Metabolism of Proteins

Metabolism of Proteins

The ingested proteins are metabolised to amino acids by peptide bond cleaving enzymes known as proteinases.

1.  General reactions of amino acids

The general reactions of amino acids include deamination, transamination and decarboxylation. The reactions of deamination and transamination bring about the formation of keto acids which can undergo a further series of changes. Inter-conversion between keto acids and amino acids results in the synthesis of many nutritionally non essential amino acids. These provide for the synthesis of protein and important non-protein nitrogenous materials. During protein synthesis the amino acids are absorbed from the blood, as the liver does not store them.

2.  Catabolism of amino acids

Although each amino acid follows its own specific metabolic pathway, a few general reactions are found to be common in the catabolism of nearly all the amino acids. Most of the amino acids are converted to a-keto acids by the removal of nitrogen in the form of ammonia which is quickly transformed into urea or it gets incorporated into some other amino acids.

Oxidative deamination

Deamination means removal of the amino groups from amino acids. This is the mechanism where in the amino acids lose two hydrogen atoms (dehydrogenation) to form keto acids and ammonia.

Oxidative deamination is accompanied by oxidation and is catalysed by specific amino acid oxidases or more appropriately, dehydrogenases present in liver and kidneys. The process of oxidative deamination takes place in two steps.

The first step is oxidation (dehydrogenation) of amino acid resulting in the formation of imino acid. The imino acid then undergoes the second step, namely hydrolysis which results in a keto acid and ammonia.

The first reaction is catalyzed by amino acid oxidase (also called dehydrogenase) and the coenzyme FAD or FMN takes up the hydrogen. There are two types of amino acid oxidases depending upon the substrate on which they act, namely,

·              L-amino acid oxidases which act on L-amino acids (FMN acts as coenzyme).

·              D-amino acid oxidases which act on D-amino acids (FAD acts as coenzyme).

FMN occurs only in the liver and kidney and FAD occurs in all animal tissues. The major site of oxidative deamination is liver but kidney and other tissues also have a role.

The oxidative deamination of L-glutamic acid is an exceptional case where the deamination needs not only the zinc-containing enzyme L-glutamic acid dehydrogenase but also NAD+ or NADP+ as coenzymes.

NADH gets oxidized to NAD+ as it passes through the electron transport chain.

As the above reaction is reversible it occurs during both amino acid catabolism and biosynthesis.

Transamination

The process of transfer of an amino group from an amino acid to an a-keto acid, resulting in the formation of a new amino acid and keto acid is known as transamination. In other words, it is deamination of an amino acid, coupled with amination of a keto acid.

Transamination is catalyzed by transaminases or aminotransferases with pyridoxal phosphate functioning as coenzyme. There are two active transaminases in tissues, catalyzing interconversions. They are

·              Aspartate aminotransferase (AST) is also known as Glutamate

-oxalo acetate transaminase (GOT)

·              Alanine aminotransferase (ALT) is also known as Glutamate

-pyruvate transaminase (GPT)

AST

ALT

It catalyses the transfer of NH₂ group from glutamate to pyruvate, resulting in the formation of a-ketoglutaric acid and alanine.

Decarboxylation

This refers to the removal of CO₂ from the carboxyl group of amino acids. The removal of CO₂ needs the catalytic action of enzymes decarboxylases and the pyridoxal phosphate coenzyme. The enzymes act on amino acids resulting in the formation of the corresponding amines with the liberation of CO₂.

There are several amino acid decarboxylases found in various tissues such as liver, kidney, intestine, spleen, lung and brain. They convert the amino acids into the respective amines and liberate CO₂. For example, histidine is converted to histamine by the action of histidine decarboxylase.

The amino acid tryptophan is converted to tryptamine, tyrosine to tyramine, etc. Such amines are called biogenic amines which are physiologically important.

Transmethylation

The transfer of methyl group from one compound to another is called transmethylation and the enzymes involved in the transfer are known as transmethylases.

Transfer of methyl group usually involves methionine (amino acid containing methyl group). By this process various important, physiologically active compounds such as epinephrine, creatine, thymine and choline are synthesised in the body. Detoxification of certain toxic substances are also carried out by this process (eg. nicotinic acid is detoxified by methionine into a nontoxic methyl derivative namely N-methyl nicotinamide).

Methionine is a principal methyl donor. It has to be activated by ATP which requires a methionine activation enzyme of liver,known as methionine adenosyl transferase. By the action of this enzyme, methionine is converted to active methionine.

The active methionine thus formed is known as S-adenosyl methionine and in the activation reaction, ATP transfers its adenosine moiety to methionine and loses three molecules of phosphate, one as orthophosphate(Pi) and two as pyrophosphate (PPi).

Active Methionine         +  Norepinephrine  ------------>  Epinephrine

Active Methionine         +  Nictoinamide      ------------> N-methyl nicotinamide

Active Methionine         +  Uracil                  ------------>  Thymine

Active Methionine         +  Guanido acetate  ------------>  Creatine

(Methyl group donor)    (Methyl group acceptor)

Active methionine contains S-methyl bond which is a high energy bond and hence methyl group is liable and can be easily transferred to a methyl group acceptor.

Catabolism of the carbon skeleton of amino acids

The carbon skeletons left behind after deamination are identified as a-keto acids. They may take any one of the following pathways.

·              Synthesis of amino acids

They may get reductively aminated by reversal of transdeamination or undergo transamination to form once again the original amino acids.

·              Glucogenic pathway

The keto acids of some amino acids may get converted to the intermediates of carbohydrate metabolism such as a-keto glutarate, oxaloacetate, pyruvate, fumarate and succinyl CoA and hence could be converted to glucose and glycogen and these amino acids are said to by glucogenic amino acids.

The pathways of three important glucogenic amino acids are shown below. Though the routes vary with each amino acid, they all converge at the stage of pyruvic acid.

Glucogenic amino acids constitute more than 50% of the amino acids, derived from animal protein. The process of conversion of the keto acids of glucogenic amino acids to carbohydrate metabolites is known as gluconeogenesis.

·              Ketogenic pathway

The keto acids formed from the deamination of certain amino acids are closely related to fats rather than carbohydrates. They metabolise to form acetyl CoA or acetoacetyl CoA or acetoacetate (ketone bodies) which are the intermediates of fatty acid metabolism and not glucose and these amino acid are said to be ketogenic amino acids.

Ketogenic amino acids constitute only a minority and follow specialised and complex pathways. Examples are leucine, isoleucine, phenyl alanine and tyrosine. Among these, leucine is purely ketogenic, whereas the other three amino acids are both ketogenic and glucogenic.