Types of Metabolic Pathways

Metabolic Pathways

Metabolic processes usually involve a number of steps that must occur in the correct sequence.

• A sequence of enzyme-controlled reactions is called a metabolic pathway.
• Typically metabolic pathways are interconnected.

Carbohydrate pathways

• Carbohydrates may enter catabolic pathways and be used as energy sources.
• When present in excess, carbohydrates may enter anabolic pathways and be converted to glycogen or fat.

Lipid pathways

• Most dietary fats are triglycerides.
• Before fats can be used as an energy source they must be converted into glycerol and fatty acids.
• Fatty acids can be changed to acetyl coenzyme A and ketone bodies which in turn can be oxidized by the citric acid cycle
• Fats can be synthesized from glycerol and fatty acids and from excess glucose or amino acids.

Protein pathways

• Proteins are used as building materials for cellular parts, as enzymes, and as energy sources.
• Before proteins can be used as energy sources, they must be decomposed into amino acids, and the amino acids must be deaminated.
• The deaminated portions of amino acids can be broken down into carbon dioxide and water, or can be converted into glucose or fat.
• About 8 essential amino acids cannot be synthesized by human cells and must be obtained in foods.

Regulation of metabolic pathways

• A metabolic pathway may be regulated by a rate-limiting enzyme. 
• The regulatory enzyme, in turn, may be controlled by a negative feedback mechanism in which the product of the pathway inhibits the enzyme.
• The rate of product production usually remains stable.