QUIZ 1 (Carbohydrate Metabolism) Flashcards by Cecilia Read

When ATP needs outpace oxygen delivery, ___ is the primary source of energy.

Glycolysis

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General reaction of Glycolysis

1 glucose —> 2 pyruvate + 2 ATP

*does NOT require oxygen = anaerobic reaction

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Glycolysis is a critical source of ATP especially in ___ because __.

RBC - they don’t have mitochondria to use aerobic respiration

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Glucose is the prefered source of fuel for ___ and ___.

RBC (because they lack mitochondria)

Brain (under non-starvation)

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The ___ is the distribution center of glucose.

Liver

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De novo synthesis of glucose is termed ___ and occurs in the ___.

Gluconeogenesis

Liver

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GLUT Transportors

GLUT1 - brain and RBC = high affinity

GLUT2 - liver and kidneys = low affinity

GLUT3 - neurons = high affinity

GLUT4 - skeletal muscle, fat, heart = low affinity
*regulated by INSULIN!

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Km is ____ proportional to ___.

Inversely

Affinity (of substrate binding)

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Glucose transportor ___ is sequestered in vesicles, but inserted into the membrane in response to ____ which induces ___.

GLUT4

Insulin binding

Glucose uptake into the cell

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Where does each occur?
Glycolysis
Gluconeogenesis

Glycolysis - cytoplasm of all cells

Gluconeogenesis - in liver / kidney / small intestine

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___ in the liver and pancreas traps glucose within the cell by___.

Glucokinase / adding Po on glucose

Hexokinase traps it for all other cells

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What enzyme is associated with the rate limiting step in glycolysis?

PK1 (Phosphofructokinase-1)

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What reactions require ATP in glycolysis?

Trapping of glucose in cell by hexokinase /glucokinase

Conversion of Fructose 6P to Fructose 1,6-BP by Phosphofructokinase-1
*RLS!!

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What enzymes are responsible for the generation of ATP in glycolysis?

Phosphoglycerate Kinase

Pyruvate Kinase

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Explain how PFK-2 affects glycolysis

Works as a kinase when dephosphorylated (favored by INSULIN)
=promotes PFK-2 activity —> activates PFK-1

Works as a phosphatase when phosphorylated (favored by GLUCAGON)
=inhibits PFK-2 activity —> deactivates PFK-1

*PFK-1 is the RLS in glycolysis

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Tarui Disease is due to a deficiency in ___, which causes ___.

PFK-1

Inhibition of ATP production via glycolysis

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Pyruvate Kinase (PK) regulation

During fed state = insulin is high
—>activates phosphatase to de-phosphorylate PK = active PK

During starvation = glucagon is high
—> activates kinase to phosphorylate PK = inactive PK

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Glycolysis is ___ during exercise to meet energy requirements.

Activated

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Defective glycolytic enzymes (causing ineffective glycolysis) especially affects ___ resulting in ___.

RBCs

Hemolytic anemia

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During extreme starvation, the brain “switches” from glucose to ___ as a fuel source.

Ketone bodies

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Carbohydrate metabolism is orchestrated by ___ and ___ in blood.

Insulin / Glucagon

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Fanconi-Bickel Syndrome

Mutation in GLUT2 transporter inhibits uptake of glucose/fructose/galactose in the liver and pancreas

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Direct glucose reserves are sufficient to need glucose needs for ___.

About 1 day

*whole body need 160g / brain needs 120 g / glycogen in body stores 190g

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Which 4 enzymes of gluconeogenesis bypass glycolysis?

Glucose 6 Phosphatase
—>bypasses Hexokinase/Glucokinase

Fructose 1,6 Biphosphatase
—>bypasses PK-1

PEP Carboxykinase + Pyruvate Carboxylase
—>bypass Pyruvate Kinase

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When is glycolysis favored over gluconeogenesis?

High [glucose] / Insulin / [AMP] / [Fru 2,6-BP] / [Fru 1,6-BP] Low Glucagon / [ATP] / [citrate] / [Glc 6-P] / [Fru 6-P]

Pyruvate Carboxylase is located where?

Mitochondria

What enzyme allows for the conversion and reconversion of OAA to Malate?

Malate Dehydrogenase

What is the rate limiting step of gluconeogenesis?

Conversion of Fructose 1,6 Biphosphate to Fructose 6P | —> by Fructose 1,6- Biphosphatase

___ is the enzyme that is associated with a GLUT transporter (___) and is responsible for converting G6P to glucose.

Glucose 6-Phosphatase GLUT7

The Cori Cycle connects ___ produces from glycolysis in RBC/muscles to gluconeogenesis in the liver.

Lactate

Precursor of gluconeogenesis:

Carbs / lipids / proteins

Von Gierke Disease (____) is due to a deficiency in ____.

