Metabolism: Transformations & Interactions

Chapter 7

Metabolism

MetabolismThe sum total of all the chemical

reactions that go on in living cells

Energy MetabolismIncludes all the reactions by which the

body obtains & spends energy from food

The Site of Metabolic Reactions

Metabolic reactions take place inside cells, especially liver cells

Anabolism is the building up of body compounds and requires energy.

Catabolism is the breakdown of body compounds and releases energy.

The Site of Metabolic Reactions

Energy Yielding Nutrients

From CHO → glucose

From fats → glycerol & FA

From proteins → AA

4 basic units used in metabolic pathway

Alcohol can disrupt normal metabolic pathways (Highlight 7)

The Transfer of Energy in Reactions - ATP ATP – adenosine triphosphate

High-energy storage compoundCaptures some energy released during

breakdown of glucose, glycerol, fatty acids, and amino acids

Adenosine Triphosphate (ATP)

The Transfer of Energy in Reactions - ATP Hydrolysis of ATP occurs simultaneously

with reactions that will use that energyCoupled reactionsEnergy released from breakdown of one

compound used to create bond in formation of another

Enzymes & Co-enzymes – Helpers in Metabolism Co-enzymes are organic molecules that work

with enzymes to facilitate their activity.

Enzymes & co-enzymes are helpers in metabolic reactions.

Some B vit serve as coenzymes to enzymes that release energy from glucose, glycerol, FA & AA.

Breaking Down Nutrients for Energy Basic units enter metabolic pathways. During

catabolism, the body separates atoms of basic units. Glucose: 6 C Glycerol: 3 C FA: even number of C (commonly 16 or 18) AA: 2, 3 or more C, with N attached

While each starts down a different path – 2 by-products are common pyruvate & acetyl-coA

Eventually all enter the TCA cycle & electron transport chain

Glycolysis Glucose splitting 6-C Glucose → two 3-C compounds → Pyruvate

Glucose

anaerobic

Pyruvate

Pyruvate to Acetyl CoA If cell needs energy (&

O2 is available), it removes COOH group from pyruvate to produce 2 C compounds that bonds with CoA to form Acetyl CoA

Pyruvate

Acetyl CoA

Absence of sufficient O2 (or mitochondria) pyruvate is converted to lactic acid

Pyruvate

anaerobic

Lactate

Lactate recycled to glucose in liver by Cori cycle

aerobic

Paths of Pyruvate & Acetyl CoA

Glycerol to Pyruvate

3-C glycerol is easily converted to pyruvate Also easily converted to glucose

Glucose

Glycerol

Pyruvate

Fatty Acids to Acetyl CoA

FA are taken apart in 2-C units through fatty acid oxidation.

2-C units split off & combine with CoA to form Acetyl CoA.

If cell doesn’t need energy, acetyl CoA molecules combine to create TG.

Amino Acids Breakdown

Deaminated first (lose N)

Catabolized in a variety of ways: Pyruvate glucose Acetyl CoA more energy or body fat Directly into TCA Cycle generate energy

Amino Acids Breakdown

Amino Acids Deamination results in two products:

Keto acid Ammonia

Transamination is the transfer of the amino group from an amino acid to a keto acid.

Ammonia is converted to urea—a much less toxic compound—in the liver.

Urea is excreted through the kidneys to rid the body of unused nitrogen.

Breaking Down Nutrients for Energy In Summary

Glucose and fatty acids are primarily used for energy, amino acids to a lesser extent.

Glucose is made from all carbohydrates, most amino acids and the glycerol portion of fat.

Protein is made from amino acids. Glucose can be made into nonessential amino acids if

nitrogen is present. All energy-yielding nutrients consumed in excess can

contribute to fat storage.

Breaking Down Nutrients for Energy – the final stepsTCA Cycle – tricarboxylic acid cycle (akaKreb’s cycle)

Oxaloacetate picks up acetyl CoA & drops off 2-C & returns to pick up another acetyl CoA.

As acetyl CoA breaks to CO2, H atoms with their electrons are removed.

Coenzymes made from B vitamins take H & electrons & transfers them to ETC.

