Carbohydrate Metabolism

Primary sources for ATP (energy) production

Carbohydrates and lipids

Broken down into amino acids, which are primarily used for building structure (muscles, enzymes) but can be used for energy if necessary

Proteins

Vitamins and minerals act as ________ and cofactors to facilitate biochemical reactions

Coenzymes

Known as complex sugars (e.g., starch)

Polysaccharides

Simple sugars (e.g., glucose, sucrose)

Monosaccharides and disaccharides

The individual “building block” molecules

Monomers

Long chains of monomers linked together

Polymers

A reaction that bonds monomers together to form a polymer; a hydrogen atom and a hydroxyl group are removed, releasing a water molecule in the process

Dehydration synthesis

The process of breaking down polymers into monomers; primary mechanism of digestion; requires the addition of a water molecule to break the chemical bond

Hydrolysis

Single sugar units; examples are glucose, fructose, and galactose

Monosaccharides

Two monosaccharides joined together

Disaccharides

Glucose + glucose

Maltose

Glucose + fructose

Sucrose

Glucose + galactose

Lactose

The specific name for the covalent bond formed between two sugar molecules via dehydration synthesis

Glycosidic linkage

Energy storage in plants; glucose units face the same direction; easily digestible by humans

Starch

Structural component in plants; every other glucose unit is “upside down”; humans lack the enzymes to digest this

Cellulose

Energy storage in animals; highly branched structure; stored primarily in the liver and muscle tissues for quick glucose release

Glycogen

The process of getting nutrients into the body; occurs across several organs using specific enzymes to break down carbohydrates, fats, and proteins

Digestion

Organ where salivary amylase begins breaking down starch into maltose

Esophagus

Organ where proteases break proteins into polypeptides; acids also contribute to chemical digestion

Stomach

Organ that secretes many enzymes into the small intestine including amylase, lipase, and endopeptidase

Pancreas

Organ that produces/stores bile salts, which aid in lipid digestion

Liver

Organ that contains membrane-bound enzymes that finalize the breakdown of nutrients for absorption

Small intestine

Carbohydrates must be broken down into ________ to pass from the intestinal lumen through the mucosal surface

Monosaccharides

________ located at the mucosal border act as catalysts to “break down molecules into components” before they enter the epithelial cells

Enzymes

Because epithelial tissue is ________, glucose must use specific transporters to reach the bloodstream

Avascular

From the lumen, glucose enters the cell via ______, a symporter that moves glucose against its gradient by pairing it with Na+

SGLT1

SGLT1 moves glucose against its gradient into cells by pairing it with ____ transport

Na+

Glucose exits the cell into the bloodstream via ______

GLUT2

Maintains the concentration gradient by pumping Na+ out of the cell, allowing the SGLT1 transporter to continue functioning

Sodium-potassium pump

Transmembrane proteins that specifically recognize and transport glucose across cell membranes, as glucose cannot readily diffuse through the lipid bilayer

GLUTs

When extracellular glucose binds to the GLUT, it undergoes a change in __________ which releases the glucose molecule into the cytosol

Conformation

GLUT expressed globally

GLUT1

GLUT expressed in the liver, kidney, and pancreas

GLUT2

GLUT found in the brain to support high energy requirements

GLUT3

First stage in the metabolism of carbohydrates, specifically the breakdown of glucose into pyruvate

Glycolysis

Pyruvate is converted to Acetyl CoA, which then enters the TCA cycle and then the ETC to produce a high yield of ATP

Aerobic respiration

The cell uses a final electron acceptor other than oxygen

Anaerobic respiration

In humans, during anaerobic respiration pyruvate is converted to _________

Lactate

Involves the transport of lactate to the liver, where it is converted back into glucose

Cori Cycle

A catabolic process that occurs in the cytoplasm of the cell; inputs are 1 glucose, 2 ATP, 2 NAD+ and outputs are 2 pyruvate, 4 ATP, and 2 NADH

Glycolysis

Glycolysis generates ___ NADH

2

Glycolysis generates ___ ATP

2

First phase of glycolysis; consumes ATP to “prime” the glucose molecule

Phosphorylation

Second phase of glycolysis; the 6-carbon glucose is split into two 3-carbon molecules (G3P and DHAP)

Cleavage

Third phase of glycolysis; energy is harvested to form NADH and ATP

Oxidation

Steps 1-3 of glycolysis; key enzymes are Hexokinase and Phosphofructokinase

Phosphorylation

Steps 4-5 of glycolysis; key enzymes are Aldolase, Triosephosphate isomerase

Cleavage

Steps 6-10 of glycolysis; key enzymes are GAPDH and pyruvate kinase

Oxidation

“Committed step” and most important regulatory point/enzyme in glycolysis; inhibited by high levels of ATP

Phosphofructokinase

Steps of glycolysis that are essentially irreversible and drive the pathway forward

Hexokinase, Phosphofructokinase, Pyruvate Kinase

The investment of energy in the _________ and cleavage phases allows for the payoff phase, where enzymes drive downstream components to produce a net surplus of energy in glycolysis

