Carbohydrate Structure
& Metabolism
Master the building blocks of biological energy: from monosaccharides to glycolysis, understand how carbohydrates fuel life at the molecular level.
MCAT Biochemistry
What Are Carbohydrates?
Carbohydrates are organic molecules with the general formula \( C_n(H_2O)_n \). They are composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. They serve as the body's primary energy source and play critical roles in cell structure and signaling.
Classification of Carbohydrates
Monosaccharides
Single sugar units: glucose, fructose, galactose. Classified by carbon count (triose, pentose, hexose) and carbonyl type (aldose vs. ketose).
Disaccharides
Two monosaccharides joined by a glycosidic bond: sucrose, lactose, maltose. Formed via dehydration synthesis.
Polysaccharides
Long chains of monosaccharides: starch (energy storage in plants), glycogen (animal storage), cellulose (structural).
Aldose vs. Ketose Sugars
Cyclic Forms & Anomers
In aqueous solution, monosaccharides cyclize via an intramolecular reaction between the carbonyl and a hydroxyl group. This creates a hemiacetal (aldoses) or hemiketal (ketoses) ring. The carbon that was the carbonyl becomes the anomeric carbon.
Glycosidic Bond Formation
\(\alpha\)-1,4
Links glucose units in starch and glycogen. Digestible by human enzymes (amylase).
\(\beta\)-1,4
Links glucose in cellulose. Humans cannot digest this bond (no cellulase enzyme).
Key Polysaccharides
Glycogen
Animal glucose storage. Highly branched: \(\alpha\)-1,4 backbone with \(\alpha\)-1,6 branch points every 8-12 residues. Stored in liver and muscle.
Starch
Plant glucose storage. Two forms: amylose (unbranched, \(\alpha\)-1,4) and amylopectin (branched, \(\alpha\)-1,6 every 24-30 residues).
Cellulose
Structural polymer in plant cell walls. Unbranched \(\beta\)-1,4 glucose chains. Forms rigid, linear fibers via hydrogen bonding.
Glycolysis: The Central Pathway
Glycolysis is a 10-step metabolic pathway occurring in the cytoplasm of all cells. It converts one molecule of glucose into two molecules of pyruvate, generating net ATP and NADH.
Energy Investment
Steps 1-5: uses 2 ATP to phosphorylate glucose and split it into two 3-carbon molecules (G3P).
Energy Payoff
Steps 6-10: generates 4 ATP and 2 NADH per glucose, for a net gain of 2 ATP.
Irreversible Steps of Glycolysis
1
Hexokinase (Step 1): Glucose \( \rightarrow \) Glucose-6-phosphate. Uses 1 ATP. Traps glucose inside the cell. Inhibited by \( \text{G6P} \) (product inhibition).
2
Phosphofructokinase-1 (PFK-1, Step 3): \( \text{Fructose-6-P} \rightarrow \text{Fructose-1,6-bisphosphate} \). The rate-limiting step. Activated by AMP and fructose-2,6-bisphosphate. Inhibited by ATP and citrate.
3
Pyruvate kinase (Step 10): \( \text{PEP} \rightarrow \text{Pyruvate} \). Generates 1 ATP per PEP (2 per glucose). Activated by fructose-1,6-bisphosphate (feedforward). Inhibited by ATP and alanine.
Fate of Pyruvate
Glycolysis vs. Gluconeogenesis
| Glycolysis | Gluconeogenesis | |
|---|---|---|
| Direction | Glucose \( \rightarrow \) Pyruvate | Pyruvate \( \rightarrow \) Glucose |
| Location | Cytoplasm | Cytoplasm + Mitochondria |
| Energy | Produces 2 ATP net | Costs \( 4\text{ ATP} + 2\text{ GTP} \) |
| Rate-limiting enzyme | PFK-1 | Fructose-1,6-bisphosphatase |
| Organ | All cells | Mainly liver, some kidney |
| Active when | Fed state (insulin) | Fasting state (glucagon) |
Glycogen Synthesis & Breakdown
Glycogenesis
Building glycogen. Key enzyme: glycogen synthase (adds glucose via \(\alpha\)-1,4 bonds). Branching enzyme creates \(\alpha\)-1,6 branches. Activated by insulin.
Glycogenolysis
Breaking glycogen. Key enzyme: glycogen phosphorylase (cleaves \(\alpha\)-1,4 bonds). Debranching enzyme removes \(\alpha\)-1,6 branches. Activated by glucagon and epinephrine.
