/reversible?

glucokinase phosphorylates and "traps" glucose in liver and pancreatic cells, and works with GLUT2 as part of the glucose sensor in B-islet cells. In liver cells, it is induced by insulin. Irreversible.


Phosphofructokinase-1: function/regulation/reversible?

PFK-1 catalyzes RLS of glycolysis, phosphorylating fructose 6-phosphate to fructose 1,6-bisphosphate using ATP. Inhibited by ATP, citrate, glucagon. Activated by AMP, fructose 2,6-bisphospahte, insulin. Irreversible.


Glyceraldehyde-3-phosphate dehydrogenase: function/reversible?

glyceraldehyde-3-phosphate dehydrogenase generates NADH while phosphorylating glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. Reversible.


3-phosphoglycerate kinase: function/reversible?

3-phosphoglycerate kinase performs a substrate-level phosphorylation, transferring a phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate. Reversible.


pyruvate kinase: function/regulation/reversible?

pyruvate kinase performs another substrate-level phosphorylation, transferring a phosphate from PEP to ADP, forming ATP and pyruvate. It is activated by fructose 1,6-bisphosphate. Irreversible.


why must pyruvate undergo fermentation for glycolysis to continue?

fermentation must occur to regenerate NAD+, which is in limited supply in cells. fermentation generates no ATP or energy carriers; it merely regenerates the coenzymes needed in glycolysis.


why is it necessary that fetal hemoglobin does not bind 2,3-BPG?

the binding of 2,3-BPG decreases hemoglobin's affinity for oxygen.


high altitude PO2

is lower, increase respiration


bisphosphoglycerate mutase

RBC use this enzyme to convert 1,3-BPG to 2,3-BPG, which decreases Hg's affinity for oxygen


how can liver continue glycolysis in order to generate pyruvate for FA synthesis and glycogenesis in the presence of ATP?

PFK-2, an enzyme found mostly in liver, converts fructose 6-phosphate to F2,6-BP, which activates PFK-1


which enzyme is responsible for trapping galactose in the cell?



what enzyme in galactose metabolism results in a product that can feed directly into glycolysis, linking the two pathways?

galactose-1-phosphate uridyltransferase produces glucose 1-phosphate, a glycolytic intermediate, thus linking the pathways


which enzyme is responsible for trapping fructose in the cell?

fructokinase, with a small contribution from hexokinase


what enzyme in fructose metabolism results in a product that can feed directly into glycolysis, linking the two pathways?

aldolase B produces DHAP and glyceraldehyde (which can be phosphorylated to glyceraldehyde 3-phosphate), which are glycolytic intermediates, thus linking the pathways


what are the reactants of the pyruvate dehydrogenase complex?

pyruvate, NAD+, CoA


what are the products of the pyruvate dehydrogenase complex?

Acetyl-CoA, NADH, CO2


how does acetyl-CoA affect PDH complex activity? why?

inhibits. signals cell is energetically satisfied. pyruvate can then be used to form other products, such as OAA for use in gluconeo


what is the structure of glycogen?

core protein glycogenin with linear chains of glycose emanating out from center. if the chains are branched, the highest density is at periphery.


what type of glycosidic links exist in a glycogen granule?

alpha-1,4 glycosidic links connect linear chains. branches are formed by alpha-1,6 glycosidic links.


what are the two main enzymes of glycogenesis, and what does each accomplish?

glycogen synthase attaches the glucose molecule from UDP-glucose to the growing glycogen chain to form alpha-1,4. branching enzyme breaks an alpha-1,4 and attaches the oligoglucose as a branch by alpha-1,6.


what are the two main enzymes of glycogenolysis, and what does each accomplish?

glycogen phosphorylase removes a glucose molecule from glycogen using a phosphate and creates glucose 1-phosphate.
Debranching enzyme breaks alpha-1,4 of a branch and forms new alpha-1,4. then, alpha-1,6 of remaining single glucose is broken to release free glucose.


under what physiological conditions should the body carry out gluconeogenesis?

When the individual has been fasting for over 12 hours. Hepatic (and renal) cells must have sufficient energy for this process, which requires sufficient fat stores for beta-oxidation.


what are the enzymes of gluconeogenesis that replace glycolytic enzymes, and which?

1. Pyruvate carboxylase and PEP carboxykinase replace pyruvate kinase
2. fructose 1,6-bisphosphatase replaces PFK-1
3. glucose 6-phosphatase replaces hexokinase


How does acetyl-CoA shift the metabolism of pyruvate?

inhibit PDC. activate pyruvate carboxylase. shift from burning pyruvate in TCA to making glucose. acetyl-CoA for this purpose comes mainly from beta-ox, not glycol.