GSD1a Glucose 6-Phosphatase *causes inefficient release of free glucose into the bloodstream from the liver after gluconeogenesis and glyconeogenesis

Monosaccharides of: - sucrose - galactose

- sucrose = glucose + fructose | - galactose = glucose + lactose

Uptake of fructose, glucose, and galactose is done by...?

SGLT1 —> glucose / galactose / Na+ GLUT5 —> fructose GLUT2 —> glucose / galactose / fructose *into the bloodstream

Glucose is converted to ___ by ___ and then converted to fructose by___.

Sorbitol / Aldose Reductase / Sorbitol Dehydrogenase

Fructose can be directly converted to Fructose 6P by ___.

Hexokinase

Fructose can be converted to Fructose 1P by ___ which feeds into glycolysis/gluconeogenesis as Glyceraldehyde 3P.

Fructokinase

Fructose can either enter glycolysis or ___.

become fats (triacyglycerols)

Fructose metabolism is faster than glucose because it ___.

bypasses the RLS of glycolysis *reason why high fructose corn syrup is NOT healthy; it leads to unregulated generation of Acetyl-CoA which is converted to fatty acids (excess fatty acids lead to obesity and fatty liver)

Galactose can be converted to Galactose 1P by ___.

Galactokinase

What is the rate limiting enzyme in Galactose metabolism?

GALT (glucose 1P uridyltransferase)

If Galactose is not converted to Glucose, what may occur?

It can be converted to Galactitol and accumulate in the eyes, giving rise to cataracts *usually due to a deficiency in Galactokinase or GALT

Galactosemia is a result in a deficiency of ___ or ___.

Galactokinase / GALT (glucose 1P uridyltransferase)

What is the main products of the Pentose Phosphate Pathway?

NADPH / Ribulose 5P

In the PPP, conversion of G6P to Ribulose 5P is a ___ reaction.

Irreversible oxidative (catabolic)

What enzyme is associated with the RLS of the PPP?

G6P Dehydrogenase

G6P Dehydrogenase deficiency is characterized by...?

Hemolytic anemia *the need for NADPH is high; NADPH can activate glutathione (antioxidant in RBC) during infections

NADPH regenerates ___ which is an important antioxidant that detoxifies hydrogen peroxide.

Glutathione

Rapidly dividing cells (including cancers) favor the production of ___ from the PPP.

Ribose 5P *used in DNA/RNA synthesis

Glycogen is formed by ___ and ___ glycosidic bonds.

alpha-1,4 (linear) / alpha-1,6 (branching) * Glycogen Synthase * Glucosyl (4:6) Transferase

Glycogen is stored in the form of ___ in ___.

granules / liver and muscles

Function of glycogen: Liver vs muscle

Liver glycogen = regulates blood glucose Muscle glycogen = provides glucose/energy for physical activity

What is the rate limiting enzyme of glycogenesis? glycogenolysis?

Glycogen Synthase *makes alpha-1,4 glycosidic bonds Glycogen Phosphorylase

Glycogen phosphorylase requires ___ as a cofactor during glycogenolysis.

Pyridoxal Phosphate (Vitamin B6)

___ is the debranching enzyme in glycogenolysis.

Transferase

___ removes glucose in the form of ___ during glycogenolysis.

Glycogen Phosphorylase / Glucose 1P

Once Glucose 1P is converted into Glucose 6P, it will...

Be converted to glucose in the liver (by Glucose 6 Phosphatase) Be used to make energy in muscles

____ is an enzyme found only in the liver.

Glucose 6 Phosphatase *reason why muscles cannot reform glucose; gluconeogenesis only occurs in the liver!

Regulation of Glycogen Synthase and Glycogen Phosphorylase:

Glycogen Synthase: -Po = ACTIVE (high insulin leads to dephosphorylation) +Po = inactive Glycogen Phosphorylase -Po = inactive +Po = ACTIVE (high glucagon leads to phosphorylation)

Glucagon does not act on ___.

Muscle

Insulin binds to ____ type receptor, while Glucagon binds to a ____ receptor.

GPCR / Tyrosine Kinase

Insulin binds receptor and then... | Glycogen synthesis or degradation?how?

Activates PkB —> activates phosphatase —> dephosphorylates Glycogen Synthase (active) and Glycogen Phosphatase (inactive) —>Glycogenesis

Glucagon binds receptor and then... | Glycogen synthesis or degradation?how?

Activates PkA (a kinase) —>phosphorylates Glycogen Synthase (inactive) and Glycogen Phosphorylase (active) —>Glycogenolysis

Insulin causes ___ while Glucagon causes ___.

Dephosphorylation / phosphorylation

Glycogen Storage Disease (1-6) - enzymes involved

0 - Glycogen Synthase 1 - Glucose 6 Phosphatase 2 - Acid Maltase 3 - Alpha-1,6-glucosidease = debrancing enzyme 4- Glucosyl (4:6) transferase = branding enzyme 5 - Glucose Phosphorylase in muscle 6 - Glucose Phosphorylase in liver