Lipids fatty acids and glycerol Proteins amino acids
Gives us the nitrogen to synthesize nitrogen-containing compounds in our cells (new proteins and nucleic acids)
Both categories are further broken down in enzymatic reactions from there.
Adipocytes = fat cells that compose our adipose tissue These store triacylglycerols Body fat is our major source of stored energy
First step of dietary fat digestion: in the small intestine, fat globules mix with bile salts, and emulsification occurs.
Second step of dietary fat digestion: pancreatic lipases hydrolyze the triacylglycerols to yield monoacylglycerols and free fatty acids. These products get reabsorbed through the lining of the small
intestine and recombine once again to form triacylglycerols, then combine with proteins to form a lipoprotein called a chylomicron.
Chylomicrons are transported through the bloodstream (now water soluble!) and can be carried whereever they are needed.
Used as energy when blood glucose is low and glycogen stores are depleted
Fat mobilization: triacylglycerols in the adipose tissue are broken down to fatty acids and glycerol
In the next sets of slides, we’ll learn how the glycerol and the fatty acids get processed once they’re broken apart
Activated by the hormones glucagon or epinephrine
Gets metabolized in the liver Glycerol is converted to dihydroxyacetone
phosphate in two steps, which we will not memorize, but this compound feeds into glycolysis and gluconeogenesis.
Yields a lot of energy! Metabolism of fatty acids: beta () oxidation, which
removes chunks of two carbons at a time. Each two carbon chunk is degraded into an acetyl-CoA unit,
which enters the citric acid cycle. And, the length of the fatty acid determines the number of
times the cycle repeats itself. For instance… myristic acid = 14 carbons = produces 7 acetyl-CoA
groups = goes through cycle 6 times
Beta oxidation: applies to saturated fatty acids with an even number of carbons. But many things we eat contain unsaturated fatty acids.
How are these processed? A cis fatty acid is turned into a trans fatty acid via an isomerase
Then, a hydration reaction adds water across the double bond.
Each cycle of beta oxidation yields one NADH, one FADH2, and one acetyl-CoA.
So, if you have a saturated fatty acid containing 16 carbons, what would be the NADH/FADH2 yield?
When we don’t have enough carbohydrates to meet our energy needs, the body will break down stored fats.
However, the breakdown of large amounts of fat will cause acetyl-CoA molecules to accumulate in the liver. These molecules will combine to form ketone bodies through the ketogenesis pathway. Ketone bodies, once produced in the liver, are then transported
to cells in the heart, brain, and skeletal muscle. Conversion back to acetyl-CoA generates small amounts of energy.
Sometimes ketone bodies are not completely metabolized – leading to ketosis – lowers pH of the blood, causes breathing difficulties.
Occurs when the body has met all of its energy needs and there is acetyl-CoA left over. In other words… a process a lot of us try to avoid, and a
process that has created a multi-billion dollar industry! Called lipogenesis
Overall: two carbon acetyl units are linked together to form palmitic acid (a 16-carbon fatty acid)
Longer or shorter fatty acids may be formed by either stopping the process short, or special enzymes that can continue the process longer
Takes place primarily in adipose tissue – site of formation and storage of triglycerides
When blood glucose is high, insulin moves glucose into the cells. Once inside the cell, insulin stimulates glycolysis and
pyruvate oxidation, which provides acetyl-CoA for synthesis of fatty acids
Why do proteins get digested? Provide amino acids for synthesis of new proteins in the
body Provide N atoms for synthesis of compounds such as
nucleotides Our major energy sources are carbohydrates and lipids, but
when they are not available, proteins can be used as an energy source
Stage 1: the stomach HCl released by glands creates an acidic environment (pH
2). Denatures proteins, activates enzymes that begin to hydrolyze peptide bonds
Stage 2: the small intestine Polypeptides from the stomach move into the small
intestine. Proteolytic enzymes (such as trypsin and chymotrypsin) complete the hydrolysis of the peptides into individual amino acids.
Then, the individual amino acids are absorbed through the intestinal walls into the bloodstream. From here, they get transported to the cells.
Or, proteins do not live forever in our bodies! Old proteins are constantly being replaced with new ones. The process of synthesizing proteins and breaking them
down = protein turnover For example, insulin has a half-life of 10 minutes, and
hemoglobin has a half-life of 120 days If a protein is damaged and/or ineffective, it is also
degraded and replaced The body cannot store nitrogen, so the excess gets
excreted as urea
It wouldn’t be the first choice. Normally, only about 10% of the body’s energy
needs is supplied by amino acids. However, under conditions of fasting or starvation,
when carbohydrate and fat stores are exhausted, amino acids are used as an energy source. If these conditions persist for a long time, breakdown of
body proteins can lead to a destruction of essential body tissues.
First, the amino group is removed, yielding a keto acid.
This keto acid can be converted into an intermediate which can feed into other metabolic pathways.
Most of the amino groups get converted to urea. Degradation of amino acids primarily occurs in the
liver.
What happens to all of the ammonium ions resulting from deamination of amino acids? The ammonium groups are toxic if they are allowed to
accumulate. A pathway called the urea cycle converts them to
urea, which is transported to the kidneys to form urine. In a typical day, an adult will excrete about 25-30 g of urea
in the urine – moreso if the diet is high in protein Occurs in the liver cells
The rest of the amino acid is sent into the citric acid cycle or another metabolic pathway. If the amino acid skeleton has 3 carbons, it can be
converted to pyruvate. If they have 4 or 5 carbons, they can be converted to
another intermediate in the citric acid cycle (oxaloacetate, or ketoglutarate).
Humans (unlike bacteria) can only synthesize 10 of the 20 naturally occurring amino acids. The ones that we can synthesize are called nonessential
amino acids The ones that we cannot synthesize – in other words, we
need to consume them – are called essential amino acids Some amino acids require merely a simple one-step
synthesis (a transamination); others require a multistep reaction. We will not memorize the steps for this class; just be aware that there are different pathways for the different amino acids.