Phosphorylation

During the phosphorylation phase, a single D-glucose is converted into __________

Fructose 1,6-bisphosphate

First step of glycolysis; converts Glucose to Glucose 6-phosphate

Hexokinase

Step of glycolysis that requires 1 ATP; takes phosphate from ATP and adds it to the glucose molecule

Hexokinase

Hexokinase is the first input of ________ and is an irreversible reaction

Energy

Hexokinase represents a ________ to processing glucose and is a primary target of regulation

Commitment

Enzyme which converts Glucose 6-phosphate to Fructose 6-phosphate

Phosphoglucose isomerase

Isomerization by phosphoglucose isomerase is a _________ reaction and allows the carbon at position 1 to be available for further phosphorylation

Reversible

Step 3 of glycolysis; converts fructose 6-phosphate to fructose 1,6-bisphosphate

Phosphofructokinase

Step where a second ATP is added to add another phosphate group

Phosphofructokinase

Phosphofructokinase is a highly ________ enzyme; the cell only wants this second phosphorylation to occur if it needs more ATP

Regulated

This enzyme’s activity is the major flux-control point for glycolysis and is governed by positive and negative feedback

Phosphofructokinase

Activators of PFK; indicate a low energy stage

ADP, AMP, and cAMP

Activator of PFK; a potent feed-forward activator of glycolysis

Fructose 2,6-bisphosphate

Inhibitor of PFK; indicates high energy; no further glucose breakdown is needed

ATP

Inhibitor of PFK; a downstream product of the TCA cycle; indicates ATP is already being produced

Citrate

Stage 4 of glycolysis; converts Fructose 1,6-bisphosphate to G3P and DHAP

Aldolase

Step 5 of glycolysis; rearranges the structure of DHAP to convert it into G3P

Triose phosphate isomerase

While the body can use G3P directly for the next stage of glycolysis, _____ cannot undergo downstream effects until it is converted to G3P

DHAP

Step 6 of glycolysis; converts G3P to 1,3-BPG

Glyceraldehyde 3-phosphate dehydrogenase

First oxidation reaction of glycolysis; generates NADH

Glyceraldehyde 3-phosphate dehydrogenase

Step 7 of glycolysis; converts 1,3-BPG to 3-phosphoglycerate

Phosphoglycerate kinase

Glycolysis; this enzyme performs the “first generation of energy”, producing 2 ATP per glucose

Phosphoglycerate kinase

Phosphoglycerate kinase produces ATP through _______-____ ________

Substrate-level phosphorylation

Step 8 of glycolysis; converts 3-phosphoglycerate to 2-phosphoglycerate

Phosphoglycerate mutase

Step 9 of glycolysis; converts 2-phosphoglycerate to phosphoenolpyruvate

Enolase

Step 10 of glycolysis; converts phosphoenolpyruvate to pyruvate

Pyruvate kinase

Second generation of energy during glycolysis via substrate-level phosphorylation

Pyruvate kinase

Glycolysis is primarily regulated by the balance between _______ and ______; this regulation ensures that the body processes glucose when energy is abundant and conserves it when blood sugar is low

Insulin and glucagon

After a meal, blood glucose rises, triggering the pancreas to increase ________ levels and decrease glucagon levels

Insulin

________ stimulates enzymes to process glucose for energy production or storage

Insulin

Insulin _________ the three irreversible steps of glycolysis

Activates

The first “priming” step of glycolysis; traps glucose inside the cell

Hexokinase

The major committed step of glycolysis

Phosphofructokinase

Final step of glycolysis; results in the net production of ATP

Pyruvate kinase

Before entering the TCA cycle, pyruvate produced in the cytosol must be transported into the _________ _______

Mitochondrial matrix

Pyruvate is converted into Acetyl-CoA via the ______ ________ complex

Pyruvate dehydrogenase

Inputs for pyruvate oxidation

2 pyruvate, 2 CoA, 2 NAD+

Outputs for pyruvate oxidation

2 Acetyl-CoA, 2 CO2, 2 NADH

Inputs for glycolysis

1 glucose, 2 ATP, 2 NAD+

Outputs for glycolysis

2 pyruvate, 4 ATP, 2 NADH

Described as a complex “carrier” molecule that delivers the acetyl group to the TCA cycle

Acetyl-CoA

A 2-carbon unit attached to Coenzyme A

Acetyl-CoA

The bond between the acetyl group and CoA is a ________ bond; breaking this bond provides the energy necessary to drive the first step of the TCA cycle forward

Thioester

_________ acts as a metabolic branch point. Depending on the cell’s energy needs and oxygen availability, it can be converted into several different molecules

Pyruvate

Converts pyruvate to Acetyl-CoA; primary route for aerobic respiration

Pyruvate dehydrogenase

Converts pyruvate to Oxaloacetate; replenishes TCA intermediates or enters gluconeogenesis

Pyruvate carboxylase

__________ serves as a link between carbohydrate metabolism and the TCA cycle

Pyruvate