Biochimica

CHOLESTEROL METABOLISM:

Cholesterol is transported in blood plasma through lipoproteins. Apoproteins

on the lipoproteins serve as address tags determining he destination and

function of each lipoprotein, for example LDL transport cholesterol from liver

to other tissues while HDL returned extra cholesterol in the liver.

Dietary cholesterol packed into chylomicrons is absorbed in the intestine

and carried through the blood stream to the liver, which packages dietary

and endogenous cholesterol and triglycerides into VLDL. It travels to other

organs in the blood, muscles and adipose tissues extract triglycerides from

VLDL turning it into LDL. Peripheral cells take LDL by endocytosis.

Cholesterol is used for cell membrane and other functions. Extra cholesterol

is exported to HDL to return in the liver through a reverse cholesterol

transport. The liver uses cholesterol to produce bile which in the intestine

helps to break fats. Part of bile is excreted through feces and part goes back

to the liver. LDL is the major cholesterol carrier.

STATINS

Competitive inhibitors of HMG-CoA reductase

BETA OXIDATION:

mitochondria

In order to enter the mitochondria the assistance of two carrier proteins is

required for fatty acids, Carnitine acyltransferase I and II.

There are four distinct stages in the oxidation of fatty acids. Once the

triglycerides are broken down into glycerol and fatty acids, the latter must be

activated before they can enter into the mitochondria and proceed on with

beta-oxidation. This is done by Acyl-CoA synthetase (FACS) to yield fatty acyl-

CoA. After the fatty acid has been acylated, it is ready to enter into the

mitochondria. There are two carrier proteins (Carnitine acyltransferase I and II), one

located on the outer membrane and one on the inner membrane of the mitochondria.

Both are required to entry the Acyl-CoA into the mitochondria.

Carnitine palmitoyltransferase 1 (CPT1) converts the long chain acyl-CoA to long chain

acylcarnitine and allows the fatty acid moiety to be transported across the inner

mitochondrial membrane via carnitine translocase (CAT) which exchanges long chain

acylcarnitine for carnitine. An inner mitochondrial membrane enzyme, CPT2, then

converts the long chain acylcarnitine back to long chain acyl-CoA. Once inside the

mitochondria the fatty acyl-CoA can begin the beta-oxidation pathway.

Oxidation

A fatty acyl-CoA is oxidized by Acyl-CoA dehydrogenase to yield a trans alkene(trans

enoyl coA). This is done with the aid of a [FAD] prosthetic group.

Hydration

The trans alkene is then hydrated with the help of Enoyl-CoA hydratase = beta-

hydroxyacyl-coA

Oxidation +

The alcohol of the hydroxyacly-CoA is then oxidized by NAD to a carbonyl with the

+

help of Hydroxyacyl-CoA dehydrogenase(beta ketoacyl-coA). NAD is used to oxidize

the alcohol rather than [FAD] because a higher difference of reduction potential is

needed to form a double bond C=O than a double bond C=C.

Cleavage

Acetyl-CoA is finally cleaved off with the help of Thiolase to yield an Acyl-CoA that is

two carbons shorter than before.(+acetyl coA) The cleaved acetyl-CoA can easily enter

the TCA and ETC because it is already within the mitochondria.

Odd carbon atoms fatty acids

Fatty acids with an odd number of carbon atoms are common in vegetal.

They follow the beta oxidation pathway too. All the steps are the same until the last

one in which, instead of two molecules of acetyl-CoA, one molecule of acetyl-CoA and

one of propionyl-CoA are produced.

The latter is carboxylated to methyl-malonyl-CoA first and after isomerized to succinyl-

CoA by an isomerase, which uses vitamin B as coenzyme.

In the cells, unsaturated fatty acids are reduced and isomerized to beta unsaturated to

enoyl-CoA and join beta-oxidation in the first step.

Ketone bodies

Ketone bodies are three water-soluble molecules that are produced by the liver from

fatty acids during periods of low food intake (fasting) or carbohydrate restriction for

cells of the body to use as energy instead of glucose.

Although termed "bodies", they are molecules, not particles. Ketone bodies are picked

up by cells and converted back into acetyl-CoA, which then enters the citric acid cycle

and is oxidized in the mitochondria for energy. In the brain, ketone bodies are also

used to make acetyl-CoA into long chain fatty acids, which cannot pass through the

blood-brain barrier.

The liver additionally produces glucose from non-carbohydrate sources other than

fatty acids by gluconeogenesis during starvation. In the brain, ketone bodies are a

vital source of energy during fasting or strenuous exercise.

Regulation of fatty acids metabolism

Ketone bodies are produced from acetyl-CoA (during the so-called ketogenesis) mainly

in the mitochondrial matrix of hepatocytes (liver tissue) when carbohydrates are so

scarce that energy must be obtained from breaking down fatty acids.

Because of the high level of acetyl CoA present in the cell:

- The pyruvate dehydrogenase complex is inhibited,

 - Whereas pyruvate carboxylase becomes activated.

 High levels of ATP and NADH inhibit the enzyme isocitrate dehydrogenase in the

TCA cycle and, as a result, cause an increase in the concentration of malate

(due to the equilibrium between itself and oxaloacetate). The malate then

leaves the mitochondrion and undergoes gluconeogenesis.

