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General metabolism

Last updated: October 21, 2021

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Metabolism is the sum of chemical reactions continuously taking place in the body to maintain its proper function. Such reactions include the conversion of nutrients into energy and substances (e.g., proteins, lipids, nucleic acids, etc.) that can be processed by cells. They can involve the breakdown of substances (catabolism), which generally releases energy, or the synthesis of substances (anabolism), which generally consumes energy. Metabolic reactions are organized into pathways (e.g., glycolysis, lipolysis), with each step involving a specific enzyme to catalyze the reaction. Dysregulation of these pathways can lead to metabolic disorders such as glycogen storage diseases.

This article gives an overview of the most important metabolic pathways, as well as the most common metabolic disorders.

This section deals with the most important metabolic pathways.

For details on the individual pathways, see the articles “Carbohydrates”, “Glycolysis and gluconeogenesis”, and “Glycogen metabolism”.

For details on this topic, see the article “Lipids and their metabolism.”

Overview of lipid metabolism
Pathway Precursor(s) End product(s) Rate-limiting enzyme Individual steps Pathway regulation
Stimulation Inhibition
Lipogenesis
Lipolysis
  • Acyl-CoA

Ketogenesis
  • Prolonged fasting
  • Alcohol consumption
  • N/A
Ketogenolysis
  • N/A
Cholesterol synthesis

For details on this topic, see the article “Amino acids”.

This section gives an overview of the most common metabolic disorders.

Disorders that affect the carbohydrate metabolism are predominantly inherited genetic conditions that are inherited in an autosomal recessive fashion. Lactose intolerance, however, is generally caused by a genetic polymorphism of the lactase-coding gene or develops secondary to other conditions (e.g., gluten-sensitive enteropathy). For details, see the article “Lactose intolerance” and “Inborn errors of carbohydrate metabolism”.

Overview of disorders affecting the carbohydrate metabolism
Condition Enzyme deficiency Pathophysiology Clinical features
Lactose intolerance
  • Diarrhea (often watery, bulky, and frothy)
  • Cramping abdominal pain (often periumbilical or in the lower abdomen)
  • Abdominal bloating, flatulence
  • Nausea
Fructose intolerance Hereditary fructose intolerance
Essential fructosuria
  • Asymptomatic
Galactosemia Galactokinase deficiency
  • ↓ Conversion of galactose to galactose-1-phosphate → accumulation of galactitol in tissues
Classic galactosemia
  • ↓ Conversion of galactose-1-phosphate to UDP-galactose → accumulation of toxic substances in tissues (e.g., galactitol in the lens)
Uridine diphosphate galactose-4-epimerase deficiency
  • Mostly asymptomatic

Glycogen storage diseases Type I (von Gierke disease) Type 1a
Type 1b
Type II (Pompe disease)
  • Hydrolyzation of α-1,4- and α-1,6-linkages in the acidic environment of the lysosome → impaired glycogenolysis
Type III (Cori disease)
Type IV (Andersen disease)
Type V (McArdle disease)
Type VI (Hers disease)

Medium-chain acyl-CoA dehydrogenase deficiency, primary carnitine deficiency, and carnitine palmitoyltransferase II deficiency are all genetic conditions that are inherited in an autosomal recessive fashion. For details on these conditions, see “Disorders of fatty acid metabolism”.

Overview of disorders affecting the fatty acid metabolism
Condition Deficiency Pathophysiology Clinical features
Medium-chain acyl-CoA dehydrogenase deficiency (MCAD deficiency)
Primary carnitine deficiency
  • Carnitine transporters
Carnitine palmitoyltransferase II deficiency (CPT II deficiency)
  • Carnitine palmitoyltransferase II

All conditions that affect the amino acid metabolism are genetic conditions that are inherited in an autosomal recessive fashion. For details on these conditions, see the article “Inborn errors of metabolism”.

Overview of disorders affecting the amino acid metabolism
Condition Deficiency Causes Pathophysiology Clinical features
Phenylketonuria
Homocystinuria
Hartnup disease
Alkaptonuria
Maple syrup urine disease
Cystinuria
Organic acidemias Propionic acidemia
Methylmalonic acidemia
Cystinosis
Histidinemia
  • Histidase
  • Mostly asymptomatic
Pyruvate dehydrogenase complex deficiency
  • Onset: early neonatal period
  • Poor feeding, lethargy
  • Tachypnea
  • Developmental delay
  • Progressive neurological symptoms

For details, see “Purine salvage deficiencies”.

Overview of disorders affecting the purine metabolism
Condition Enzyme deficiency Causes Pathophysiology Clinical features
Lesch-Nyhan syndrome
Adenosine deaminase deficiency

For details, see “Urea cycle disorders”.

Overview of disorders affecting the urea cycle
Condition Enzyme deficiency Causes Pathophysiology Clinical features
Ornithine transcarbamylase deficiency
  • Symptoms commonly manifest in the first days of life but can develop at any age.
  • Nausea, vomiting, irritability, poor feeding
  • Delayed growth and cognitive impairment
  • Metabolic encephalopathy
Arginase deficiency
Carbamoyl phosphate synthetase 1 (CPS1) deficiency
N-acetylglutamate synthase deficiency

For details, see the article “Inborn errors of metabolism”.

Overview of miscellaneous disorders
Condition Defect Causes Pathophysiology Clinical features
Alpha-1 antitrypsin deficiency
  • Gene mutation → conformational change in the structure of AAT protein → dysfunctional (or absent) AAT
Mitochondrial myopathies

Mitochondrial Encephalomyopathy, Lactic Acidosis, Stroke-like episodes (MELAS)

Myoclonic epilepsy with ragged red fibers (MERRF)
Chronic progressive external ophthalmoplegia (CPEO)
Kearns-Sayre syndrome
Leber hereditary optic neuropathy (LHON)
  • Painless acute or subacute bilateral vision
Leigh syndrome
Pyruvate dehydrogenase complex deficiency
  • Onset: early neonatal period
  • Poor feeding, lethargy
  • Tachypnea
  • Developmental delay
  • Progressive neurological symptoms
Orotic aciduria

  1. Bhagavan NV, Ha C-E. Carbohydrate Metabolism II. Elsevier ; 2015 : p. 205-225
  2. Liao Y, Davies NA, Bogle IDL. Computational Modeling of Fructose Metabolism and Development in NAFLD. Frontiers in Bioengineering and Biotechnology. 2020; 8 . doi: 10.3389/fbioe.2020.00762 . | Open in Read by QxMD