Biology Review: Proteins and Enzymes


Proteins are long chains of amino acids joined by peptide bonds that fold up into complex three-dimensional structures.

The functions of proteins are so diverse and important that almost every function of every living organism depends on proteins.

This helps explain why evolutionary biologists tend to believe that protein synthesis preceded the evolution of the first cells.

In cells, proteins account for about half of the dry mass.

Proteins are used in facilitating chemical processes, structural support, transport, communication, movement, and defense against harmful substances.

As enzymes, proteins catalyze chemical reactions and are essentially the engines of life.

Proteins are extraordinarily diverse in both function and structure. Form indeed follows function as proteins have very complex three-dimensional structures that determine which substrates it affects and what it does.

They are made up of amino acid chains held together by peptide chains. Thus, they are made up of polypeptide chains.

Each polypeptide chain has an amino end (N-terminus) and a carboxyl end (C-terminus). Jutting out from the polypeptide backbone are side groups that are important in forming the structure of the protein.

Peptide Bonds

These are covalent bonds formed by dehydration synthesis between the carboxyl group of one molecule and the amino group of the other molecule.

As with all bonds made by dehydration synthesis, a water molecule is produced.


Enzymes are macromolecules that are most often proteins but can also be RNA enzymes.

The main functions of enzymes are to speed up the rate of chemical reactions and lower the activation energy needed for those reactions to happen. In doing so, the enzyme is not consumed in the process but can be reused over and over again.

Although spontaneous chemical reactions can take place without enzymes, their speed may be far too slow to allow life to function. A process that may take years might only take seconds with the help of an enzyme.

Activation Energy Barrier

When chemical reactions happen, bonds are always broken and made.

In order to get to the point where bonds can be broken and the chemical reaction started, energy must first be invested.

This requirement is usually met by absorbing heat from the surrounding environment. When absorbed, the reactants reach their energy summit called the transition state, where they are unstable and their bonds can be broken.

However, without the help of enzymes, the amount of energy needed to start the reactions may only happen sporadically and thus slow down the rate at which cycles of the reactions happen.

How Enzymes Lower This Requirement

Proteins catalyze reactions by lowering the energy investment needed so that the reactions can take place even at room temperature.

Since proteins are specific to certain substrates and can be turned on and off, this allows cells to exactly control their metabolism.

This is better than simply heating up the organism because that might kill off the cells and it speeds up all reactions indiscriminately.

Enzymes bind the substrate they are matched for in their active site and work to orientate the substrate correctly, stabilize the transition state, stress the substrates, and help directly in the reaction.

All of these actions lower the activation energy needed for the reaction to happen.

Enzymes work extremely fast: A single enzyme usually works through a thousand substrate molecules per second, with some enzymes being much faster.

Since enzymes are unharmed and unaltered through each cycle, a small number of enzymes can greatly catalyze chemical reactions.

Article Source: Luke T Liu

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