How to Approach Carbohydrate Questions
1
Identify the metabolic state. Is the organism fed (insulin dominant) or fasting (glucagon dominant)? This determines which pathways are active.
2
Locate the pathway. Is the question about glycolysis, gluconeogenesis, glycogenesis, glycogenolysis, or the pentose phosphate pathway?
3
Identify the regulatory enzyme. Focus on the rate-limiting step and its allosteric regulators. Most MCAT questions target enzyme regulation.
4
Track the energy currency. Count ATP consumed vs. produced. Remember: NADH from glycolysis must shuttle into mitochondria (malate-aspartate or glycerol-3-phosphate shuttle).
5
Check the tissue. Liver vs. muscle behave differently. Liver has glucokinase and glucose-6-phosphatase; muscle does not.
Example: Net ATP from Glycolysis
Problem: How many net ATP molecules are produced per glucose molecule during glycolysis?
1
Investment phase: Hexokinase uses 1 ATP (Step 1). PFK-1 uses 1 ATP (Step 3). Total invested: 2 ATP.
2
Payoff phase: Each G3P molecule generates 2 ATP (Steps 7 and 10). Since 1 glucose yields 2 G3P: 4 ATP total.
3
Net calculation: \( 4\text{ ATP (produced)} - 2\text{ ATP (invested)} = \) 2 ATP net.
Example: Enzyme Regulation
Problem: A patient has elevated blood glucose after a meal. Which enzyme is most active, and what signal activates it?
1
Identify the state: Post-meal = fed state = high blood glucose = insulin release from pancreatic \( \beta \)-cells.
2
Insulin activates: Glycolysis (via PFK-1 activation through fructose-2,6-bisphosphate) and glycogenesis (glycogen synthase activated by dephosphorylation).
3
Answer: Glycogen synthase is most active, storing excess glucose as glycogen. Glycogen phosphorylase is simultaneously inhibited.
Common Mistakes
✕ Confusing \( \alpha \) and \( \beta \) linkages
\( \alpha \)-linkages (starch, glycogen) are digestible; \( \beta \)-linkages (cellulose) are not. The anomeric OH orientation determines the bond type.
✕ Forgetting the investment phase
Glycolysis produces 4 ATP total but invests 2 ATP first. The net yield is 2 ATP, not 4. Always subtract the investment.
✕ Mixing up phosphorylation effects
Phosphorylation activates glycogen phosphorylase but inactivates glycogen synthase. These opposing effects ensure only one pathway runs at a time.
Question 1 of 5
Which enzyme catalyzes the rate-limiting step of glycolysis?
PFK-1 catalyzes the committed, rate-limiting step of glycolysis: \( \text{fructose-6-phosphate} \rightarrow \text{fructose-1,6-bisphosphate} \). It is allosterically regulated by ATP (inhibitor), AMP (activator), and fructose-2,6-bisphosphate (activator).
Question 2 of 5
Under anaerobic conditions, pyruvate is converted to lactate primarily to:
Lactate dehydrogenase converts pyruvate to lactate while oxidizing NADH back to \(\text{NAD}^+\). Without this regeneration, glycolysis would halt at step 6 (glyceraldehyde-3-phosphate dehydrogenase requires \(\text{NAD}^+\)).
Question 3 of 5
Glycogen is more branched than amylopectin. What is the biological advantage of this increased branching?
More branch points mean more non-reducing ends where glycogen phosphorylase can simultaneously cleave glucose-1-phosphate residues. This allows rapid glucose release during periods of high energy demand (e.g., exercise).
Question 4 of 5
A patient with a deficiency in glucose-6-phosphatase (Von Gierke disease) would be expected to have:
Glucose-6-phosphatase is the final enzyme of both gluconeogenesis and glycogenolysis in the liver, converting G6P to free glucose for release into blood. Without it, the liver cannot export glucose, causing fasting hypoglycemia and accumulation of glycogen in the liver (hepatomegaly).
Question 5 of 5
Which of the following correctly describes the effect of glucagon on carbohydrate metabolism in the liver?
Glucagon signals fasting and raises blood glucose. It activates glycogen phosphorylase (glycogenolysis) via a cAMP/PKA cascade and promotes gluconeogenesis by activating fructose-1,6-bisphosphatase and inhibiting PFK-1. GLUT4 translocation is an insulin effect in muscle/adipose tissue.