Keywords: amino acids, dextrorotatory, levorotatory, racemic mixture, proteins, ribosomes, cell wall, osmoregulation, biology, organisms
A recent science news article discussed the importance of D-amino acids in helping bacteria adapt to changing environments.1 However, it is a commonly held idea that D-amino acids are not associated with life. What is the truth?
What Are D-amino Acids?
Amino acids, the individual building blocks that comprise proteins, have two possible orientations in space: dextrorotatory (D) and levorotatory (L). D and L can also be designated as right-handed (D) and left-handed (L). Just like your right and left hands are mirror images of one another, amino acids can also have configurations in space that are mirror images.
Although a majority of people are right-handed and only a minority are left-handed, the majority of amino acids in living organisms are left-handed. In fact it has only been in the last few decades with the advancement of technology to detect right-handed amino acids that roles have been discovered for D-amino acids in living things.
Orientation of Amino Acids and Its Relevance to the Origin of Life Debate
In the hypothetical primordial soup from which life arose, D- and L-amino acids would likely have been present in nearly equal amounts (racemic mixture).2 So, why would living things that arose by purely natural means preferentially have used L-amino acids in their proteins when both L and D were equally available?
An article from the Howard Hughes Medical Institute states, in reference to the preference, “[A] chance occurrence established this orientation bias billions of years ago, in the common ancestor of modern life.”1 Some believe that certain chemical processes were favored energetically leading to increased levels of L-amino acids on the early earth.3 Others believe that meteorites brought L-amino acids to earth, which led to an imbalance that may have favored the formation of life.4
No matter how you look at it, all of these ideas are merely humanity’s opinions about the unobservable world of the past. Many creationists enthusiastically use and endorse the “handedness” problem when referring to the origin of life issue.5 However the soundness of the argument against the naturalistic origin of life should not lead us to dismiss the evidence that D-amino acids play important roles in living things.
D-amino Acids and Living Organisms
The following is a list of organisms in which D-amino acids have been discovered and their possible roles.
In a recent study, D-amino acids were found to play a role in bacterial cell wall formation during a non-growth phase (stationary phase).6 The bacteria release and use the D-amino acids to decrease cell wall formation when resources are scarce. Thus, the D-amino acids help the bacteria adapt to adverse environmental conditions.
Previous studies have shown that D-amino acids are an important part of the bacterial cell wall and may contribute to antibiotic resistance in some bacteria. Microbes are likely the major source of D-amino acids in fermented foods (e.g., bread, cheese).3
The major source of D-amino acids for marine invertebrates (e.g., crab, shrimp, lobster) is likely seawater. It has been postulated that these organisms may use D-amino acids for osmoregulation and/or a source of L-amino acids under adverse conditions. D-amino acids are metabolized slowly and could serve as a reservoir of amino acids that can be converted to the L form when the L form is not readily available from the environment.3
The platypus, funnel web spider, and cone snail all have D-amino acids in their venom. Isomerases (enzymes that convert the D form to the L form) in these organisms are thought to produce the D-amino acids needed for the venom.7
The source of D-amino acids for humans is mainly diet.3 Food processing such as high heat converts L-amino acids to D-amino acids. D-amino acids naturally occur in fresh produce such as apples, carrots, and citrus fruits. Fermented foods (e.g., bread, cheese, vinegar) are high in D-amino acids mainly due to microbial activity.
D-amino acids are absorbed by the intestine and travel to tissues where they are metabolized. Enzymes such as amino acid oxidase break down the D-amino acids to ammonia, hydrogen peroxide, and a keto acid, which can then be used by other enzymes to make L-amino acids.8
D-amino acids may be important in the brain for neurotransmission9 and in tumor inhibition. Schizophrenic patients showed increased cognitive function and performance when given a specific D-amino acid. D-amino acids may also have analgesic (pain reduction) effects.3
Although D-amino acids may not be as common as their mirror image, they are important in living things and more studies need to be done to elucidate their exact roles. Creationists should not dismiss the importance of D-amino acids even though racemic mixtures of amino acids are a clear problem for a naturalistic origin of life. Amino acids serve important functions in living things (both in proteins and individually) and organisms were designed and created with complex mechanisms to utilize both L- and D-amino acids.