Metabolism

Animals rely on energy from the sun to do work. Light energy to chemical bond energy. Heterotrophs or chemotrophs (that’s us) extract energy in the form of chemical bond energy to do work. reduce CO 2  glucose oxidize glucose  CO 2 Plants Animals A Little Biology

Ultimately the carbon atoms from glucose  CO 2 In the conversion of glucose to CO 2 energy is extracted in the form of chemical bond energy in discrete steps. The Big Picture Biochemists like to use the word “fate” for “what happens to”. What is the fate of glucose under aerobic conditions? What is the fate of pyruvate during strenuous exercise? What is the fate of medical students after their biochemistry final exam?

Metabolism The sum total of all the chemical and physical changes that occur in a living system, which may be a cell, a tissue, an organ, or an organism. –The reactions of metabolism are almost all enzyme- catalyzed. transformation of nutrients excretion of waste products energy transformations synthetic and degradative processes

Catabolism vs. Anabolism Catabolism is the phase of metabolism that encompasses the breaking down and energy yielding reactions. The cellular breakdown of complex substances and macromolecules

Catabolism vs. Anabolism Anabolism is the phase of metabolism that encompasses the making of biological molecules and require energy. The cellular synthesis of complex substances and macromolecules smaller molecules.

The Really Big Picture

The Stages of Cellular Metabolism: A Preview Metabolic Respiration is a cumulative function of three metabolic stages –Glycolysis –The citric acid cycle –Oxidative phosphorylation

Glycolysis (glyco= glucose and lysis= split) –Breaks down glucose into two molecules of pyruvate The citric acid cycle –Completes the breakdown of glucose

Oxidative phosphorylation –Is driven by the electron transport chain –Generates ATP (Cell energy)

2 H 1 / 2 O 2 (from food via NADH) 2 H e – 2 H + 2 e – H2OH2O 1 / 2 O 2 Controlled release of energy for synthesis of ATP ATP Electron transport chain Free energy, G (b) Cellular respiration + Figure 9.5 B

A animal cell Rough ERSmooth ER Centrosome CYTOSKELETON Microfilaments Microtubules Microvilli Peroxisome Lysosome Golgi apparatus Ribosomes In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm) Nucleolus Chromatin NUCLEUS Flagelium Intermediate filaments ENDOPLASMIC RETICULUM (ER) Mitochondrion Nuclear envelope Plasma membrane Figure 6.9

Mitochondria are enclosed by two membranes –A smooth outer membrane –An inner membrane folded into cristae Mitochondrion Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Mitochondrial DNA Inner membrane Cristae Matrix 100 µm Figure 6.17

An overview of cellular respiration Figure 9.6 Electrons carried via NADH Glycolsis Glucose Pyruvate ATP Substrate-level phosphorylation Electrons carried via NADH and FADH 2 Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis ATP Substrate-level phosphorylation Oxidative phosphorylation Mitochondrion Cytosol

Both glycolysis and the citric acid cycle –Can generate ATP by substrate-level phosphorylation Figure 9.7 Enzyme ATP ADP Product Substrate P +

Glycolysis harvests energy by oxidizing glucose to pyruvate Glycolysis –Means “splitting of sugar” –Breaks down glucose into pyruvate –Occurs in the cytoplasm of the cell

Glycolysis consists of two major phases –Energy investment phase –Energy payoff phase Glycolysis Citric acid cycle Oxidative phosphorylation ATP 2 ATP 4 ATP used formed Glucose 2 ATP + 2 P 4 ADP + 4 P 2 NAD e H + 2 NADH + 2 H + 2 Pyruvate + 2 H 2 O Energy investment phase Energy payoff phase Glucose 2 Pyruvate + 2 H 2 O 4 ATP formed – 2 ATP used 2 ATP 2 NAD e – + 4 H + 2 NADH + 2 H + Figure 9.8

Before the citric acid cycle can begin –Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis CYTOSOLMITOCHONDRION NADH + H + NAD CO 2 Coenzyme A Pyruvate Acetyle CoA S CoA C CH 3 O Transport protein O–O– O O C C CH 3 Figure 9.10

An overview of the citric acid cycle ATP 2 CO 2 3 NAD + 3 NADH + 3 H + ADP + P i FAD FADH 2 Citric acid cycle CoA Acetyle CoA NADH + 3 H + CoA CO 2 Pyruvate (from glycolysis, 2 molecules per glucose) ATP Glycolysis Citric acid cycle Oxidative phosphorylatio n Figure 9.11