The elevated level of NADH and ATP result from β-oxidation of fatty acids.

Unable to be used in the citric acid cycle, the excess acetyl-CoA is therefore

rerouted to ketogenesis. Such a state in humans is referred to as the fasted

state.

Acetone is produced by spontaneous decarboxylation of acetoacetate, meaning

this ketone body will break down if it is not used for energy and be removed as

waste or converted to pyruvate.

This "use it or lose it" factor may contribute to the weight loss found in

ketogenic diets.

Acetone cannot be converted back to acetyl-CoA, so it is excreted in the urine,

or (as a consequence of its high vapor pressure) exhaled unless first converted

to Pyruvate.

Acetone is responsible for the characteristic "Sweet & fruity" odor of the breath of

persons in ketoacidosis.

PROTEIN DIGESTION:

Proteins, like other dietary macromolecules, are broken down by hydrolysis of

specific peptide bonds and hence the enzymes involved are termed

‘peptidases’

• These enzymes can either cleave internal peptide bonds (endopeptidases) or

cleave off one amino acid at a time from either the – COOH or –NH2 terminal of

the polypeptide (they are exopeptidases subclassified into carboxypeptidases,

and aminopeptidases, respectively). The endopeptidases cleave the large

polypeptides to smaller oligopeptides, which can be acted upon by the

exopeptidases to produce the final products of protein digestion, amino acids,

di- and tripeptides, which are then absorbed by the enterocytes

Depending on the source of the peptidases, the protein digestive process

 can be divided into gastric, pancreatic and intestinal phases

Stomach acid and enzymes facilitate the digestion of protein.

 They’re first denatured, then broken down to polypeptides.

 The small intestine continues to break down protein into smaller peptides

 and amino acids so it can be absorbed

From the stomach, the mixture of smaller peptides pass into the small

 intestine.

The low pH stimulates secretion of SECRETIN into the blood. ENDOPEPTIDASE

EXOPEPTIDASE

pepsin trypsin chymotrypsin ENDOPEPTIDASE

carboxypeptidase

aminopeptidase dipeptidase EXOPEPTIDASE

SECRETIN stimulates the release of bicarbonate from the pancreas into

 the small intestine: pH around 7

The digestion goes on in the SMALL INTESTINE

 In the duodenum CCK(cholecistochinine) stimulates secretion of pancreatic

 enzymes as trypsin, elastase, chimorypsin, carboxypeptidase (pH 7 to 8) and

Free amino acids, di- and tripeptide are absorbed

gallbladder contraction.

across the enterocyte membrane by specific carrier- mediated transport

Amino acids are transported by specific active transporters showing

 mechanisms which are similar to ones active in glucose transport

•These Na dependent symporters are located at the brush- border membrane

•This is an indirect active process

•At least six specific symporter systems have been identified as follows:

-Neutral amino acid symporter for a.a. with short or polar side- chains (Ser,

Thr, Ala)

-Neutral amino acid symporter for aromatic or hydrophobic

side chains (Phe, Tyr,Tryp, Met,Val,Leu,Ileu)

-Basic amino acid symporter (Lys, Arg, Cys )

-Acidic amino acid symporter (Asp, Glu)

Di- and tripeptides are further hydrolyzed to their constituent amino acids

inside the enterocyte

The final transfer is therefore of free amino acids across the

 contraluminal plasma membrane into the portal blood system

Na- independent transporters are present in the contraluminal surface,

 allowing a.a. facilitated transport to the portal vein

After a protein-rich meal, protein digestion takes place in the small

 intestine

• The amino acids released are absorbed by intestinal epithelial cells

 • A large proportion of amino acids are transaminated to alanine, which is

 released into the portal vein and taken to the liver

• Therefore, ALANINE is the major amino acid secreted by the gut and the

 principal carrier of nitrogen in the plasma

Amino Acid Metabolism

Sources of amino acids

– Dietary ~100 gr/day

– Endogenous protein turnover 35-200 g/day

• Use it or lose it – amino acids can’t be stored

– Protein biosynthesis

to essential metabolites

-Conversion

– Oxidation for energy

– Excretion

Amino Acids Not Used in Biosynthetic Reactions Undergo Oxidative Degradation

• Amino acids as fuel may be derived from

– A diet rich in protein

– Cellular protein turnover

– Abnormal protein turnover (starvation or diabetes)

• Energy comes from the α-keto acid carbon skeleton, after removal of the

amino group, into either the TCA cycle or gluconeogenesis

• Removal of the amino group (waste) requires

expenditure of energy

Nitrogen catabolism:

General reactions are deamination(+NH4+), TRANSAMINATION(NH2),OXIDATIVE

DECARBOXYLATION(+NH3)

The amino group is removed from all amino acid first

Fate of nitrogen:

Plants conserve almost all the nitrogen

 Many aquatic vertebrates release ammonia to their environment



– Passive diffusion from epithelial cells

• Many terrestrial vertebrates excrete nitrogen in the form of urea

– Urea is far less toxic that ammonia – Urea has very high solubility

Some animals, such as birds and reptiles excrete nitrogen as uric acid

 (insoluble)

Humans and great apes excrete both urea (from amino acids) and uric

 acid (from purines)

Tra