Carbohydrate Metabolism: Glycolysis, HMP Shunt, and Glycogenolysis

Vocabulary flashcards covering the key terms, enzymes, pathways, and reactions of glycolysis, the hexose monophosphate (HMP) shunt, and glycogenolysis.

Glycolysis

A metabolic pathway taking place in the cytosol of cells that involves the systematic breakdown of glucose into two molecules of pyruvate to produce energy.

First Phase of Glycolysis

The preparatory phase of glycolysis where glucose is converted into $2$ molecules of Glyceraldehyde-3-Phosphate, consuming energy ($2$ molecules of $\text{ATP}$) to add phosphate groups rather than producing energy.

Hexokinase

The enzyme that catalyses the phosphorylation of glucose into Glucose-6-phosphate by transferring a phosphate group from $\text{ATP}$, utilizing $1$ molecule of $\text{ATP}$.

Phosphofructokinase

The enzyme that catalyses the transfer of a phosphate group from $\text{ATP}$ to Fructose-6-phosphate to form Fructose-1,6-Bisphosphate, utilizing $1$ molecule of $\text{ATP}$.

Aldolase

The enzyme that splits Fructose-1,6-Bisphosphate into two $3$-carbon compounds: Glyceraldehyde-3-phosphate and Dihydroxyacetone phosphate.

Glyceraldehyde-3-phosphate Dehydrogenase

The enzyme that converts Glyceraldehyde-3-phosphate into 1,3-Bisphosphoglycerate via the addition of a phosphate group at the first position, reducing $\text{NAD}^+$ to $\text{NADH}$.

Phosphoglycerate Kinase

The enzyme that transfers the phosphate group from the first position of 1,3-Bisphosphoglycerate to $\text{ADP}$, generating a molecule of $\text{ATP}$ and producing 3-Phosphoglycerate.

Phosphoglycerate Mutase

The enzyme that catalyses a simple rearrangement reaction converting 3-Phosphoglycerate into 2-Phosphoglycerate.

Enolase

The enzyme requiring $\text{Mg}^{2+}$ that catalyses the conversion of 2-Phosphoglycerate into Phosphoenolpyruvate.

Pyruvate Kinase

The enzyme that transfers the phosphate group from Phosphoenolpyruvate to $\text{ADP}$, converting it into $\text{ATP}$ and yielding Pyruvate.

Net Reaction of Glycolysis

$$\text{Glucose} + 2\,\text{ADP} + 2\,\text{P}_i + 2\,\text{NAD}^+ \rightarrow 2\,\text{Pyruvate} + 2\,\text{ATP} + 2\,\text{NADH}$$

HMP Shunt Pathway

Also known as the pentose phosphate shunt pathway, a pathway entered by $10\%$ of glucose that is essential for the synthesis of $\text{NADPH}$ and pentose sugars.

Glucose 6-phosphate Dehydrogenase

An enzyme in the oxidative phase of the HMP shunt requiring $\text{Mg}^{2+}$ that oxidizes Glucose 6-phosphate into 6-phosphogluconolactone while reducing $\text{NADP}^+$ to $\text{NADPH} + \text{H}^+$.

Glucanolactone Hydrolase

An enzyme in the oxidative phase of the HMP shunt that catalyses the conversion of 6-phosphogluconolactone into 6-phosphogluconate.

Phosphogluconate Dehydrogenase

An enzyme in the oxidative phase of the HMP shunt requiring $\text{Mg}^{2+}$ that oxidatively decarboxylates 6-phosphogluconate into Ribulose 5-phosphate, producing $\text{CO}_2$ and $\text{NADPH} + \text{H}^+$.

Transketolase

A $\text{TPP}$-dependent enzyme in the non-oxidative branch of the HMP shunt that transfers two-carbon units between ketose and aldose sugars.

Transaldolase

An enzyme in the non-oxidative branch of the HMP shunt that catalyses the conversion of Sedoheptulose 7-phosphate and Glyceraldehyde 3-phosphate into Fructose 6-phosphate and Erythrose 4-phosphate.

Glycogen

An animal polysaccharide stored primarily in the liver and muscle; broken down in muscle to support aerobic or anaerobic metabolism during exercise, and in liver during fasting between meals.

$$\alpha(1\rightarrow 4)\,\text{Glycosidic Bond}$$

The glycosidic linkage connecting carbon $1$ and carbon $4$ of adjacent glucose units in the linear chains of glycogen.

$$\alpha(1\rightarrow 6)\,\text{Glycosidic Bond}$$

The glycosidic linkage connecting branch points in glycogen.

Glycogen Phosphorylase

The enzyme in glycogenolysis that cleaves glucose residues from glycogen chains using inorganic phosphate ($\text{P}_i$).

Phosphoglucomutase

The enzyme that catalyses the interconversion of Glucose 1-phosphate and Glucose 6-phosphate.

Glucose-6-phosphatase

An enzyme present in the liver ER membrane that hydrolyses Glucose 6-phosphate to free glucose and $\text{P}_i$ to maintain blood glucose levels; absent in skeletal muscle.

$$G6P\,\text{Transporter (T1)}$$

A transporter protein in the endoplasmic reticulum membrane that imports Glucose 6-phosphate from the cytosol into the ER lumen for hydrolysis.

$$\text{Glucose Transporter (T2)}$$

A transporter protein in the endoplasmic reticulum membrane that transports free glucose produced by Glucose-6-phosphatase out of the ER lumen toward capillaries.