There are three main processes in this metabolic enterprise Electron shuttles span membrane CYTOSOL 2 NADH 2 FADH 2 2 NADH 6 NADH 2 FADH 2 2 NADH Glycolysis Glucose 2 Pyruvate 2 Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis MITOCHONDRION by substrate-level phosphorylation by substrate-level phosphorylation by oxidative phosphorylation, depending on which shuttle transports electrons from NADH in cytosol Maximum per glucose: About 36 or 38 ATP + 2 ATP + about 32 or 34 ATP or Figure 9.16

The catabolism of various molecules from food Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3- P Pyruvate Acetyl CoA NH 3 Citric acid cycle Oxidative phosphorylation Fats Proteins Carbohydrates Figure 9.19

The control of cellular respiration Glucose Glycolysis Fructose-6-phosphate Phosphofructokinase Fructose-1,6-bisphosphate Inhibits Pyruvate ATP Acetyl CoA Citric acid cycle Citrate Oxidative phosphorylation Stimulates AMP + – – Figure 9.20

ATP Chemical energy of the cell The cell takes up glucose and converts it to cell energy (ATP) Various forms of cell energy –ATP, ADP, AMP, Creatine phosphate

The structure of ATP, ADP, and AMP ATP is most commonly hydrolyzed to ADP or AMP adenine ribose

The structural Basis of High Phosphoryl Transfer Potential of ATP

Creatine phosphate is a reservoir of high potential phosphoryl groups. Creatine kinase transfers phosphate to ADP to form ATP. This reaction is important in heart muscle after an Myocardial Infarction.

Other Activated Carriers Just as ATP carries and transfers phosphate other molecules carry electrons and participate in oxidation reduction reactions (i.e. NADH, NADH2, FADH2).

The electrons are not directly transferred to O 2. Electron Transfer System Electron carriers (i.e. NADH) Deliver Energy To ETS

Nicotine Adenine Dinucleotide NADPH

Generally, NAD + participates in reactions where alcohols are converted to ketones/aldehydes and organic acids.

Synthetic and degradative pathways are distinct. If [ATP] is low, degradative pathways are stimulated. If [ATP] is high, degradative pathways are inhibited. Synthesis Degradation

High [NADH] is indirectly equivalent to high[ATP]. This means that the cell is high in “energy”. High [NAD + ] or [ADP or AMP] means that the cell is low in “energy”. These molecules (and others) can act as allosteric effectors stimulating or inhibiting allosteric enzymes which are usually at the beginning or branch-points of a specific pathway. Regulation of the degradation and synthesis of glucose and glycogen depends on the energy state of the cell

Synthetic and Degradative Pathways Don’t Happen at the Same Time They can share some common steps but they are never simply the reverse of one another. Synthetic pathways always use more ATP than a degradative pathway will produce. If both synthetic and degradative pathways occurred at the same time, “wasteful” hydrolysis of ATP would result. This is termed a “futile cycle.”

Regulation of synthetic and degradative pathways. For example, phosphorylation activates glycogenolysis (breakdown of glucose) whereas phosphorylation inactivates glycogenesis (glycogen synthesis). Put differently: Phosphorylation activates glycogenolysis whereas dephosphorylation activates glycogenesis. On the same theme, the action of insulin is opposite to that of glucagon. Insulin decreases blood glucose levels whereas glucagon increases blood glucose levels.

In Summary

Intrinsic Regulation Molecules such as NAD +, NADH, ATP, ADP, AMP etc. are important intrinsic regulators of cellular metabolism. –the concentrations of these molecules mirror the energy charge of the cell and act as regulators of the cells’ metabolism. This is only one level of regulation.

Extrinsic Regulation Hormones are a higher order of regulation involving communication between cells, tissues, and the environment. Many hormones (not all) interact with cell surface receptors and set off a cascade of molecular events which:  stimulate or repress the activity of key enzymes.  AND/OR  stimulate or repress the transcription of specific genes.

Two hormones that are of particular importance and involve the regulation of catabolic and anabolic pathways are: INSULIN & GLUCAGON

Insulin vs. Glucagon In general: –Insulin operates through dephosphorylation mechanisms. –Glucagon operates through phosphorylation mechanisms.

A fatty acid molecule (subunit of fat) is in a more reduced state than a molecule of glucose. Thus, more energy is extracted from the FA than CHO.

What’s Next? The endocrine lectures will deal with the degradation and synthesis of carbohydrates. This will be followed by lectures dealing with the degradation and synthesis of fatty acids. Some integrative of metabolism will then be discussed within a frame work of - “feeding, fasting, and exercising”