Alternative Metabolic Pathways

Carbohydrates are the first nutrient most organisms catabolise for energy, however, living things must metabolize other energy-rich nutrients to obtain energy in times of starvation

Protein Catabolism

– first digested into amino acids

– amino groups are removed (deamination; amino group à ammonia [NH₃])

– remaining portion of amino acid converted into various components of glycolysis or the Krebs cycle

– the type of amino acid determines the point of entry into these metabolic pathways,

– For example, leucine is converted into acetyl-CoA, alanine is converted into pyruvate

Lipid Catabolism

– first triglycerides are digested into glycerol and fatty acids

– glycerol may be converted into glucose (glucaoneogenesis), or it may be changed into DHAP or G3P à fed into glycolysis

– fatty acids transported to mitochondria (b-oxidation)

– b-oxidation is the removal  of two-carbon acetyl groups, starting at the carboxyl end

– acetyl groups combine with coenzyme A to form acetyl-CoA à enter the Krebs cycle

Ethanol Fermentation

– allows NAD+ to be recycled and glycolysis to continue

Lactic Acid Fermentation

– allows NAD+ to be recycled and glycolysis to continue

– lactate in muscle tissue causes stiffness, soreness and fatigue

– lactate is oxidized back to pyruvate, which then goes through the Krebs cycle when vigorous activity ceases