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Metabolic Disorders
and Pathway Defects

Explore how enzyme deficiencies disrupt biochemical pathways, leading to conditions like PKU and diabetes: the molecular basis of metabolic disease.

Biochemistry

What Is Metabolism?

Metabolism is the complete set of chemical reactions in living organisms that convert food into energy and building blocks. These reactions are organized into metabolic pathways: ordered sequences where the product of one reaction becomes the substrate for the next.

Substrate A Enzyme 1 Product B Enzyme 2 Product C Enzyme 3 Product D Each step requires a specific enzyme catalyst

Enzymes as Gatekeepers

Enzymes are biological catalysts that lower the activation energy \( E_a \) of reactions. Each enzyme is highly specific: it binds one substrate at its active site and converts it into a product.

Specificity
Lock-and-key model: each enzyme fits only its specific substrate
Catalysis
Lowers \( E_a \), speeding reactions by factors of \( 10^6 \) to \( 10^{12} \)
Regulation
Feedback inhibition, allosteric control, and gene expression modulate activity

What Are Metabolic Disorders?

Metabolic disorders arise when a genetic defect impairs an enzyme in a metabolic pathway. The consequences follow a predictable pattern:

1
Substrate accumulation: the molecule before the blocked step builds up to abnormally high concentrations.
2
Product deficiency: the molecule after the blocked step is not produced in adequate amounts.
3
Alternative pathway activation: the accumulated substrate may be shunted into normally minor side-reactions, producing toxic byproducts.

The Enzyme Block Model

Normal Pathway Phe PAH Tyr Melanin, etc. Downstream Defective Pathway (PKU) Phe ACCUMULATES BLOCKED Tyr DEFICIENT Phenylpyruvate (toxic byproduct via alternative pathway)

Phenylketonuria: PKU

PKU is an autosomal recessive disorder caused by mutations in the PAH gene on chromosome 12. It affects approximately 1 in 10,000 births worldwide.

Cause
Loss-of-function mutation in PAH enzyme: cannot convert Phe to Tyr
Effect
Phe accumulates in blood and brain, causing intellectual disability if untreated
Treatment
Lifelong low-Phe diet; Tyr supplementation; newborn screening via Guthrie test

Type 1 vs Type 2 Diabetes

Both types disrupt glucose metabolism, but through fundamentally different mechanisms:

Type 1 Diabetes Type 2 Diabetes
Root cause Autoimmune destruction of beta cells Insulin resistance + beta cell exhaustion
Insulin level Absent or very low Normal to high (initially)
Onset Usually childhood Usually adulthood
Genetics HLA-linked susceptibility Polygenic + lifestyle factors
Treatment Insulin injections (essential) Diet, exercise, oral drugs, sometimes insulin
Metabolic defect No insulin signal at all Impaired signal transduction

Insulin Signaling Pathway

Insulin Insulin Receptor IRS / PI3K / Akt GLUT4 Translocation Glucose enters cell Glycogen Synthesis Glucose stored as glycogen Type 2: Resistance here Type 1: No insulin

Blood Glucose Regulation

Time (hours) Blood Glucose Normal Meal 2h 4h Normal Diabetic High

How to Analyze a Metabolic Disorder

Use this systematic approach whenever you encounter a metabolic disorder in biochemistry:

1
Identify the defective enzyme. Which specific enzyme is missing, reduced, or non-functional?
2
Locate the block. Where in the metabolic pathway does the blockage occur? Draw the pathway and mark the blocked step.
3
Predict accumulation. Which substrate(s) build up upstream of the block?
4
Predict deficiency. Which product(s) are missing downstream of the block?
5
Check for shunting. Does the accumulated substrate enter an alternative pathway producing toxic byproducts?
6
Connect to symptoms. How do the biochemical changes (accumulation, deficiency, toxicity) explain the clinical signs?

Worked Example: PKU Analysis

Problem: A newborn's blood test shows phenylalanine at 20 mg/dL (normal: < 2 mg/dL). Apply the 6-step framework.

