Carbohydrate Metabolism

Glycolysis is the sequence of 10 enzymatic reactions that converts one molecule of glucose (6C) into two molecules of pyruvate (3C), generating a net yield of 2 ATP and 2 NADH. It occurs in the cytoplasm of virtually all cells and does not require oxygen.

The Two Phases of Glycolysis

Energy Investment Phase (Steps 1–5):

Two ATP molecules are consumed to phosphorylate glucose and its intermediates, trapping them inside the cell and preparing them for cleavage.

• Step 1: Glucose → Glucose-6-phosphate (enzyme: hexokinase in most tissues; glucokinase in liver and pancreatic β-cells). This is the first committed step.
• Step 3: Fructose-6-phosphate → Fructose-1,6-bisphosphate (enzyme: phosphofructokinase-1, PFK-1). This is the rate-limiting step — the most important regulatory point of glycolysis. PFK-1 is activated by AMP, fructose-2,6-bisphosphate and inhibited by ATP, citrate.
• Step 4: Fructose-1,6-bisphosphate is cleaved by aldolase into two 3-carbon molecules: DHAP and glyceraldehyde-3-phosphate (G3P).

Energy Payoff Phase (Steps 6–10):

Each of the two 3-carbon molecules generates 2 ATP (by substrate-level phosphorylation) and 1 NADH.

• Step 10: Phosphoenolpyruvate → Pyruvate (enzyme: pyruvate kinase). This is the third irreversible step and another key regulatory point.

Net Yield of Glycolysis

Per molecule of glucose: 2 ATP (net), 2 NADH, and 2 pyruvate.

Fate of Pyruvate

Pyruvate's fate depends on oxygen availability and the metabolic needs of the cell:

• Aerobic conditions: Pyruvate enters mitochondria and is converted to acetyl-CoA by the pyruvate dehydrogenase complex (PDH), then enters the citric acid cycle.
• Anaerobic conditions: Pyruvate is reduced to lactate by lactate dehydrogenase (LDH), regenerating NAD⁺ so glycolysis can continue. This occurs in exercising muscle, red blood cells (which lack mitochondria), and in tumors (the Warburg effect).
• Alcoholic fermentation (in yeast): Pyruvate → ethanol + CO₂.

The citric acid cycle (also called the Krebs cycle or TCA cycle) is a series of 8 enzymatic reactions in the mitochondrial matrix that oxidizes the acetyl group of acetyl-CoA to two molecules of CO₂, generating high-energy electron carriers (NADH and FADH₂) that feed into the electron transport chain.

The Pyruvate Dehydrogenase Complex (PDH)

Before entering the TCA cycle, pyruvate must be converted to acetyl-CoA. This irreversible reaction is catalyzed by the pyruvate dehydrogenase complex, a large multi-enzyme complex requiring five coenzymes — remember them with the mnemonic "Tender Loving Care For Nancy":

1. Thiamine pyrophosphate (TPP, from vitamin B1)
2. Lipoic acid
3. Coenzyme A (CoA, from vitamin B5/pantothenic acid)
4. FAD (from vitamin B2/riboflavin)
5. NAD⁺ (from vitamin B3/niacin)

PDH is regulated by phosphorylation: it is inactivated by PDH kinase (stimulated by ATP, acetyl-CoA, NADH) and activated by PDH phosphatase (stimulated by Ca²⁺, insulin).

Key Reactions of the TCA Cycle

Three reactions in the cycle are irreversible and represent the key regulatory steps:

1. Citrate synthase: Acetyl-CoA + Oxaloacetate → Citrate. Inhibited by citrate, ATP, NADH.
2. Isocitrate dehydrogenase: Isocitrate → α-Ketoglutarate + CO₂ + NADH. The rate-limiting step. Activated by ADP; inhibited by ATP, NADH.
3. α-Ketoglutarate dehydrogenase: α-Ketoglutarate → Succinyl-CoA + CO₂ + NADH. This enzyme complex is structurally similar to the PDH complex and requires the same five coenzymes.

Net Yield per Acetyl-CoA

One turn of the TCA cycle produces: 3 NADH, 1 FADH₂, 1 GTP (equivalent to 1 ATP), and 2 CO₂.

Since each glucose produces 2 acetyl-CoA, the total TCA cycle yield per glucose is: 6 NADH, 2 FADH₂, 2 GTP.

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. It occurs primarily in the liver (and to a lesser extent the kidneys) during fasting, starvation, and prolonged exercise to maintain blood glucose levels.

Substrates for Gluconeogenesis

• Lactate — From anaerobic glycolysis in muscle and RBCs. Converted back to pyruvate by LDH in the liver (the Cori cycle).
• Amino acids — Especially alanine (via the glucose-alanine cycle) and glutamine. Glucogenic amino acids can be converted to TCA cycle intermediates or pyruvate.
• Glycerol — From triglyceride hydrolysis in adipose tissue. Enters gluconeogenesis as DHAP.
• Note: Fatty acids cannot be converted to glucose in humans (acetyl-CoA cannot be converted to pyruvate or OAA in net terms because the PDH reaction is irreversible and humans lack the glyoxylate cycle).

Bypass Reactions

Gluconeogenesis is NOT simply the reverse of glycolysis. Three irreversible glycolytic reactions must be bypassed by different enzymes:

1. Pyruvate → Oxaloacetate → Phosphoenolpyruvate: Catalyzed by pyruvate carboxylase (in mitochondria, requires biotin) and PEPCK (phosphoenolpyruvate carboxykinase, in cytoplasm). These two steps bypass pyruvate kinase.
2. Fructose-1,6-bisphosphate → Fructose-6-phosphate: Catalyzed by fructose-1,6-bisphosphatase. Bypasses PFK-1.
3. Glucose-6-phosphate → Glucose: Catalyzed by glucose-6-phosphatase (found only in liver and kidney — this is why only these organs can release free glucose into the blood). Bypasses hexokinase/glucokinase.

Glycogen Metabolism

Glycogen is a branched polymer of glucose stored primarily in liver (maintains blood glucose) and skeletal muscle (provides fuel for contraction).

• Glycogenesis (synthesis): Glycogen synthase adds glucose residues in α-1,4 linkages. Branching enzyme creates α-1,6 branch points. Stimulated by insulin.
• Glycogenolysis (breakdown): Glycogen phosphorylase cleaves α-1,4 bonds, releasing glucose-1-phosphate. Debranching enzyme handles α-1,6 branch points. Stimulated by glucagon (liver) and epinephrine (muscle).