test 2: study guide 5

Glycolysis – cytosolic pathway 

Glycolysis takes place in the cytosol of a cell, and it can be broken down into two main phases: 

1. Energy-______ phase (uses 2 ATP). In this phase, the starting molecule of glucose is rearranged, and two phosphate groups are attached. The phosphate groups make the modified sugar—now called fructose 1,6-bisphosphate—unstable, allowing it to split in half and form two phosphate-bearing three-carbon sugars. The three-carbon sugars formed when the sugar is cleaved by aldolase B are:  glyceraldehyde-3-phosphate and DHAP. Only glyceraldehyde-3-phosphate is directly oxidized; DHAP is readily converted into glyceraldehyde-3-phosphate. 

2. Energy-_____ phase (produces 4 ATP and 2 NADH). In this phase, each three-carbon sugar is converted into another three-carbon molecule, pyruvate, through a series of reactions. In these reactions, two ATP molecules and one NADH molecule are made. Because this phase takes place twice, it makes four ATP and two NADH overall.

1. requiring

2. releasing

______ pathway: Glucose is oxidized to pyruvate and oxidation continues in the mitochondria to generate acetyl-CoA

-NAD+ is regenerated by transport of reducing equivalents across the mitochondrial membrane using one of two shuttles

- Glycerolphosphate shuttle (week 3)

- Malate shuttle (week 3) 

______ pathway: Glucose is oxidized to lactate

• This will occur either in the absence of oxygen 

• In cells that lack mitochondria (red blood cells)

• Skeletal muscle oxidation of glycogen under anaerobic conditions

• Impaired activity of the pyruvate dehydrogenase complex

• NAD⁺ is regenerated by the reduction of pyruvate to lactate 

Review glucose uptake (Week 4)

• aerobic

• anaerobic

Focus on the three regulatory steps: 

1.  Conversion of Glucose to Glucose 6-phosphate

-  Uses ____ 

• Enzymes: Glucokinase or Hexokinase (Compare the locations and activity of these two enzymes)

• Glucokinase vs. hexokinase  

-   Glucokinase and hexokinase are ______. They catalyze the same enzymatic reaction. Isozymes are enzymes that differ in amino acid sequence but catalyze the same chemical reaction.

-   Hexokinase has a ____ Km for glucose and is found in tissues other than the liver.  It is rapidly saturated. 

-  Glucokinase has a ____ Km for glucose and is found liver or pancreas.  It is not rapidly saturated and can remove glucose steadily from the bloodstream as a means of allowing for excess glucose to be stored as glycogen (triacylglycerol).

• ATP

• isozymes

• low

• high

Focus on the three regulatory steps: 

2.  Conversion of Fructose 6-phosphate to Fructose 1,6-bisphosphate

-  Uses ___ 

• Enzyme: Phosphofructokinase-1 (PFK1)

• Fructose 6-phosphate can also be converted to fructose 2,6-bisphosphate by Phosphofructokinase-2 (PFK2) – NOTE: this is not considered part of glycolysis but is a shunt of the pathway.

-PFK2 is regulated through phosphorylation. 

- When PFK2 is dephosphorylated, it is ACTIVE as a ____ 

• Fructose 2,6-bisphosphate enhances the activity of PFK1 (allosteric activator)

Fructose 1,6-bisphosphate is a _______ _____of Pyruvate kinase (See week 2)

• ATP

• kinase

• feedforward activator

Focus on the three regulatory steps: 

3. Conversion of phosphoenolpyurvate (PEP) to pyruvate

• Enzyme: Pyruvate Kinase (PK)

• Catalyzes the ______ reaction of PEP to pyruvate

•  Regulated by ______ regulation 

  • Allosterically activated by fructose 1,6-bisphosphate

• irreversible

• feedfoward

Pyruvate generated through this process can either:

• Enter the TCA cycle through the pyruvate dehydrogenase complex

• Be converted to ____ under anaerobic conditions

• Be transaminated to ____

• lactate

• alanine

Connections to other pathways

____ _____ synthesis 

• Citrate is shuttled out of the TCA 

  • In the cytosol it is cleaved by citrate lyase to generate: acetyl-CoA and oxaloacetate (OAA)

  • Acetyl-CoA is used as a substrate for fatty acid synthesis

  • OAA is reduced to malate → malate is decarboxylated back to pyruvate which can reenter the TCA 

This conversion by malic enzyme produces _____

• fatty acid

• NADPH

Connections to other pathways

_______ reactions 

• Reactions that transfer net carbon into the TCA cycle

  • Carboxylation of pyruvate → oxaloacetate (by pyruvate carboxylates)

  • Transamination reactions (specifics of this reaction are covered in Unit 4)

    • Glutamate → alpha-ketoglutarate

    • Alanine → pyruvate

    • Aspartate → oxaloacetate

Products metabolized to propionyl-CoA

• anaplerotic

Review shuttles from week 2 

Shuttle systems

1) _________ shuttle (Glycerol 3-phosphate shuttle) 

• Moves reducing equivalents of NADH from the cytosol to an FAD in the mitochondrion 

  • FAD is a required cofactor glycerol-3 phosphate dehydrogenase 

  • FAD is tightly or covalently bound to the dehydrogenase. 

• Cytosolic dihydroxyacetone phosphate is reduced to glycerol 3-phosphate -->glycerol 3-phosphate moves into the mitochondria and transfers the e- to FAD  bound to the dehydrogenase 

Glycerophosphate

Review shuttles from week 2 

Shuttle systems

2) _____-_____ shuttle 

• Oxaloacetate DOES not move across the mitochondrial membrane! 

• Starting in the cytosol:

  • Oxaloacetate is REDUCED to malate by cytosolic malate dehydrogenase

  • The shuttle moves malate (reduced form of oxaloacetate) into the mitochondria 

• Once in the mitochondria:

  • Malate can be OXIDIZED back to oxaloacetate by mitochondrial malate dehydrogenase 

• The resulting oxaloacetate can undergo a TRANSAMINATION REACTION (See week 1) with glutamate 

  • The reaction transfers the NH3⁺ from glutamate TO oxaloacetate to generate 

• → alpha-ketoglutarate and aspartate 

• Aspartate can move in / out of the mitochondria 

  • Aspartate can be converted to oxaloacetate. 

• For each NADH equivalent →  2.5 molecules of ATP are generated 

Malate-Aspartate

Which of the following metabolic shifts would reduce the activity of the glycolytic pathway?

a)Decreased alanine

b)Increased glucose

c)Increased ATP

d)Decreased ATP

c)Increased ATP

A deficiency in which of the following cofactors would result in decreased activity of the pyruvate dehydrogenase complex?

A)Thiamine

B)Biotin

C)Vitamin C

D)Folate

A)Thiamine

Alcohol metabolism increases the production of NADH, increasing the ratio of NADH/NAD+ in the cell.

How would you expect increased consumption of alcohol to impact the activity of the TCA cycle?

decreased activity of the TCA cycle