1
Defective enzyme: Phenylalanine hydroxylase (PAH)
2
Block location: Phe → Tyr conversion is blocked
3
Accumulation: Phenylalanine builds up in blood (20 mg/dL vs normal < 2)
4
Deficiency: Tyrosine is not produced: reduced melanin, dopamine, and thyroid hormones
5
Shunting: Phe is transaminated to phenylpyruvate (excreted in urine: "phenylketonuria")
6
Symptoms: Intellectual disability, fair skin/hair (low melanin), musty odor (phenylpyruvate)

Worked Example: Type 1 Diabetes

Problem: A child presents with blood glucose of 350 mg/dL (normal fasting: 70-100 mg/dL), ketones in urine, and weight loss. Analyze using the framework.

1
Defective component: Beta cells of the pancreas are destroyed by autoimmune attack: no insulin production.
2
Block: Insulin-dependent glucose uptake into muscle and fat cells is absent.
3
Accumulation: Glucose accumulates in blood (hyperglycemia: 350 mg/dL).
4
Deficiency: Cells are starved of glucose: they cannot perform glycolysis or produce sufficient ATP.
5
Shunting: Cells switch to fatty acid oxidation, producing excess acetyl-CoA, which forms ketone bodies (diabetic ketoacidosis).
6
Symptoms: Polyuria, polydipsia, weight loss, fruity breath (ketones), fatigue.

Worked Example: Type 2 Diabetes

Problem: A 55-year-old with BMI 34 has a fasting glucose of 180 mg/dL and elevated insulin levels. How does this differ from Type 1?

Insulin Present Receptor RESISTANCE GLUT4 / Akt Glucose stays in blood Cells resist insulin's signal despite high insulin levels

Other Metabolic Disorders

Galactosemia
Deficient galactose-1-phosphate uridylyltransferase: cannot metabolize galactose from milk. Causes liver damage and cataracts.
Maple Syrup Urine Disease
Branched-chain alpha-ketoacid dehydrogenase deficiency: accumulation of leucine, isoleucine, valine. Sweet-smelling urine.
Tay-Sachs
Hexosaminidase A deficiency: GM2 ganglioside accumulates in neurons, causing progressive neurodegeneration.
Gaucher Disease
Glucocerebrosidase deficiency: glucocerebroside accumulates in macrophages. Treatable with enzyme replacement therapy.

Common Mistakes

✕ Confusing Type 1 and Type 2
Type 1: no insulin produced. Type 2: insulin produced but cells resist it. They are mechanistically distinct despite similar symptoms.
✕ Thinking PKU means "can't eat protein"
PKU patients restrict phenylalanine specifically, not all protein. They need other amino acids for growth, including supplemental tyrosine.
✕ Forgetting alternative pathways
Students often focus on substrate buildup but miss that the accumulated substrate can enter alternative pathways producing additional toxic products (e.g., phenylpyruvate in PKU, ketone bodies in T1D).

Summary: Key Disorders at a Glance

Disorder Defective Component Accumulates Treatment
PKU PAH enzyme Phenylalanine Low-Phe diet
Type 1 Diabetes Beta cells (destroyed) Blood glucose Insulin injections
Type 2 Diabetes Insulin receptor signaling Blood glucose Diet, metformin
Galactosemia GALT enzyme Galactose-1-P Lactose-free diet
Tay-Sachs Hex A enzyme GM2 ganglioside No cure (supportive)

Question 1 of 5

In phenylketonuria (PKU), the defective enzyme is phenylalanine hydroxylase. What is the immediate biochemical consequence of this deficiency?

Question 2 of 5

A patient has high blood glucose, high insulin levels, and is obese. Which condition is most likely?

Question 3 of 5

In Type 1 diabetes, cells are starved of glucose and switch to fatty acid oxidation. What toxic byproduct results from this metabolic shunting?

Question 4 of 5

Archibald Garrod's concept of "inborn errors of metabolism" established which foundational principle?

Question 5 of 5

A metabolic disorder causes accumulation of GM2 ganglioside in neurons due to a deficiency in hexosaminidase A. Which disorder is this, and why is it currently untreatable?