Breaking Down Nutrients for Energy – the final stepsElectron Transport Chain (ETC) Consist of a series of proteins that serve

as electron carriers. These carriers are inside inner membrane of mitochondria.

In ETC, energy is captured in bonds of ATP molecules.

ATP leaves mitochondria & enters cytoplasm, where it can be used for energy.

Kcal/gram Each ATP holds energy & Kcal measures

energy, so the more ATP generated the more Kcal collected. One glucose molecule yield 36-38 ATP when

oxidized completely. One 16 carbon FA yield 129 ATP when oxidized

completely.

Energy Balance - Surplus

When energy intake exceeds energy output, there is a gain in weight.

Fat cells enlarge & multiply regardless of whether the excess comes from protein, CHO or fat.

Most direct & efficient pathway to body fat is dietary fat.

Energy Balance

Body needs energy all the time Relies on energy stores between meals

glycogen fatty acids lean mass tissue .. least preferred

Energy Balance – Fasting/Starvation Fasting—Inadequate Energy

Glucose needed for the brainProtein meets glucose needsThe shift to ketosis

Ketones are produces when glucose is not available.

Ketosis causes a suppression of the appetite.Slowing of metabolism

Energy Balance – Fasting/Starvation Fasting—Inadequate Energy

Symptoms of starvation Muscle wasting Decreased heart rate, respiratory rate, metabolic

rate, and body temperature Impaired vision Organ failure Decreased immunity Depression, anxiety, and food-related dreams

Alcohol & Nutrition

Alcohol

For most adults, moderate consumption of alcohol is no more than 1 drink a day & no more than 7 drinks a week.

Pregnant women are advised to avoid alcohol.

Alcohol in body

Doesn’t need digestion & quickly absorbed.

About 20% is absorbed directly across walls of an empty stomach & can reach brain within minutes.

Stomach breaks down alcohol with its alcohol dehydrogenase enzyme.

Can reduce amount of alcohol entering blood by about 20%.

Alcohol in small intestine

Rapidly absorbed

From then on, it’s absorbed & metabolized before most nutrients which ensures a speedy disposal.

Alcohol in liver

Liver cells make alcohol dehydrogenase enzyme to oxidize alcohol.

It can process about ½ ounce of ethanol per hour Rate is set by amount of alcohol dehydrogenase

available.

Extra alcohol circulates until liver enzymes are available to process it.

Liver deterioration

1st stage – fatty liver

2nd stage – fibrosis

3rd stage – cirrhosis (damage least reversible)

Fig. H7-3, p. 241

Fat (triglycerides)

Fatty acidsNAD+ NADH + H+ NAD+ NADH + H+

Alcohol (ethanol)

Alcohol dehydrogenase

Acetaldehyde dehydrogenase

Acetaldehyde Acetate Acetyl CoA

CoA

TCA Cycle

Acetyl CoA molecules are blocked from getting into the TCA cycle by the high level of NADH. Instead of being used for energy, the acetyl CoA molecules become building blocks for fatty acids.

Alcohol Metabolism

Alcohol & Metabolism

Alcohol alters amino acid & protein metabolism

Synthesis of protein important to immune system slows down, weakening body’s defense against infection.

Protein deficiency can develop.

Fig. H7-4, p. 242

Alcohol Effects

Alcohol Effects

Alcohol reduces production of Anti-diuretic hormone which retains H2O.

Drinking alcoholic beverages – ↑ water loss - leads to dehydration.

Alcohol Effects

Alcohol can contribute to body fat promote obesity.

High in calories

Most often associated with central obesity

More Kcal from alcohol - fewer Kcal from nutritious food

Alcohol Effects

Deficiency in folate & thiamin

Acetaldehyde (intermediate in alcohol metabolism) interferes with nutrient use.

Alcohol Effects

Alcohol causes stomach cells to over secrete gastric acid & histamine.

Beer stimulates gastric secretion, irritating stomach & esophagus linings making them vulnerable to ulcer formation.

Next

Nutrients involved in fluid & electrolyte balance - Chapter 12