Introduction to Metabolism

Metabolism The sum of the chemical changes that convert nutrients into energy and the chemically complex products of cells Hundreds of enzyme reactions organized into discrete pathways Substrates are transformed to products via many specific intermediates Metabolic maps portray the reactions

A Common Set of Pathways Organisms show a marked similarity in their major metabolic pathways Evidence that all life descended from a common ancestral form There is also significant diversity Autotrophs use CO 2 ; Heterotrophs use organic carbon; Phototrophs use light; Chemotrophs use Glc, inorganics use S and obtain chem energy through food generated by phototrophs.

The Sun is Energy for Life Phototrophs use light to drive synthesis of organic molecules Heterotrophs use these as building blocks CO 2, O 2, and H 2 O are recycled

Metabolism Metabolism consists of catabolism and anabolism Catabolism: degradative pathways Usually energy-yielding! “destructive metabolism” FUELS -> -> CO 2 + H 2 O + useful energy Anabolism: biosynthetic pathways energy-requiring! “constructive metabolism” Useful energy + small molecules --> complex molecules

Organization in Pathways Pathways consist of sequential steps The enzymes may be: Separate Form a multienzyme complex A membrane-bound system New research indicates that multienzyme complexes are more common than once thought

Catabolism and Anabolism Catabolic pathways converge to a few end products Anabolic pathways diverge to synthesize many biomolecules Some pathways serve both in catabolism and anabolism and are called amphibolic pathways

Comparing Pathways Anabolic & catabolic pathways involving the same product are not the same Some steps may be common to both Others must be different - to ensure that each pathway is spontaneous This also allows regulation mechanisms to turn one pathway and the other off

METABOLIC REGULATION Regulated by controlling: 1.Amounts of enzymes 2.Catalytic activities 3.Accessibility of substrates

Digestion of food polymers:  enzyme-catalyzed hydrolysis Glycolysis:  glucose catabolism  generate ATP without consuming oxygen (anaerobic) Citric Acid Cycle:  metabolism of acetyl-CoA derived from pyruvate, fatty acids, and amino acids  acetyl oxidized to CO 2  operates under aerobic conditions  reduction of coenzymes NAD + and FAD; energy used to produce ATP Oxidative phosphorylation:  reduction of molecular oxygen by NADH and FADH 2  energy of reduced compounds used to pump protons across a cell membrane  potential energy of electrochemical gradient drives phosphorylation of ADP to ATP

The ATP Cycle ATP is the energy currency of cells In phototrophs, light energy is transformed into the chemical energy of ATP In heterotrophs, catabolism produces ATP, which drives activities of cells ATP cycle carries energy from photosynthesis or catabolism to the energy-requiring processes of cells

Redox in Metabolism NAD + collects electrons released in catabolism Catabolism is oxidative - substrates lose electrons, usually H - ions Anabolism is reductive - NADPH provides the electrons for anabolic processes, and the substrates gain electrons

WHY ATP? Free energy is released when ATP is hydrolyzed. This energy drives reactions that need it (eg. muscle contraction) Recall coupled reactions ATP has a higher phosphoryl transfer potential

RECURRING MOTIFS IN METAB Certain compounds keep on recurring or appearing in metabolic reactions and their functions are the same in the processes Metab looks complicated but reactions are actually limited and repeating.

ACTIVATED CARRIERS These species help carry out the metabolic reactions, even nonfavorable ones, at times Example: ATP (activated carrier of phosphoryl groups)

Activated carriers of electrons for fuel oxidation: e - acceptors! Aerobic systems: O 2 is the final e - acceptor, but this does not occur directly Fuels first transfer e - to carriers: pyridine molecules or flavins. NAD + : nicotinamide adenine dinucleotide

Activated carriers of electrons for fuel oxidation: e - acceptors! FAD: Flavin adenine dinucleotide

Activated carrier of electrons for reductive biosynthesis: e - donors! NADPH: common electron donor R is phosphate group

Activated carrier of carbon fragments COENZYME A: carrier of acyl groups

Activated carrier of two-carbon fragments

VITAMINS Many vitamins are "coenzymes" - molecules that bring unusual chemistry to the enzyme active site Vitamins and coenzymes are classified as "water-soluble" and "fat-soluble" The water-soluble coenzymes exhibit the most interesting chemistry

Key Reactions in Metabolism

1. REDOX reactions Electron carriers are needed!

2. LIGATION reactions Bond formation facilitated by ATP cleavage

3. ISOMERIZATION reactions

4.GROUP TRANSFER

5.HYDROLYTIC reactions Bond cleavage by addition of H 2 O

6.ADDITION of functional groups to double bonds or REMOVAL of groups to form double bonds Uses lyases